Notes on Writing Portable Programs in C

A. Dolenc
A. Lemmke
Helsinki University of Technology
D. Keppel
CS&E, University of Washington
G. V. Reilly
Dept. of Computer Science, Brown University

Nov 1990, 8th Revision

Note: This document was placed in the public domain by the authors. This version was converted to XHTML and is hosted by Jason R. Blevins. The original LaTeX source is also available: cport.tar.gz.


This documents describes the features and non-features of different C preprocessors, compilers, and environments. As such, it is an incomplete document, growing as information is gathered. It contains some material concerning ANSI C but it is not a substitute for the Standard itself; neither are related textbooks. We assume the reader is familiar with the C programming language.


  1. Foreword
  2. Introduction
  3. Standardization Efforts
  4. Preprocessors
  5. The Language
  6. Unix Flavors: System V and BSD
  7. Header Files
  8. Run-time Library
  9. Using Floating-Point Numbers
  10. VMS
  11. General Guidelines
  12. Further Reading
  13. Acknowledgements
  14. Trademarks
  15. References

1. Foreword

We will call a program portable if adapting it to a new environment is easier than rewriting it for that environment. This document is mainly for those who have never ported a program to another platform—a specific hardware and software environment—and, evidently, for those who plan to write large systems which must be used across different vendor machines. If you have already done some porting, you may not find the information herein very useful.

We suggest that Recommended C Style and Coding Standards [1] be read in conjunction with this document.1 Posters to the newsgroup comp.lang.c have repeatedly recommended Portable C Software [4] and C Traps and Pitfalls [8] (none of the information herein has been taken from those two references).

Disclaimer: We will attempt to keep the information herein updated, but it can happen that some of it may be incorrect at the time of reading. The code fragments presented are intended to make applications “more” portable, meaning that they may fail with some compilers and/or environments.

This document can be obtained via anonymous FTP from [] in ~ftp/pub/CompSciLab/doc. The files portableC.tex, portableC.sty, portableC.bib, and are the LaTeX source and style files, BibTeX and the compressed PostScript, respectively. Alternatively, there is a site in the US from which one can obtain all four files, [] in ~ftp/pub/cport.tar.Z. All files are in the public domain. Comments, suggestions, flames, eggs, and requests for copies via e-mail should be directed to

2. Introduction

The aim of this document is to collect the experience of several people who have had to write and/or port programs written in C to more than one platform.

In order to keep this document within reasonable bounds, we must restrict ourselves to programs which must execute under Unix-like operating systems and those which implement a reasonable Unix-like environment. The only exception we will consider is VMS.

A wealth of information can be obtained from programs that have been written to run on several platforms. This is the case of publicly available software such as that developed by the Free Software Foundation and the MIT X Consortium.

When discussing portability, one focuses on two issues:

The language,
which includes the preprocessor and the syntax and the semantics of the language.
The environment,
which includes the location and contents of header files and the run-time library.

We include in our discussions the standardization efforts upon the language and the environment. Special attention will be given to floating-point representations and arithmetic, to limitations of specific compilers, and to VMS.

Our main focus will be boiler-plate problems. Systems programming, e.g., raw I/O from terminals, and twisted code associated with bizarre interpretations of the ANSI C Standard [13]—henceforth referred to as the Standard—are not extensively covered in this document.2

3. Standardization Efforts

All standards have a good side and an evil side. Due to the nature of this document, we are forced to focus our attention on the latter.

The American National Standards Institute (ANSI) has recently approved of a standard for the C programming language [13]. The Standard concentrates on the syntax and semantics of the language and specifies a minimum environment (the name and contents of some header files and the specification of some run-time library functions).

Copies of the ANSI C Standard (ANSI X3.159-1989) can be obtained from the following address:

American National Standards Institute
Sales Department
1430 Broadway
New York, NY 10018
(Voice) (212) 642-4900
(Fax) (212) 302-1286

3.1. ANSI C

3.1.1. Translation Limits

We first bring to the reader’s attention the fact that the Standard states some environmental limits. These limits are lower bounds, meaning that a correct (compliant) compiler may refuse to compile an otherwise-correct program that exceeds one of those limits.3

Below are the limits that we judge to be the most important. The ones related to the preprocessor are listed first.

It is really unfortunate that some of these limits may force a programmer to code in a less elegant way. We are of the opinion that the remaining limits stated in the Standard can usually be obeyed if one follows “good” programming practices.

However, these limits may break programs that generate C code such as compiler-compilers and many C++ compilers.

3.1.2. Unspecified and Undefined Behavior

The following are examples of unspecified and undefined behavior:

  1. The order in which the function designator and the arguments in a function call are evaluated.
  2. The order in which the preprocessor concatenation operators # and ## are evaluated during macro substitution.
  3. The representation of floating-point types.
  4. An identifier is used that is not visible in the current scope.
  5. A pointer is converted to something other than an integral or pointer type.

The list is long. One of the main reasons for explicitly defining what is not covered by the Standard is to allow the implementor of the C environment to make use of the most efficient alternative.

3.2. POSIX

The objective of the POSIX working group P1003.1 is to define a common interface for Unix. Granted, the ANSI C standard does specify the contents of some header files and the behavior of some library functions but it falls short of defining a useful environment. This is the task of P1003.1.

We do not know how far P1003.1 addresses the problems presented in this document as at the moment we lack proper documentation. Hopefully, this will be corrected in a future release of this document.

4. Preprocessors

Preprocessors can behave differently in several ways. For those who need them, there are good publicly available preprocessors that are ANSI C-compliant. One such preprocessor is the one distributed with the X Window System developed by the MIT X Consortium.

4.1. Command Options

The interpretation of the -I command option can differ from one system to another. Besides, it is not covered by the Standard. For example, the directive #include "dir/file.h" in conjunction with -I.. would cause most preprocessors in a Unix-like environment to search for file.h in ../dir, but under VMS, file.h is only searched for in the subdirectory dir in the current working directory.

4.2. #pragma and #elif

Directives are very much the same in all preprocessors, except that some preprocessors may not know about the defined operator in a #if directive nor about the #pragma and #elif directives.

The #pragma directive should pose no problems even to old preprocessors if it comes indented.4 Furthermore, it is advisable to enclose them with #ifdefs in order to document under which platform they make sense:

#ifdef <platform-specific-symbol>
   #pragma ...

Beware of #pragma directives that alter the semantics of the program and consider the case when they are not recognized by a particular compiler. Evidently, if the behavior of the program relies on their correct interpretation then, in order for the program to be portable, all target platforms must recognize them properly.

4.3. Concatenation

Concatenation of symbols has two variants. One is the old K&R [9] style that simply relied on the fact that the preprocessor substituted comments such as /**/ for nothing. Obviously, that does not result in concatenation if the preprocessor includes a space in the output. The ANSI C Standard defines the operators ## and (implicit) concatenation of adjacent strings. Since both styles are a fact of life it is useful to include the following in one's header files:5

#ifdef  __STDC__
#  define  GLUE(a,b)  a##b
#  define  GLUE(a,b)  a/**/b

If needed, one could define similar macros to GLUE several arguments.6

4.4. Token Substitution

Some preprocessors perform token substitution within quotes while others do not. Therefore, this is intrinsically non-portable. The Standard disallows it but provides a mechanism to obtain the same results. The following should work with ANSI-compliant preprocessors or with the ones that perform token substitution within quotes:

#ifdef  __STDC__
#  define  MAKESTRING(s)  # s
#  define  MAKESTRING(s)  "s"

4.5. Miscellaneous

5. The Language

5.1. The Syntax

The syntax defined in the Standard is a superset of the one defined in K&R [9]. It follows that if one restricts oneself to the former, there should be no problems with an ANSI C-compliant compiler with respect to syntax. The semantics are, however, another problem altogether and is covered superficially in the next section.

The Standard extends the syntax with the following:

  1. The inclusion of the keywords const, enum, signed, void, and volatile.
  2. The inclusion of additional constant suffixes to indicate their type.
  3. The ellipsis (“...”) notation to indicate a variable number of arguments.
  4. Function prototypes.
  5. Trigraph notation for specifying otherwise-unobtainable characters in restricted character sets.

We encourage the use of the reserved words const and volatile since they aid in documenting the code. It is useful to add the following to one's header files if the code must be compiled by a non-conforming compiler as well:

#ifndef __STDC__
#  define const
#  define volatile

However, one must then make sure that the behavior of the application does not depend on the presence of such keywords. (Evidently, programs that contain identifiers with those names must be modified to conform to the Standard.)

The trigraph notation can bring unexpected results when a program is compiled by an ANSI-compliant compiler, e.g., strings such as "??!" will produce "|". Watch out!

5.2. The Semantics

The syntax does not pose any problem with regard to interpretation because it can be defined precisely. However, programming languages are always described using a natural language, e.g., English, and this can lead to different interpretations of the same text.

Evidently, K&R [9] does not provide an unambiguous definition of the C language otherwise there would have been no need for a standard. Although the Standard is much more precise, there is still room for different interpretations in situations such as f(p=&a, p=&b, p=&c). Does this mean f(&a,&b,&c) or f(&c,&c,&c)? Even “simple” cases such as a[i] = b[i++] are compiler-dependent [1].

As stated in the Introduction, we would like to exclude such topics. The reader is instead directed to the Usenet newsgroups comp.std.c or comp.lang.c where such discussions take place and from where the above example was taken. The Journal of C Language Translation7 could, perhaps, be a good reference. Another possibility is to obtain a clarification from the Standards Committee and the address is:

X3 Secretariat, CBEMA 311 1st St NW Ste 500 Washington DC, USA

Finally, we mention that a complete list of the differences between “ordinary” C and ANSI C can be found in the Second Edition of K&R [10]. A slightly less up-to-date list can also be found in C: A Reference Manual [5].

6. Unix Flavors: System V and BSD

A long time ago (1969), Unix said “papa” for the first time at AT&T (then called Bell Laboratories, or Ma Bell for the intimate) on a PDP-7. Everyone liked Unix very much and its widespread use we see today is probably due to the relative simplicity of its design and of its implementation. (It is written, of course, mostly in C.)

However, these facts also contributed to everyone developing their own dialect. In particular, the University of Berkeley at California distribute the so-called BSD8 Unix whereas AT&T now distribute (sell) System V Unix. All other versions of Unix are descendants of one of these major dialects.

The differences between these two major flavors should not upset most application programs. In fact, we would even say that most differences are just annoying.

BSD Unix has an enhanced signal handling capability and implements sockets. However, all Unix flavors differ significantly in their raw I/O interface (that is, the ioctl system call), and this should be avoided if possible.

The reader interested in knowing more about the past and future of Unix can consult "A Single Standard Emerges from the UNIX Tug-Of-War" [11] and "Interview With Five Technologists" [6].

7. Header Files

Many useful system header files are in different places in different systems, or they define different symbols. We will assume henceforth that the application has been developed on a BSD-like Unix and must be ported to a System V-like Unix or VMS or a Unix-like system with header files that comply with the Standard.

In the following sections, we show how to handle the most simple cases that arise in practice. Some of the code that appears below was derived from the header file Xos.h which is part of the X Window System distributed by MIT. We have added changes, e.g., to support VMS.

