Thread: Checking if the macro _POSIX_C_SOURCE is defined doesn't work

Hello,
I wrote a C program that relies on the sqrtf function to calculate the square root of a float, so I check if the macro _POSIX_C_SOURCE is defined before including any header. The problem is that the macro is defined (its value is 200809L) and it refuses to compile. Here's an example:

Code:

#ifdef _POSIX_C_SOURCE
#include <stdio.h>
#include <math.h>
#endif

int main(void) {
    float foo = sqrtf(3.5);
    printf("%f\n", foo);
    return 0;
}

And this is what I get when trying to compile it:

Code:

$ gcc -std=c99 -lmath -o test test.c
test.c: In function 'main':
test.c:7:17: warning: implicit declaration of function 'sqrtf' [-Wimplicit-function-declaration]
     float foo = sqrtf(3.5);
                 ^~~~~
test.c:7:17: warning: incompatible implicit declaration of built-in function 'sqrtf'
test.c:7:17: note: include '<math.h>' or provide a declaration of 'sqrtf'
test.c:8:5: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
     printf("%g\n", foo);
     ^~~~~~
test.c:8:5: warning: incompatible implicit declaration of built-in function 'printf'
test.c:8:5: note: include '<stdio.h>' or provide a declaration of 'printf'
/usr/bin/ld: cannot find -lmath
collect2: error: ld returned 1 exit status

Why aren't the headers included?
Thanks in advance.

sqrtf is in C99. You don't need POSIX for that function.

Besides, _POSIX_C_SOURCE is a user-defined macro; it's up to you to define that macro to tell the implementation to conform to a particular version of POSIX.

Lastly, gcc takes -lm (not -lmath) to link in the math library.

It appears that gcc is being somewhat "helpful" by defining _POSIX_C_SOURCE when you include one of the standard headers if _GNU_SOURCE is defined (see /usr/include/sys/features.h). _GNU_SOURCE is defined when you use -std=gnu99 but not -std=c99.

Still, from POSIX's standpoint, the application (the program you're writing) is responsible for defining _POSIX_C_SOURCE. From what I can see, gcc simply relaxes that requirement by defining it for you when you compile with one of the "gnuXX" standards.

If you want full control over it, use "-std=c99" and define the macro yourself, either in the source code or with the option "-D_POSIX_C_SOURCE=200809L" when you compile.

You're absolutely right christop, the sqrtf function is available in C99 according to cppreference.com: sqrt, sqrtf, sqrtl - cppreference.com. I should have looked there before making any assumptions when reading the POSIX math.h header documentation.

And it seems I completely misunderstood the documentation. This is what misled me to believe the macro is already defined on POSIX systems:

In the compilation of an application that #defines a feature test macro specified by POSIX.1-2017, no header defined by POSIX.1-2017 shall be included prior to the definition of the feature test macro. This restriction also applies to any implementation-provided header in which these feature test macros are used. If the definition of the macro does not precede the #include, the result is undefined.

But when I read it again I notice it clearly states it's me the one who has to define the macro. I'm sorry I asked this just because of this.
And thank you all for your help.