Building POSIX Compliant C Programs: A Straightforward Guide


2 min read

Building POSIX Compliant C Programs: A Straightforward Guide

Welcome to this concise tutorial on writing and compiling POSIX-compliant C programs. Whether you're a seasoned coder or just starting, this guide aims to simplify the process for you. Let's break down the essential steps and understand why they matter.

TLDR: Quick Compilation Command

For those in a hurry, here's the quick command to compile your POSIX-compliant C program:

clang-3.7 -std=c99 -pedantic -Wall -Werror -o yourprogram yourfile.c

But if you're curious about the details, keep reading!

Dive into POSIX Compliant C Programming

Choosing the Right Standard: C99

When compiling a C program, using the -std option is crucial for portability. I prefer the C99 standard as C11 is not as widely implemented. Here's the basic compilation command:

CC -std=c99 -pedantic -Wall -Werror -o program program.c

Beyond ISO C: The Need for POSIX

Sometimes, we require functions not specified in the ISO C standard, like getaddrinfo, getnameinfo, and freeaddrinfo for socket operations. These functions are part of the POSIX.1-2008 standard.

Using -std=c99 restricts system headers to ISO C99 standards, which is generally good. However, it can cause issues when using non-standard functions. For example:

$ CC=clang-3.7 make
clang-3.7 -std=c99 -pedantic -Wall -Werror -o hostinfo hostinfo.c
hostinfo.c:58:21: error: variable has incomplete type 'struct addrinfo'
    struct addrinfo hints, *addri;
hostinfo.c:58:12: note: forward declaration of 'struct addrinfo'
    struct addrinfo hints, *addri;
hostinfo.c:62:18: error: implicit declaration of function 'getaddrinfo' is
      invalid in C99 [-Werror,-Wimplicit-function-declaration]
    int gairet = getaddrinfo(argv[1], NULL, &hints, &addri);
hostinfo.c:64:40: error: implicit declaration of function 'gai_strerror' is
      invalid in C99 [-Werror,-Wimplicit-function-declaration]
        fprintf(stderr, "Error: %s\n", gai_strerror(gairet));
hostinfo.c:64:40: note: did you mean 'strerror'?
/usr/include/string.h:413:14: note: 'strerror' declared here
extern char *strerror (int __errnum) __THROW;

Enabling POSIX Features: Feature Test Macros

To access POSIX.1-2008 features, we need to define feature test macros explicitly:

#define _POSIX_C_SOURCE 200809L

This macro ensures that header files expose definitions according to the POSIX.1-2008 base specification, enabling our program to compile consistently on POSIX-compliant systems.


Building a POSIX-compliant C program is like assembling a puzzle with pieces from different sets. By selecting the correct standard (C99) and explicitly enabling POSIX features, we ensure our program is both portable and compliant with contemporary systems.

Remember, in the world of programming, the right tools and knowledge make all the difference. Happy coding, and may your journey in C be as smooth and error-free as possible!