Many header files are unprotected in many systems, notably those derived from BSD version 4.2 and earlier. By "unprotected" we mean that an attempt to include a header file more than once will either cause compilation errors (e.g., due to recursive or nested includes) or, in some implementations, warnings from the preprocessor stating that symbols are being redefined. It is good practice to protect header files.

7.1. ctype.h

ctype.h provides almost the same functionality on all systems, except that some symbols must be renamed.

#ifdef SYSV
#  define  _ctype_  _ctype
#  define  toupper  _toupper
#  define  tolower  _tolower

Under Sys V, toupper and tolower are also defined and will check the validity of their arguments and perform the conversion only if necessary. Under BSD-derived systems, one must normally remember to check the validity of the arguments. The following solution might be acceptable to most:

#ifdef SYSV
#  define  TOUPPER(c)  toupper(c)
#else /* !SYSV */
#  define  TOUPPER(c)  (islower(c)?toupper(c):(c))

The definitions in <ctype.h> are not portable across character sets.

7.2. fcntl.h and sys/file.h

Many files that a BSD-like system expects to find in the sys directory are placed in /usr/include in System V. Other systems, such as VMS, do not even have a sys directory.9

The symbols used in the open function call are defined in different header files in the two types of systems:

#ifdef  SYSV
#  include <fcntl.h>
#  include <sys/file.h>

In some systems, e.g., BSD 4.3 and SunOS, it does not make a difference which one is used because both define the O_xxxx symbols.

7.3. errno.h

The semantics of the error number may differ from one system to another and the list may differ as well (e.g., BSD systems have more error numbers than System V). Some systems, e.g., SunOS, define the global symbol errno which will hold the last error detected by the run-time library. This symbol is not declared in most systems, although it is required by the Standard that such a symbol be defined (see section 4.1.3 of the Standard [13]). It is, of course, available in all Unix implementations.

The most portable way to print error messages is to use perror.

7.4. math.h

System V has more definitions in this header file than BSD-like systems. The corresponding library has more functions as well. This header file is unprotected under VMS and Cray, and in that case we must do it ourselves:

#if defined(CRAY) || defined(VMS)
#  ifndef  __MATH__
#    define  __MATH__
#    include <math.h>
#  endif

7.5. strings.h vs. string.h

Some systems cannot be treated as System V or BSD, but are really special cases, as one can see in the following:

#ifdef  SYSV
#  ifndef SYSV_STRINGS
#    define  SYSV_STRINGS
#  endif

#ifdef  _STDH_  /* ANSI C Standard header files */
#  ifndef SYSV_STRINGS
#    define  SYSV_STRINGS
#  endif

#ifdef  macII
#  ifndef SYSV_STRINGS
#    define  SYSV_STRINGS
#  endif

#ifdef  vms
#  ifndef SYSV_STRINGS
#    define  SYSV_STRINGS
#  endif

#  include <string.h>
#  define  index   strchr
#  define  rindex  strrchr
#  include <strings.h>

As one can easily observe, System V-like Unix systems use different names for index and rindex and place them in different header files. Although VMS supports better System V features, it must be treated as a special case.

7.6. time.h and types.h

When using time.h, one must also include types.h. The following code does the trick:

#ifdef macII
#  include <time.h>   /* on a Mac II we need this one as well */

#ifdef  SYSV
#  include <time.h>
#  ifdef vms
#    include <time.h>
#  else
#    ifdef CRAY
#      ifndef __TYPES__   /* it is not protected under CRAY */
#        define __TYPES__
#        include <sys/types.h>
#      endif
#    else
#      include <sys/types.h>
#    endif /* of ifdef CRAY */
#    include <sys/time.h>
#  endif  /* of ifdef vms  */

The above is not sufficient in order for the code to be portable since the structure that defines time values is not the same in all systems. Different systems have vary in the way time_t values are represented. The Standard, for instance, only requires that it be an arithmetic type. Recognizing this difficulty, the Standard defines a function called difftime to compute the difference between two time values of type time_t, and mktime which takes a string and produces a value of type time_t.

7.7. varargs.h vs. stdarg.h

In some systems the definitions in both header files are contradictory. For instance, the following will produce compilation errors, e.g., under VMS:

#include <varargs.h>
#include <stdio.h>

This is because <stdio.h> includes <stdarg.h> which in turn redefines all the symbols (va_start, va_end, etc.) in <varargs.h>. This is incorrect behavior because Standard header files should not include other Standard header files. Furthermore, the method used in <varargs.h> for defining variadic functions is incompatible with the Standard (see section 11.2.3 for more information on variadic functions).

The solution we adopt is to always include <varargs.h> last and not to define in the same module both functions that use <varargs.h> and functions that use the ellipsis notation.

7.8. sys/wait.h

This one is lacking in some systems (e.g., Altos and Xenix). HP-UX does define it but one must use macros to access the fields of the wait struct, instead of using the names of the fields. The wait struct uses bit-fields and if the platform does not define it one must do it oneself and care must be taken with respect to byte ordering (see Byte ordering in section 11.1).

8. Run-time Library

This section admittedly contains very little information if compared to Portable C Software [4]. We direct the reader to that reference for more information.

Time and time again, it happens that the target platform does not have all the library functions needed by a given application. This is particularly true with mathematical functions. We would like to remind the reader that the sources to 4.3BSD are publicly available, and may be obtained at several sites, e.g., [] in ~ftp/pub/bsd-sources, the contents of which are cloned from Read the copyright notices before using them.

