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

Meaning

Old C assumed an undeclared function returns int and accepts anything. That hides bugs such as truncated pointers on 64-bit systems. GCC 14 and Clang 16 turn it into a hard error, so old code that merely warned before may now fail to build.

Common causes

• Missing #include <stdlib.h>, <string.h>, <unistd.h> or similar
• Calling a function defined later in the file without a prototype
• Misspelled function name
• Header not found because of a missing feature-test macro such as _GNU_SOURCE

Detailed fix by platform

Code examples

#include <stdlib.h>      /* declares malloc, free, atoi */
#include <string.h>      /* declares strlen, memcpy     */

int main(int argc, char** argv) {
    int n = argc > 1 ? atoi(argv[1]) : 0;   /* without stdlib.h: implicit declaration of function 'atoi' */
    return n;
}

How to diagnose

1. Name — Spelling of the function?
2. Header — Which header does man say?
3. Standard — Is the file compiled as C99 or newer?

More related errors