SIGCHLD is sent on SIGCONT on Linux but not on macOS

Question

In the main process I listen to SIGCHLD:

signal(SIGCHLD, &my_handler);

Then I fork(), execv() and let it run in background (/bin/cat for example).

When I try from terminal to send SIGSTOP to the child process, my_handler() gets called. But when I try to send SIGCONT to it, the the handler isn't called on macOS but it's executed on my Ubuntu.

Man:

SIGCHLD: child status has changed.

Am I missing something? Is it an expected behaviour? I wrote my app on Ubuntu and expected it to work on mac as well.

I tried with sigaction() as well, but with the same results.

Here's a sample code to demonstrate:

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

void    my_handler(int signum)
{
    printf("\t SIGCHLD received\n");
    fflush(stdout);
}

void    my_kill(pid_t pid, int signum)
{
    printf("Sending %d\n", signum);
    fflush(stdout);

    kill(pid, signum);

    printf("Sent %d\n\n", signum);
    fflush(stdout);
}

int main()
{
    pid_t   pid;
    char    *cat_args[2] = {"/bin/cat", NULL};

    signal(SIGCHLD, &my_handler);
    pid = fork();

    if (pid == 0)
    {
        execv("/bin/cat", cat_args);
    }
    else
    {   
        my_kill(pid, SIGSTOP);
        my_kill(pid, SIGCONT);
        wait(NULL);
    }
    return 0;
}

With the output on macOS:

Sending 17
         SIGCHLD received
Sent 17

Sending 19
Sent 19

Answer 1

That behavior is optional. An implementation need not generate a SIGCHLD upon continuation. The language used in POSIX.1-2008 (2016 edition) is "may" rather than "shall":

When a stopped process is continued, a SIGCHLD signal may be generated for its parent process, unless the parent process has set the SA_NOCLDSTOP flag.

- System Interfaces, 2.4.3 Signal Actions

...a SIGCHLD signal may be generated for the calling process whenever any of its stopped child processes are continued.

- System Interfaces sigaction "Description"

Emphases added.

Answer 2

Am I missing something?

Probably not.

Is it an expected behaviour?

Probably yes.

OSX is based on 4.4 BSD, and this BSD simply does not support sending a SIGCHLD to the parent when the child continues. Earlier versions of Linux lacked this support as well.