8.1. Mathematical Functions

8.1.1. cbrt and pow

cbrt(x) evaluates the cube root of its argument, that is, x1/3. pow(x,y) evaluates xy. Some systems implement neither of these, or just the latter. In that case, one can define pow as a function of exp and log, and if one has pow but not cbrt, one can write the latter as a function of the former:

#define  pow(x,y)  (exp(log(x)*(y)))
#define  cbrt(x)   (pow((x),1./3.))

Thus defined, pow only admits strictly positive arguments. If the argument x is negative, then a result can be evaluated if y is an integer and one must implement such a function oneself (a predicate which determines if y is an integer is usually not available).

The definitions given above are a “poor man’s” solution to the problem but acceptable in many situations. In order to obtain numerically robust and accurate results one must investigate other alternatives such as obtaining the source code for the 4.3BSD implementation via anonymous FTP as mentioned at the beginning of this Section.

It should be mentioned that if the argument y is zero then implementations differ on the result. The 4.3BSD implementation returns always 1.0; others may return undefined values, flag an error, or return not-a-number.

8.1.2. rand

rand returns a pseudo-random integer in the range 0 to RAND_MAX, which is guaranteed only to be at least 32,767. Do not rely on rand returning results over a much wider range.

8.2. Memory allocation and initialization

8.2.1. alloca

alloca(n) allocates the amount of bytes specified by n and returns a pointer to the allocated memory. This space is—for all practical purposes—automatically deallocated (freed) when the block scope is exited. More specifically, the storage is deallocated no sooner than the exit from the block scope; the implementation is allowed to do the freeing at function exit, upon the next call to alloca, or at any other moment deemed appropriate. The example below illustrates incorrect usage of alloca:

foo ()
 char *sto;
      sto = alloca (10);
      use (sto);  /* Correct. */
    use (sto); /* Error: storage may have been freed. */

Conceptually, the space is allocated on a stack, so allocation can be as fast as just adjusting the stack pointer if the machine has one, and several regions can be freed at once by simply readjusting the stack pointer. However, it is hard to implement alloca both portably and efficiently. alloca is not available on all platforms and as such is not required by the Standard. However, there are public domain implementations that work in a wide variety of cases, but which can be slow and which can delay freeing arbitrarily.10

Thus, while it is very desirable to use alloca when it is available, because of efficiency considerations, it is highly recommended that the code be written so that malloc and free can easily replace it, if and when necessary.

8.2.2. bcopy vs. memcpy and memmove

bcopy(s1,s2,n) copies the string s1 into s2, whereas memcpy(s1,s2,n) copies s2 into s1. bcopy can be found in BSD-like systems, and some implementations handle overlapping strings, while others do not. memcpy and memmove are implemented in the other camp (System V); memcpy does not handle overlapping strings, whereas memmove does.

The normal solution is to use macros.

8.2.3. bzero vs. memset

bzero(s,n) is equivalent to memset(s,0,n). The former is implemented in BSD-like systems, whereas the latter is implemented in System V-like systems and is required by the Standard.

See also Initialization in section 11.2.4.

8.2.4. malloc and free

malloc is available in all C implementations and its behavior is very well defined except in boundary conditions. Not all implementations accept a zero-sized request. There are other minor differences such as the return type being char * in some implementations and void * in others.

In a similar vein, some implementations of free do not accept NULL as an argument. Worse, though, is that some implementations allowed the caller to use the pointer even after it had been freed so long as no other call to malloc was performed. Relying on such behavior is bad.

8.2.5. realloc

realloc(sto,n) takes a pointer to a region allocated with malloc and grows or shrinks the region so that it is of size n. The return value from realloc is a pointer to the resized storage; if the storage was grown “in place”, the return value is the same as sto. If the region was moved, then the old contents are copied to the new storage (if n is smaller than the old size, then only the first n units are copied). If the region is grown, the new storage at the end is uninitialized and may contain garbage.

Under ANSI C:

For non-ANSI versions of realloc, specifying NULL as the storage or 0 as the new size causes undefined behavior. Thus, it is recommended that portable programs, even those written in ANSI C, not use these features. If it is necessary to rely on those features, use a macro or write a function that can be configured to check for those cases explicitly.

8.3. Miscellaneous

8.3.1. scanf

scanf can behave differently on different platforms because its descriptions, including the one in the Standard, allows for different interpretations under some circumstances. The most portable input parser is the one you write yourself.

Some versions of the scanf family modify and then restore arguments which are string constants. These implementations cause problems when string constants are placed in read-only memory (see “String constants” in section 11.2.4). If the string is actually a constant, then some workaround is needed; usually a compiler flag may be used to indicate that such constants should be placed in writable memory instead. If such a flag is not available then the code must be modified.

8.3.2. setjmp and longjmp

Quoting anonymously from comp.std.c, “pre-X3.159 implementations of setjmp and longjmp often did not meet the requirements of the Standard. Often they didn't even meet their own documented specs. And the specs varied from system to system. Thus it is wise not to depend too heavily on the exact standard semantics for this facility…”

In other words, it is not that you should not use them but be careful if you do. Furthermore, the behavior of a longjmp invoked from a nested signal handler11 is undefined.

Finally, the symbols _setjmp and _longjmp are only defined under SunOS, BSD, and HP-UX. Some systems do not implement setjmp and friends at all.

8.3.3. Signal Handling

We would like to point out one problem when handling signals generated by hardware, such as SIGFPE and SIGSEGV. There are two possibilities on a normal exit from the signal handler: (i) the offending instruction is re-executed, or (ii) it is not.

The first possibility may cause an infinite loop, and the only portable solution is to longjmp out of the signal handler.

9. Using Floating-Point Numbers

To say that the implementation of numerical algorithms that exhibit the same behavior across a wide variety of platforms is difficult, is an understatement. This section provides very little help but we hope it is worth reading. Any additional suggestions and information are very much appreciated as we would like to expand this section.

