C error: implicit declaration of function 'foo' [-Wimplicit-function-declaration]

implicit declaration of function 'foo' means the C compiler reached a call to foo without having seen a declaration of it. C99 removed implicit declarations, and current compilers treat them as errors: GCC 14 reports error: implicit declaration of function 'foo' [-Wimplicit-function-declaration], and Clang 16+ reports error: call to undeclared function 'foo'; ISO C99 and later do not support implicit function declarations. Include the header that declares the function, or add a prototype before the call.

Example

#include <stdio.h>

int main(void) {
    printf("%zu\n", strlen("hello"));  /* strlen is declared in <string.h> */
    printf("%d\n", square(4));         /* square is defined below main */
    return 0;
}

int square(int n) { return n * n; }

Compiling with gcc main.c reports both calls. Older GCC versions printed a warning and assumed the function returned int, which silently breaks functions that return pointers, long or double.

Fix

#include <stdio.h>
#include <string.h>        /* declares strlen */

int square(int n);         /* prototype before the first call */

int main(void) {
    printf("%zu\n", strlen("hello"));
    printf("%d\n", square(4));
    return 0;
}

int square(int n) { return n * n; }

Which header?

The man page of each function lists its header under SYNOPSIS (man 3 strlen). Common ones: