如何将管道从父进程发送到子进程

Question

I am trying to use the fork to create three child process, two of them will write string of char to pipe will the third one will read it from pipr and output it one the screen. We are supposed to create 4 files and call exec system call to access those file. but I have no idea how to send the pipe to other process. Since I tried in the main process to create fd[2] and pipe(fd) in the child process call close(fd[0]), but it gives me undeclared? how could I do this. here's my code

   int main(void){
int fd[2],z,status,i;
pid_t childB, childC, childD;
char *arg[1] = {0};
z = pipe(fd);
if(z <0 ){perror("create pipe"); exit(0);}

childB = fork();
if(childB==0){execv("PipeW1",arg);}
if(childB<0){printf("fork failed\n");exit(0);}
....

and in the PipeW1 method, I did this:

    void main (int argc, char *argv[]){
    int i;
    char str[6];
    close(fd[0]);   
    for(i=1;i<=500;i++){
    sprintf(str,"%03daaa",i);
    z = write(fd[1],str,6);
    if(z<0) {perror("write process B"); exit(1);}
    if(i%100==0){usleep(100000);}
}
close(fd[1]);
exit(0);
}

any suggestions will be helpful!

Thanks

Answer 1

Pipes remain open through an execve - but you cannot use a variable declared in one program in another (fd[]), it's not only in a different C scope (compilation unit), but it's an entirely different /process image/.

The numeric value, however, will remain - that is what identifies the pipe. So, pass fd[0], the number which represents the "read" end of the pipe, to the child process. Ordinarily it would be natural to use fork(), but since you exec(), the best way to do this is to pass the number fd[0] as a comand line arg, in the argv[] array. Note that this will be a string, so you'll need to use something like snprintf() to create the argument.

Answer 2

One way to approach the problem is to redirect stdout in the child process. This can be done with dup2 . After the fork the child process can call dup2 to connect stdout to the write end of the pipe. Then the file descriptors for the pipe can be closed. After the exec , the child writes to the pipe by writing to stdout , eg with printf .

Here is a complete example which demonstrates the concepts.

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

int main( void )
{
    int fd[2];
    pid_t pid;

    if ( pipe( fd ) < 0 )
    {
        perror( "pipe" );
        exit( 1 );
    }

    if ( (pid = fork()) < 0 )
    {
        perror( "fork" );
        exit( 1 );
    }

    if ( pid == 0 )
    {
        dup2( fd[1], STDOUT_FILENO );   // attach the write end of the pipe to stdout
        close( fd[0] );                 // close the pipe (read end)
        close( fd[1] );                 // close the pipe (write end)

        char *args[] = { "./test", "hello", "world", NULL };
        execv( args[0], args );
    }

    // the following code is only for the parent, since the child has exec'd
    close( fd[1] );         // close the write end of the pipe
    char buffer[100];
    ssize_t length = read( fd[0], buffer, sizeof(buffer) - 1 );     // read from the child
    if ( length < 0 )
    {
        perror( "read" );
        exit( 1 );
    }
    buffer[length] = '\0';
    printf( "parent: %s", buffer );
    close( fd[0] );
}

test.c

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

int main( int argc, char *argv[] )
{
    if ( argc < 2 )
    {
        printf( "no args\n" );
        exit( 1 );
    }

    for ( int i = 1; i < argc; i++ )
        printf( "%s%s", argv[i], (i+1 < argc) ? ", " : "\n" );
}

The expected output is

parent: hello, world