9.1. Machine Constants

One problem when writing numerical algorithms is obtaining machine constants. Typical values one needs are:

On Suns, they can be obtained in <values.h>. The ANSI C Standard recommends that such constants be defined in the header file <float.h>.

Suns and standards apart, these values are not always readily available, e.g., in Tektronix workstations running UTek. One solution is to use a modified version of a program that can be obtained from the network which is called machar. Machar is described in Algorithm 665, MACHAR: A Subroutine to Dynamically Determine Machine Parameters [2] and can obtained by anonymous FTP from the netlib.13

It is straightforward to modify the C version of machar to generate a C preprocessor file that can be included directly by C programs.

There is also a publicly available program called config.c that attempts to determine many properties of the C compiler and machine that it is run on. It can generate the ANSI C header files <float.h> and <limits.h> among other useful features. This program was submitted to comp.sources.misc.14 The latest version, 4.2, is available by FTP from in directory misc and is called config42.c (the next version, 4.3, will be called enquire.c). Version 4.2 is also distributed with gcc, where it is called hard-params.c.

9.2. Floating-Point Arguments

In the days of K&R [9] one was “encouraged” to use float and double interchangeably15 since all expressions with such data types where always evaluated using the double representation—a real nightmare for those implementing efficient numerical algorithms in C. This rule applied, in particular, to floating-point arguments and for most compilers around, it does not matter whether one defines the argument as float or double.

According to the ANSI C Standard, such programs will continue to exhibit the same behavior as long as one does not prototype the function. Therefore, when prototyping functions, make sure that the prototype is included when the function definition is compiled so the compiler can check if the arguments match.

9.3. Floating-Point Arithmetic

Be careful when using the == and != operators to compare floating-point types. Expressions such as

if (float_expr1 == float_expr2)

will seldom be satisfied due to rounding errors. To get a feeling about rounding errors, try evaluating the following expression using your favorite C compiler [7]:

10^50 + 812 - 10^50 + 10^55 + 511 - 10^55 = 812 + 511 = 1323

Most computers will produce zero regardless of whether one uses float or double. Although the absolute error is large, the relative error is quite small and probably acceptable for many applications.

It is rather better to use expressions such as

|float_expr1 - float_expr2| <= K


||float_expr1 / float_expr2| - 1.0| <= K

(if float_expr2 != 0.0), where 0 < K < 1 is a function of:

  1. The floating type, e.g., float or double,
  2. the machine architecture (the machine constants defined in the previous section), and
  3. the precision of the input values and the rounding errors introduced by the numerical method used.

Other possibilities exist and the choice depends on the application.

The development of reliable and robust numerical algorithms is a very difficult undertaking. Methods for certifying that the results are correct within reasonable bounds must usually be implemented. A reference such as NUMERICAL RECIPES in C [12] is always useful.

9.4. Exceptions

Floating-point exceptions (overflow, underflow, division by zero, etc) are not signaled automatically in some systems. In that case, they must be explicitly enabled.

Always enable floating-point exceptions, since they may be an indication that the method is unstable. Otherwise, one must be sure that such events do not affect the output.

10. VMS

In this section, we will report some common problems encountered when porting a C program to a VMS environment and which we have not mentioned previously.

10.1. File Specifications

Under VMS, one can use two flavors of command interpreters: DCL and DEC/Shell. The syntax of file specifications under DCL differs significantly from the Unix syntax.

Some C run-time library functions in VMS that take file specifications as arguments or return file specifications to the caller, will accept an additional argument indicating which syntax is preferred. It is useful to use these run-time library functions via macros as follows:

#ifdef  VMS
#  ifndef VMS_CI        /* Which Command Interpreter to use */
#    define VMS_CI  0   /* 0 for DEC/Shell, 1 for DCL */
#  endif

#  define  Getcwd(buff,siz)   getcwd((buff),(siz),VMS_CI)
#  define  Getname(fd,buff)   getname((fd),(buff),VMS_CI)
#  define  Fgetname(fp,buff)  fgetname((fp),(buff),VMS_CI)

#else  /* !VMS */
#  define  Getcwd(buff,siz)   getcwd((buff),(siz))
#  define  Getname(fd,buff)   getname((fd),(buff))
#  define  Fgetname(fp,buff)  fgetname((fp),(buff))

#endif /* !VMS */

More pitfalls await the unaware who accept file specifications from the user or take them from environment values (e.g., using the getenv function).

10.2. Miscellaneous

end, etext, edata:

these global symbols are not available under VMS.

struct assignments:

VAX C allows assignment of different types of structs if both types have the same size. This is not a portable feature.

The system function:

the system function under VMS has the same functionality as the Unix version, except that one must take care that the command interpreter also provides the same functionality. If the user is using DCL, then the application must send a DCL-like command.

The linker:

what follows applies only to modules stored in libraries.16 If none of the global functions are explicitly used (referenced by another module), then the module is not linked at all. It does not matter whether one of the global variables is used. As a side effect, the initialization of variables is not done.

The easiest solution is to force the linker to add the module using the /INCLUDE command modifier. Of course, there is the possibility that the command line may exceed 256 characters...(*sigh*).

11. General Guidelines

11.1. Types and Pointers

Type sizes:

Never make any assumptions about the size of a given type, especially pointers [1]. Statements such as x &= 0177770 make implicit use of the size of x. If the intention is to clear the lowest three bits, then it is best to use x &= ~07. The first alternative will also clear the high-order 16 bits if x is 32 bits wide.

