implicit declaration of function free is invalid in c99

c xcode

You should include <stdlib.h>.

c xcode

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.

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