警告：函数“kill”的隐式声明

Question

I am making these inclusions:

#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

But still getting this warning.

Answer 1

You are probably passing the "-ansi -Wall" switches to the gcc compiler. You can remove "-ansi" if you don't need it, otherwise try to set the proper feature define macro.

Something like:

#define _POSIX_SOURCE
#include <sys/types.h>
#include <signal.h>
#include <unistd.h>
#include <sys/wait.h>

Answer 2

You can also define it on the compile time by adding this flag with gcc.

-D_POSIX_C_SOURCE

ex:

-g -D_POSIX_C_SOURCE -Wall -std=c99

Answer 3

According to the kill(2) Linux manual page, _POSIX_C_SOURCE must be #define d before any includes in order for the prototype of the function kill() to be included through <signal.h> if you are using glibc.

Your code will probably compile either way, since the binary is available for linking regardless of the defines, but that's the proper way to silence this warning according to the documentation.