Byte ordering:

There are two possibilities for byte ordering: little-endian and big-endian architectures. This problem is illustrated by the code below:

long int str[2] = {0x41424344, 0x0}; /* ASCII "ABCD" */
printf ("%s\n", (char *)&str);

A little-endian (e.g., VAX) will print “DCBA” whereas a big-endian (e.g., MC68000 microprocessors) will print “ABCD”. (As a side note, there is also PDP-endian that would print “BADC”, followed by many smileys.)

Note: The example will only function correctly if sizeof(long int) is 32 bits. Although not portable, it serves well as an example for the given problem.

Alignment constraints:

Beware of alignment constraints when allocating memory and using pointers. Some architectures restrict the addresses that certain operands may be assigned to (that is, addresses of the form 2^k E, where k > 0). Code such as

char *s = "bla"; /* allocated by compiler */
int  *v = (int *)s;

would most probably fail if the alignment constraints of int types are more strict than those of char types (the usual case for RISC architectures). The code would not fail due to alignment constraints if the memory indicated by s had been allocated by malloc and friends.

Pointer formats [1]

Pointers to objects may have the same size but different formats. This is illustrated by the code below:

int *p = (int *) malloc(...); ... free(p);

This code may malfunction in architectures where int * and char * have different representations because free expects a pointer of the latter type.

Pointers to different types of objects may have different sizes as well. For instance, there are platforms where a char * is larger than an int * or where a pointer to a function will not fit in, e.g., char * or void * (although such cross-assignments work on many platforms, void * is only guaranteed to be large enough to hold a pointer to any data object). Therefore, it is not portable to assign to an object of type void * a pointer to a function. Pointers to functions are further discussed below.

Pointers to functions

If you need a generic function pointer, then use void(*)(void). Be sure to cast the pointer back to the original type before using it. That is, the type signature of the function pointer at the point that the function is called must exactly match the type signature at the point at which the function is defined.

For example, it is not possible to (portably) use varargs functions17 (that is, functions that take a variable number of arguments) and fixed-argument functions interchangeably, even if the overlapping types match (that is, even if the first n arguments to the fixed-argument function are the same as the first n arguments to the varargs function). For instance, a function that is declared as having an integer as the first argument and an optional (integer) second argument cannot be called as a function that takes two integer arguments. Similarly, varargs functions of various type signatures cannot be interchanged. Such type cheating will break on systems that use different conventions for calling fixed-argument and varargs functions and on systems that use different conventions for passing the fixed and varargs parts of the argument lists.

As a corollary, it is necessary that the definitions of external variadic functions be available at the point of their usage, e.g., library functions such as printf.

Pointer operators [1]

Only the operators == and != are defined for all pointers of a given type. The remaining comparison operators (<, <=, >, and >=) can only be used when both operands point into the same array or to the first element after the array. The same applies to arithmetic operators on pointers.18

NULL pointer:

Never redefine the NULL symbol. The NULL symbol should always be the constant zero. A null pointer of a given type will always compare equal to the constant zero, whereas comparison with a variable with value zero or to some non-zero constant has implementation-defined behavior. (In other words, the constant zero has two meanings.)

A null pointer of a given type will always convert to a null pointer of another type if implicit or explicit conversion is performed. (See 'Pointer Operators' above.)

The contents of a null pointer may be anything the implementor wishes, and dereferencing it may cause strange things to happen…

11.2. Compiler Differences

11.2.1. Conversion Rules

In arithmetic expressions, integral types may be converted in two ways: unsigned-preserving or value-preserving. In the unsigned-preserving model, chars, shorts, and bit-fields are converted to unsigned int or signed int if the original types have the modifiers unsigned or signed, respectively.

The Standard determines that the value-preserving model must be used, meaning that unsigned values are promoted to signed int, or simply int, if it can represent all the values of the original type; otherwise it is converted to unsigned int. (See section 3.2 of the Standard.)

The following example illustrates the problem. On a machine with a 16-bit short int, and 32-bit int, the code fragment

unsigned short int x = 1;
if (x < -1) printf ("unsigned-preserving");
else printf ("value-preserving");

prints unsigned- or value-preserving accordingly. Plenty of other examples can be derived, such as initializing x with 2^15 and using the predicate (x*x*2 > 0). The expression x*x*2 would probably result in the same bit pattern in both models but would cause arithmetic overflow in the value-preserving model.

11.2.2. Compiler Limitations

In practice, much too frequently one runs into several, unstated compiler limitations:

11.2.3. ANSI C

The Standard has introduced and officialized current practice, but as we all know not many compilers conform to the Standard. Among the features that are not yet widely supported, we mention here only a few:

Constant suffixes:

Many compilers allow for suffixes to be appended to constants, such as 10L to indicate a long constant. The Standard allows further typing of constants, such as 10UL to indicate an unsigned long constant. However, multiple suffixes are not supported by many compilers.

New types:

Besides the type void * which is mentioned in the next section, the Standard has introduced the type long double.

Variadic functions:

Variadic functions, as defined by the Standard, differ significantly from <varargs.h>. Besides the ellipsis notation, it is required by the Standard that the first argument be identified and that <stdarg.h> be used instead (see section 7.7). Therefore, it is not possible to define a variadic function which takes no arguments.

11.2.4. Miscellaneous

char types:

When char types are used in expressions, most implementations will treat them as unsigned but there are many others that treat them as signed (e.g., VAX C and HP-UX). It is advisable to always cast chars when they are used in arithmetic expressions.


Do not rely on the initialization of auto variables and of memory returned by malloc. In particular, since not all NULL pointers are represented by a bit pattern of all-zeroes, it is good practice to always initialize pointers appropriately.

The calloc library function returns an area of memory that has been cleared to zero. Although this can be used to initialize arrays and structs on many architectures, not all architectures represent NULL pointers internally with a zero bit-pattern. Similarly, it is not safe to assume that all architectures represent the floating-point constant 0.0 using a zero bit-pattern.

