WEXITSTATUS returning invalid exit status of popen() on success case

Question

I am trying to get the exist status of the child process using popen().

Case 1: Calling function with the shell command returning error. This is working as expected.

func("du -sh _invalid_file_");

Output:

du: cannot access '_invalid_file_': No such file or directory
Child exit value: 1

Here the child exist status is same as the exit value of du which ran in bash.

$ du -sh _invalid_file_
du: cannot access '_invalid_file_': No such file or directory
$ 
$ echo $?
 1
$

Case 2: (Error case) Calling function with the below shell command returning success. In my code, WEXITSTATUS() returning non-zero value but the same command in bash returning 0.

func("du -sh");

Output:

Child exit value: 141

Please suggest to fix this issue. Pasted the code below.

int func(char *cmd)
{
    FILE *pfp = NULL;
    int retval, status;

    pfp = popen(cmd, "r");
    if (pfp == NULL) {
        perror("popen failed");
        exit(1);
    }

    status = pclose(pfp);
    if (status < 0) {
        perror("pclose error");
    } else {
        if (WIFEXITED(status)) {
            retval = WEXITSTATUS(status);
            printf("Child exit value: %d\n", retval);
        }
    }

    return 0;
}

Answer 1

You closed the pipe before actually reading any data from it. This would cause the child process to receive SIGPIPE (13) and be killed.

popen() would use /bin/sh -c to run the command so it's actually sh -c "du ..." running. When du was killed by SIGPIPE (13), sh would set the exit status to 128+13=141 and since du is the last command in the shell session so 141 would be the final exit status of the sh -c ... command and you got 141.

To fix it you need to read data from the pipe before pclose().

int func(char *cmd)
{
    FILE *pfp = NULL;
    char buf[1024];
    int retval, status;

    pfp = popen(cmd, "r");
    if (pfp == NULL) {
        perror("popen failed");
        exit(1);
    }

    /* read from the pipe or close() may cause the process to receive SIGPIPE */
    while (fgets(buf, sizeof buf, pfp) ) {
        printf("%s", buf);
    }

    status = pclose(pfp);
    if (status < 0) {
        perror("pclose error");
    } else {
        if (WIFEXITED(status)) {
            retval = WEXITSTATUS(status);
            printf("Child exit value: %d\n", retval);
        }
    }

    return 0;
}

---

The following is from man 7 signal:

   Signal     Value     Action   Comment
   ----------------------------------------------------------------------
   ... ...

   SIGPIPE      13       Term    Broken pipe: write to pipe with no
                                 readers; see pipe(7)
   ... ...

---

This SIGPIPE behavior happens every day though it is not obvious. You can try like this:

$ od /dev/urandom | head -n1
0000000 067255 052464 166240 113163 122024 054232 015444 110641
$ declare -p PIPESTATUS    # see PIPESTATUS in `man bash'
declare -a PIPESTATUS=([0]="141" [1]="0")
$
$ { od /dev/urandom; echo EXIT=$? >&2; } | head -n1
0000000 020553 063350 025451 116300 006505 151644 122151 175763
EXIT=141

---

Regarding the shell's 128+signal behavior, the following is from man bash ^{(Bash is sh-compatible so this also applies for sh.)}:

When a command terminates on a fatal signal N, bash uses the value of 128+N as the exit status.