[SOLVED] Why child process returns exit status = 32512 in unix?

Issue

In my program I'm executing given command and getting result (log, and exit status). Also my program have to support shell specific commands (i.e. commands which contains shell specific characters ~(tild),|(pipe),*). But when I try to run sh -c ls | wc in my home directory via my program it failed and its exit status was 32512, also in stderr stream "sh: ls | wc: command not found" was printed.

But the interesting thing is that the command sh -c ls | wc works correct if I run it in shell.

What is the problem? Or more preferable how can I run shell specific commands via my program (i.ec which command with which parameters should I run)?

The code part bellow is in child part after fork(). It executs the command.

tokenized_command is std::vector<std::string> where in my case "sh", "-c", "ls", "|", "wc" are stored, also I have tried to store there "sh", "-c", "\"ls | wc\"" but result is same. command is char * where full command line is stored.

        boost::shared_array<const char *> bargv(new const char *[tokenized_command.size() + 1]);
        const char **argv = bargv.get();
        for(int i = 0; i < tokenized_command.size(); ++i)
        {
            argv[i] = tokenized_command[i].c_str();
            printf("argv[%d]: %s\n", i, argv[i]); //trace
        }
        argv[tokenized_command.size()] = NULL;

        if(execvp(argv[0], (char * const *)argv) == -1)
        {
            fprintf(stderr, "Failed to execute command %s: %s", command, strerror(errno));
            _exit(EXIT_FAILURE);
        }

P.S.

I know that using system(command) instead execvp can solve my problem. But system() waits until command is finished, and this is not good enough for my program. And also I'm sure that in implementation of system() one of exec-family functions is used, so the problem can be solved via exec as well, but I don't know how.

Solution

execvp takes a path to an executable, and arguments with which to launch that executable. It doesn't take bourne shell commands.

ls | wc is a bourne shell command (among others), and it can't be broken down into the path to an executable and some arguments due to the use of a pipe. This means it can't be executed using execvp.

To execute a bourne shell command using execvp, one has to execute sh and pass -c and the command for arguments.

char *const argv[] = {
    "sh",
    "-c", "ls | wc",  // Command to execute.
    NULL
};

execvp(argv[0], argv)

---

About your attempts

You apparently tried

char *const argv[] = {
    "sh",
    "-c", "ls",  // Command to execute.
    "|",         // Stored in called sh's $0.
    "wc",        // Stored in called sh's $1.
    NULL
};

That would be the same as bourne shell command sh -c ls '|' wc.

And both are very different than shell command sh -c ls | wc. That would be

char *const argv[] = {
    "sh",
    "-c", "sh -c ls | wc",  // Command to execute.
    NULL
};

You seem to think | and wc are passed to the sh, but that's not the case at all. | is a special character which results in a pipe, not an argument.

---

About the exit code

The following is the format of the exit code:

Bits 15-8 = Exit code.
Bit     7 = 1 if a core dump was produced.
Bits  6-0 = Signal number that killed the process.

You got 32512, or 0x7F00.

So it didn't die from a signal, a core dump wasn't produced, and it exited with code 127 (0x7F).

What 127 means is unclear, which is why it should accompanied by an error message. You tried to execute program ls | wc, but there is no such program.

Answered By - ikegami

Answer Checked By - Marie Seifert (WPSolving Admin)