The semantics of many library functions differ from system to system. Also, the specifications of some library functions have been changed in the ANSI C Standard. For example, realloc is now required to behave like malloc when called with a NULL argument; formerly, many implementations would dump core if handed NULL.

Bit fields:

Some compilers, e.g., VAX C, require that bit fields within structs be of type int or unsigned. Furthermore, the upper bound on the length of the bit field may differ among different implementations.

  • The result of sizeof may be unsigned or signed.
  • If p is a pointer, then sizeof(*p) is allowed by the Standard and many compilers even if p does not contain a valid address such as NULL. However, some compilers dereference the pointer causing programs to crash.
void and void *:

Some very old compilers do not recognize void [sic]. Although required by the Standard, some compilers recognize void but fail to recognize void *. The following code might prove useful:

#if __STDC__
#  define  HAS_VOIDP
#ifdef HAS_VOIDP
   typedef void *voidp;
   typedef char *voidp;
Functions as arguments:

When calling functions passed as arguments, always dereference the pointer. In other words, if f is a pointer to a function, use (*f)() instead of simply (f)(), because some compilers may not recognize the latter.

String constants:

Do not modify string constants since many implementations place them in read-only memory. Furthermore, that is what the Standard requires—and that is how a constant should behave!

Note: In statements such as "char *s = "string"", "string" is a string constant, whereas in "char s[] = "string" it is not and it is legal to modify s.

struct comparisons:

Some compilers might allow for structs to be compared for equality or inequality. Such an extension is not included in the Standard (meaning it is not portable).

Initialization of aggregates:

Some compilers cannot initialize auto aggregate types. Statements such as:

   typedef struct {double x,y} Interval;
   Interval range = {0.0,0.0};

are not allowed by some compilers unless the modifier static is used or if range has file scope. Although declaring all such variables static would handle most situations, the most portable solution is to add code that performs the initialization.

Nested comments:

Nested comments were never allowed in the C language, but they are allowed by some compilers. Nested comments are used by some to comment out source code containing comments. However, the same effect can be obtained using an #if 0 and #endif pair.

Shift operators:

When shifting signed ints right, the vacated bits might be filled with zeroes or with copies of the sign bit. unsigned ints will be filled with zeroes.

Division and remainder:

When both operands are non-negative, then the remainder is non-negative and smaller than the divisor; if not, it is guaranteed only that the absolute value of the remainder is smaller than the absolute value of the divisor.

11.3. Files

11.3.1. General Guidelines

Remember that not all operating systems share Unix’s simple notion of a file as a stream of bytes. MS-DOS, for instance, has text files and binary files; it is important to open files in the correct mode. VMS has many different file types and each file is viewed as being a collection of structured records.

MS-DOS provides a “poor man’s” implementation of pipes and redirection. It does not expand wildcards, however. The user must do the wildcard expansion using findfirst and findnext. Under VMS, the user must also expand wildcards, and parse argv for redirection directives manually.

Different operating systems use widely different syntax to specify pathnames. This is a potential source of problems. Some compilers may provide run-time pathname translation to translate between Unix syntax and the host’s syntax.

11.3.2. Source Files

11.4. Miscellaneous

System dependencies:

Isolate system-dependent code in separate modules and use conditional compilation.


Utilities for compiling and linking such as Make simplify considerably the task of moving an application from one environment to another. Even better, use Imake since Make files are very unportable. Imake is distributed with the X Window System by MIT. One of the authors of this document has used it extensively with very good results.

Many of the tools and libraries that one takes for granted on Unix, such as lex, yacc, curses, sed, awk, and the various shells, are often not available on other operating systems. Public-domain versions of most of the useful tools are available at many archive sites. However, the so-called copyleft restrictions on many of these programs may prove to be problematic to some would-be porters.

Name space pollution:

Minimize the number of global symbols in the application. One of the benefits is the lower probability that any conflicts will arise with system-defined functions.

Character sets:

Do not assume that the character set is ASCII. If the character set in question is not [American] English, then other characters will also be alphabetic, and their lexicographic ordering will not necessarily have any relationship to their positions within the character set. If the character set is Asian, then “characters” may be of type wchar_t, not char, and will, in general, require two or more bytes of storage each. The library string functions should be capable of handling these correctly. Code that iterates through arrays of chars may need to be changed to handle multibyte characters correctly.

If the program’s messages are likely to be translated into other languages, take care to modularize the code for easy translation. Consider keeping all text in a “language” file. Be aware that carefully formatted reports and printing routines may need major surgery.

Binary Data:

Great care must be taken when reading and writing binary data. For example, a file of floating-point numbers in binary format written by machine A is unlikely to be usable on machine B.

11.5. Writing Portable Code

Write code under the assumption that it will be ported to many strange machines. It is considerably easier to port code to a new environment when the code has been written with porting in mind, than it is to “retrofit” portability.

One school of thought advocates “Port early, port often.” That is, whenever the code reaches a certain level of stability on the development system, port it to other systems. This method has the advantage that portability problems are discovered early, and the possible disadvantage that potentially far more time could be spent in porting than would be the case if the code were just ported once, when complete.

Code in ANSI C whenever possible. Many of the extensions—prototypes, stronger type-checking, etc.—enhance portability. The more widely ANSI C is used, the quicker it will gain acceptance. Of course, this may not be an option if the code must be ported to platforms without ANSI C compilers. The short-term solution is to use the various tricks discussed in Recommended C Style and Coding Standards [1] and elsewhere; the long-term solution is to force vendors to release ANSI C compilers for their systems. Alternatively, a converter such as protoize (available via anonymous FTP from can convert between ANSI and non-ANSI programs.

