Implicit declaration of function ‘wait’

Question

I am getting a warning > Implicit declaration of function ‘wait’ < and when I run the program it works correctly, I would like to understand why I am getting this warning?

Thanks in advance

Edit: I forgot to add the library included

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>


void create (char* program, char** arg_list)
{
  /* put your code here */
  pid_t childPid;
  int status;

  if((childPid = fork()) < 0){
    printf("Failed to fork() --- exiting...\n");
    exit(1);
  }
  else if (childPid == 0){ // --- inside the child process
    if(execvp(program, arg_list) < 0){ // Failed to run the command
      printf("*** Failed to exec %s\n", program);
      exit(1);
    }
  }
  else{ // --- parent process
    while(wait(&status) != childPid)
      printf("...\n");
  }
}

Solution 1

You are probably missing the headers for wait(2):

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

Solution 2

You need to put:

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

at the top of the program to get the declaration of the function.

This is shown in the man page

Comments

[Barmar]

You're missing the #include line for the function.

[AyeJay]

Thank you, I forgot the wait library

[MichaelM]

Is sys/types.h necessary if you aren't utilizing any of the types returned from a wait function call?

[P.P]

@MichaelM I answered based on what the Linux man page had at that time .But it was removed since in this commit. The reason it was included earlier was due to types used by wait (such as pid_t). [continued]

[P.P]

@MichaelM But including necessary types should be handled by sys/wait.h itself rather than requiring users to include them. POSIX doesn't require sys/types.h either. In summary, sys/types.h isn't required for using wait!

[MichaelM]

@P.P thanks for the clarification! I just wanted to make sure I was understanding correctly. I was able to compile without sys/types.h and didn't run into any complications, but I wanted to ensure there were no "hidden" issues I wasn't aware of.