dup2() is blocking with child processes? C

Question

I'm writing a function that echo an input to a sed and then another sed. I thinck i used all my wait signal in the right way but the last print i can get is before the call to dup2() in my first child process in the echo.

void sendbc (char * str_ ) {
    int fd[2];
    int fd1[2];
    int pid,pid1;
    char* echo[] = {"echo", str_,NULL};
    char* sed1[] = {"sed","s/[^:]*;"" " "//",NULL};
    char* sed2[] = {"sed","s/[^:]*."" " "//",NULL};
    int status,er;
    FILE *f;
    if(pipe(fd) < 0){
        exit(100);
    }
    if(pipe(fd1) < 0){
        exit(100);
    }

    pid = fork();
    if (pid == 0) {
        dup2(fd[1], 1) //last command before blocking
        close(fd[1]);
        close(fd[0]);
        execvp(echo[0], echo);
        printf("Error in execvp1\n");
    }else{
        wait(&status);
        pid = fork();
        if (pid == 0){
            dup2(fd[0], 0);
            dup2(fd1[1], 1);
            dup2(fd1[1], 2);
            close(fd[1]);
            close(fd[0]);
            close(fd1[1]);
            close(fd1[0]);
            execvp(sed1[0],sed1);
            printf("Error in execvp2\n");
        }else{
            wait(&status);
            dup2(fd1[0],0);
            dup2(1,2);
            //dup2(1,1);
            close(fd1[1]);
            close(fd1[0]);
            execvp(sed2[0],sed2);
            printf("Error in execvp3\n");
        }
    }

    if(pid!=0)
        wait(&status);
    close(fd[0]);
    close(fd[1]);
    close(fd1[1]);
    close(fd1[0]);
}

I can imagine 2 possibilities... dup2 is blocking or i need to create more process because it end process on call, but this sounds not right after a quick read on his man page... what could it be?

Answer 1

General Problem

You aren't closing enough file descriptors in the various processes.

---

Rule of thumb : If you dup2() one end of a pipe to standard input or standard output, close both of the original file descriptors returned by pipe() as soon as possible. In particular, you should close them before using any of the exec*() family of functions.

The rule also applies if you duplicate the descriptors with either dup() or fcntl() with F_DUPFD or F_DUPFD_CLOEXEC .

---

If the parent process will not communicate with any of its children via the pipe, it must ensure that it closes both ends of the pipe early enough (before waiting, for example) so that its children can receive EOF indications on read (or get SIGPIPE signals or write errors on write), rather than blocking indefinitely. Even if the parent uses the pipe without using dup2() , it should normally close at least one end of the pipe — it is extremely rare for a program to read and write on both ends of a single pipe.

Note that the O_CLOEXEC option to open() , and the FD_CLOEXEC and F_DUPFD_CLOEXEC options to fcntl() can also factor into this discussion.

If you use posix_spawn() and its extensive family of support functions (21 functions in total), you will need to review how to close file descriptors in the spawned process ( posix_spawn_file_actions_addclose() , etc.).

Note that using dup2(a, b) is safer than using close(b); dup(a); close(b); dup(a); for a variety of reasons. One is that if you want to force the file descriptor to a larger than usual number, dup2() is the only sensible way to do that. Another is that if a is the same as b (eg both 0 ), then dup2() handles it correctly (it doesn't close b before duplicating a ) whereas the separate close() and dup() fails horribly. This is an unlikely, but not impossible, circumstance.

---

Specific Issues

• You aren't closing enough file descriptors for safety.
• Your regexes are dubious.
• You should not make processes in a pipeline wait for each other.

^{Pet peeve: I prefer to use fd1 and fd2 when I have two closely related variables like the pairs of pipe file descriptors; I find fd and fd1 and the like silly. You may, however, choose to ignore this.}

Working Code

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static void dump_argv(char **argv)
{
    printf("%d:\n", getpid());
    while (*argv != NULL)
    {
        printf("%d: <<%s>>\n", getpid(), *argv++);
    }
}

static void sendbc(char *str)
{
    int fd1[2];
    int fd2[2];
    int pid;
    char *echo[] = {"echo", str, NULL};
    char *sed1[] = {"sed", "s/[^:]*[;]//", NULL};
    char *sed2[] = {"sed", "s/[^:]*[.]//", NULL};
    if (pipe(fd1) < 0)
        exit(100);
    if (pipe(fd2) < 0)
        exit(101);

    printf("%d: at work\n", getpid());
    pid = fork();
    if (pid < 0)
        exit(102);
    else if (pid == 0)
    {
        printf("%d: child 1 - echo\n", getpid());
        dump_argv(echo);
        dup2(fd1[1], 1);
        close(fd1[1]);
        close(fd1[0]);
        close(fd2[0]);
        close(fd2[1]);
        execvp(echo[0], echo);
        fprintf(stderr, "Error in execvp1\n");
        exit(103);
    }
    else
    {
        printf("%d: parent - before second fork\n", getpid());
        pid = fork();
        if (pid == 0)
        {
            printf("%d: child 2 - sed 1\n", getpid());
            dump_argv(sed1);
            dup2(fd1[0], 0);
            dup2(fd2[1], 1);
            close(fd1[1]);
            close(fd1[0]);
            close(fd2[1]);
            close(fd2[0]);
            execvp(sed1[0], sed1);
            fprintf(stderr, "Error in execvp2\n");
            exit(104);
        }
        else
        {
            printf("%d: parent - sed 2\n", getpid());
            dump_argv(sed1);
            dup2(fd2[0], 0);
            close(fd1[1]);
            close(fd1[0]);
            close(fd2[1]);
            close(fd2[0]);
            execvp(sed2[0], sed2);
            fprintf(stderr, "Error in execvp3\n");
            exit(105);
        }
    }
    fprintf(stderr, "Reached unexpectedly\n");
    exit(106);
}

int main(void)
{
    char message[] =
        "This is the first line\n"
        "and this is the second - with a semicolon ; here before a :\n"
        "and the third line has a colon : before the semicolon ;\n"
        "but the fourth line has a dot . before the colon\n"
        "whereas the fifth line has a colon : before the dot .\n"
    ;

    sendbc(message);
    return 0;
}

Example output

$ ./pipe29
74829: at work
74829: parent - before second fork
74829: parent - sed 2
74829:
74829: <<sed>>
74829: <<s/[^:]*[;]//>>
74830: child 1 - echo
74830:
74830: <<echo>>
74830: <<This is the first line
and this is the second - with a semicolon ; here before a :
and the third line has a colon : before the semicolon ;
but the fourth line has a dot . before the colon
whereas the fifth line has a colon : before the dot .
>>
74831: child 2 - sed 1
74831:
74831: <<sed>>
74831: <<s/[^:]*[;]//>>
This is the first line
 here before a :
and the third line has a colon :
 before the colon
whereas the fifth line has a colon :

$

Apart from the diagnostic printing, the primary differences are that this code rigorously closes all the unused ends of the pipes and it contains no calls to wait() or its relatives — they are not needed and in general are harmful when they block concurrent execution of the processes in the pipeline.