Accessing exit codes from a c program

Question

I have tried to find solution all over the internate, but forums keep saying it is impossible, so here I a with my question. How does a shell (any shell like bash) keep track of exit codes. Is it by tracking their child process? (if so how could I go about implementing such a thing in a program where I am creating and killing many children processes) Or Is there a global variable that is equivalent to $? that I can access in c? Or do they store it in a file?

Answer 1

Here is an example of executing grep on a path that doesn't exist and getting the return code in the parent:

code:

#include <errno.h>
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>


int main(int argc, char* argv[]) {

    pid_t childPid;
    switch(childPid = fork()) {
    case -1:
        fprintf(stderr, "Error forking");
        return 1;
    case 0:
        printf("CHILD: my pid is: %d\n", getpid());
        int ret = execlp(argv[1], argv[1], argv[2], argv[3], (char *) NULL);
        if (ret == -1) {
            printf("CHILD: execv returned: %d\n", errno);
            return errno;
        }
        break;
    default:
        printf("I am the parent with a child: %d\n", childPid);
        int childRet;
        wait(&childRet);
        printf("PARENT, child returned: %d\n", childRet >> 8);
    }
    return 0;
}
 

Execution:

# Example of Failure execution:

[ttucker@zim c]$ cc -o stackoverflow so.c && ./stackoverflow grep test /does/not/exists
I am the parent with a child: 166781
CHILD: my pid is: 166781
grep: /does/not/exists: No such file or directory
PARENT, child returned: 2

# Example of a successful execution:

[ttucker@zim c]$ cc -o stackoverflow so.c && ./stackoverflow echo foo bar
I am the parent with a child: 166809
CHILD: my pid is: 166809
foo bar
PARENT, child returned: 0