Make complete, correct declarations; don’t let parameters default to int. Include all of the necessary header files. Declare functions with no return value as void. Check the results of system calls.

Use lint. Programs that fail to pass lint quietly will undoubtedly be difficult to port. Compile code with as many different compilers as possible with all warnings enabled.

Recommended C Style and Coding Standards [1] has more to say about this.

12. Further Reading

One can argue that portability and “well-written” code go hand-in-hand. Loosely defined, well-written code is one that is “easy” to understand and “easy” to maintain, and there are several style guides in the public domain expressing various views on the subject.

Besides the style guide mentioned in the foreword, there are a few more that can be obtained in [] in ~ftp/doc/programming. We also recommend standards.text from the Free Software Foundation which can be found in various sites, e.g., [] in ~ftp/pub/gnu.

For those who have access to the Usenet newsgroup comp.lang.c, we highly recommend reading the Frequently Asked Questions List (known as the FAQL) which is posted at the beginning of every month.

13. Acknowledgements

We are grateful for the early help of A. Louko (HTKK/Lsk) and J. Helminen (HTKK). The following persons have commented on and corrected previous revisions of this document: Geoffrey H. Cooper and Guy Harris. Special thanks go to Steven Pemberton, the main author of config.c, for making available such a useful tool. We thank all the contributors to the Usenet newsgroups comp.std.c and comp.lang.c from where we have taken a lot of information. Some information within was obtained from HP-UX Portability Guide [3].

14. Trademarks


  1. Recommended C Style and Coding Standards [1] can be obtained via anonymous FTP from in ~ftp/pub/cstyle.tar.Z.
  2. We regard this document as a living entity growing as needed and as information is gathered. Future versions of this document may contain a lot of such information.
  3. Maybe there are people out there who still write compilers in FORTRAN after all...
  4. Old preprocessors only take directives that begin with # in the first column.
  5. Some have suggested using #if __STDC__ instead of simply #ifdef __STDC__ to test if the compiler is ANSI-compliant because of compilers that are not, but define __STDC__ equal to zero.
  6. GLUE(a,GLUE(b,c)) would not result in the concatenation of a, b, and c.
  7. Address is 2051, Swans Neck Way, Reston, Virginia 22091, USA.
  8. Berkeley Software Distribution
  9. Under VMS, since a path such as <sys/file.h> will evaluate to sys:file.h, it is sufficient to equate the logical name sys to sys$library.
  10. A public domain implementation of alloca can be obtained from the Free Software Foundation (GNU); try in ~ftp/pub/gnu.
  11. That is, a function invoked as a result of a signal raised during the handling of another signal. See section, paragraph 15 in the Standard [13].
  12. Some representations have reserved values for +inf and -inf.
  13. Email (Internet) address is For more information, send a message containing the line send index to that address.
  14. The archive site of comp.sources.misc is
  15. In fact one wonders why they even bothered to define two representations for floating-point numbers considering the rules applied to them.
  16. This does not really belong in this document, but whenever one is porting a program to a VMS environment one is bound to come across this strange behavior which can result in a lot of wasted time.
  17. There is a difference between variadic functions defined by the Standard and the pre-Standard varargs as defined by varargs.h which is still widely used. Here we are referring to the former, and the differences between both are explored in section 11.2.3.
  18. One of the reasons for these rules is that in some architectures, pointers are represented as a pair of values and only equality is a well-defined operator for arbitrary pairs of values. The other operators are only well-defined when one of the values of both pairs is guaranteed to match, in which case the situation is analogous to "ordinary" architectures.
  19. Programs that generate other programs, e.g., yacc, can generate, for instance, very large switch statements.


[1] L. W. Cannon, R. A. Elliot, L. W. Kirchoff, J. H. Miller, J. M. Milner, R. W. Mitze, E. P. Schan, N. O. Whittinton, Henry Spencer, David Keppel, and Mark Brader. Recommended C Style and Coding Standards. Technical report, in the public domain, June 1990.

[2] W. J. Cody. Algorithm 665, MACHAR: A Subroutine to Dynamically Determine Machine Parameters. ACM Transactions on Mathematical Software, 14(4):303-311, December 1988.

[3] Hewlett-Packard Company. HP-UX Portability Guide, 1988.

[4] Mark Horton. Portable C Software. Prentice-Hall, 1990.

[5] Samuel P. Harbison and Guy L. Steele Jr. C: A Reference Manual. Prentice-Hall, Inc., second edition, 1987.

[6] Interviews. Interview With Five Technologists. UNIX Review, 8(1):41-89, January 1990.

[7] U. W. Kulish and W. L. Miranker. The Arithmetic of the Digital Computer: A New Approach. SIAM Review, 28(1):1-40, March 1986.

[8] Andrew Koenig. C Traps and Pitfalls. Addison-Wesley Publishing Co., Reading, Massachusetts, 1989.

[9] Brian W. Kernighan and Dennis M. Ritchie. The C Programming Language. Prentice-Hall, Inc., first edition, 1978.

[10] Brian W. Kernighan and Dennis M. Ritchie. The C Programming Language. Prentice-Hall, Inc., second edition, 1988.

[11] Tom Manuel. A Single Standard Emerges from the UNIX Tug-Of-War. Electronics, pages 141-143, January 1989.

[12] William H. Press, Brian P. Flannery, Saul A. Teukolsky, and William T. Vetterling. NUMERICAL RECIPES in C: The Art of Scientific Computing. Cambridge University Press, 1988.

[13] X3J11. Draft Proposed American National Standard for Information Systems -- Programming Language C. Technical Report X3J11/88-158, ANSI Accredited Standards Committee, X3 Information Processing Systems, December 1988.