clang "error: call to undeclared function; ISO C99 ... implicit declarations" in CI

Recent clang makes calling a function with no visible declaration an error in C, not a warning. The fix is to include the header that declares it rather than letting C assume an int-returning prototype.

What this error means

C code that built on older toolchains fails on newer clang with "call to undeclared function", pointing at a call whose header was never included.

clang

main.c:5:9: error: call to undeclared function 'strdup'; ISO C99 and later
do not support implicit function declarations [-Wimplicit-function-declaration]
    5 |   char *p = strdup(s);
      |             ^

Common causes

How to fix it

Include the declaring header

Find which header declares the function and include it.
Recompile so the prototype is visible at the call site.

#include <string.h>   /* declares strdup */

Define the required feature macro

Some POSIX or GNU functions need a feature-test macro before any include so their prototypes are exposed.

#define _GNU_SOURCE
#include <string.h>

How to prevent it

Include the correct headers and define needed feature-test macros so prototypes are visible; do not rely on implicit declarations that newer C compilers reject.