隐含的自由函数声明在c99中无效

Question

In xcode 5 I get this warning:

"implicit declaration of function free is invalid in c99"

How should I free my c structures if I can't use the function free()?

Answer 1

你应该包括<stdlib.h> 。

Answer 2

You get that warning because you're calling a function without first declaring it, so the compiler doesn't know about the function.

All functions need to be declared before being called, there are no "built-in" functions in C.

It's true that free() is a function defined in the standard, but it's still not built-in, you must have a prototype for it.

To figure out which header has the prototype, try searching for "man free" and look for a Linux manual page . Close to the top, it says:

#include <stdlib.h>

void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);

This tells you that in order to use the listed functions, you should add:

#include <stdlib.h>

to your source code.