为什么我不能检查我是否在 tdm-gcc 编译器中包含了 stdlib.h？

Question

I'm writing a header file in C and need stdlib.h for it to work. But, when I check if _STDLIB_H is defined, the pre-processor says it's not, even if I include the file. I have tried it on multiple compilers, where it works on most but not on tdm-gcc . How can I fix this?

Answer 1

Looking at stdlib.h source code, it seems like the macro to look for in tdm-gcc might be _TR1_STDLIB_H .
So you can try something like:

#if defined _STDLIB_H || defined _TR1_STDLIB_H

Answer 2

For a safer way to check if stdlib.h is properly included, you should check for a macro that the C standard requires the file to define.

I may be missing something, but I don't see any requirement in the C standard for stdlib.h to define _STDLIB_H . I think this may just be a common way compilers decided to guard against multiple inclusion.

try something like

#include <stdlib.h>

#ifndef NULL
#error "stdlib.h not included"
#endif

because the C standard requires that stdlib.h defines NULL

But none of this should be technically necessary... I don't know of a preprocessor that won't throw a fatal error if it can't find a file you tried to #include

EDIT:

According to the C standard stdio.h also defines NULL , so perhaps it would be better to check for EXIT_SUCCESS or EXIT_FAILURE