Consistent Build in C

In this quick tutorial, I will show you how to write and compile POSIX compliant C program. For TLDR, you need to use the following flags:

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

If you want to learn more, feel free to continue below.

Compiling C program using -std option will save me from a bunch of problems about portability of my program. I use C99 standart, since C11 is not widely implemented yet.

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

Sometimes I need a function definitions that are not specified in ISO C standart. Such as getaddrinfo, getnameinfo and freeaddrinfo for socket stuff. Those functions are specified in POSIX.1-2008 standart.

Naturally -std=c99 option will force a system headers to only expose a definitions that valid in ISO C99 standarts, and it’s good. Compiling a program that include a system header and use a functions that are not specified in the standart will yield the following errors:

% 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;
....

I need to define  feature test macros for POSIX.1-2008 explicitly in order to tell compiler that I need a features from POSIX.1-2008 standart:

#define _POSIX_C_SOURCE 200809L

This macro causes header files to expose definitions corresponding to the POSIX.1-2008 base specification (excluding the XSI extension) when program is compiled.

Now, my program will consistently compiled in POSIX-compliant system.