unnamed pipe between parent and child

main.c

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

void capture_output(char * cmd, char ** outstr, int max_lines)
{
    // we must pass a len 2 array to pipe
    int pipefds[2];
    // fork() returns a process id
    int pid;
    FILE* output;
    char line[256];
    char * tempstr;

    pipe(pipefds);
    pid = fork();
    if (pid == 0) {
        // child process
        // close the read half, child will only write.
        close(pipefds[0]);
        // redirect both streams to the write half of the pipe.
        dup2(pipefds[1], STDOUT_FILENO);
        dup2(pipefds[1], STDERR_FILENO);
        // we MUST pass a null pointer to signal the end of our variadic args.
        execl(cmd, cmd, "branch", "current", (char*) NULL);
    } else {
        // parent process
        // we HANG FOREVER if we don't close the write half of the pipe. Dang!
        char * splitted;
        close(pipefds[1]);
        output = fdopen(pipefds[0], "r");
        for (int i = 0; i <= max_lines; i++) {
            if (fgets(line, sizeof(line), output)) {
                char * brkb;
                if ((splitted = strtok_r(line, "\n", &brkb)))
                    tempstr = splitted;
            }
        }
        // this will only be reached by the parent
        int status;
        if (waitpid(pid, &status, 0) == -1)
            asprintf(outstr, "%s ", "internal error");
        if (status == 0) 
            asprintf(outstr, "%s ", tempstr);
        
    }
}

int main(int argc, char const *argv[])
{
    // asprintf will extend this for us
    char * prompt = NULL;
    char cwd[1024];
    capture_output("/usr/local/bin/fossil", &prompt, 1);
    // Get pwd
    getcwd(cwd, sizeof(cwd));
    printf("%s$", prompt);
    return 0;
}