Thread: Help with fork() and execl() running synchronously

So I have 2 command line programs that are made to work together. The first one receives 2+ files as parameters. The first file is a list of commands, the following parameters are files to call the commands on.

My first program create a fork for each file to be modified and execute the second program with execl().

Here's a part of the code

Main program:

Code:

int main(int argc, char const *argv[]){


    ...


    // Multiple forks
    for(i=2; i < argc; i++){
        
        ...


        pipe(pipes[forks]);
        forks++;


        pid = fork();
        if(pid == 0) break;


        ...
    }
    
    if(pid == 0){
        dup2(pipes[forks-1][0], STDIN_FILENO);


        // Execute the second program in the fork, 
        // pass a file to be read as a single argument
        execl("second_program", "second_program" , argv[i], NULL);
        fprintf(stderr, "execl() failed\n");
        return EXIT_FAILURE;
        
    } else {


        ...


        // Reads a file containing commands, normalize it and output a string
        char *cmds = getcmds(argv[1]);
        int f;
        for(f=0; f < forks; f++){
            // Send the list of commands to the second program
            write(pipes[f][1], cmds, strlen(cmds));
        }
        
        wait(NULL);
    }


    return 0; 
}

Child program:

Code:

int main(int argc, char const *argv[]){


    ...


    char buffer[READ_SIZE];
    do {


        scanf("%s",buffer);
        if(strcmp(buffer,"COMMAND_A") == 0){
            
            ...
            printf("`COMMAND_A on %s\n", filename);


        } if(strcmp(buffer,"COMMAND_b") == 0){
            
            ...
            printf("`COMMAND_b on %s\n", filename);


        } else {
            break;
        }


        ...                             
        
    } while(1);
    
    return 0;
}

My problem is that rather than running concurrently, each fork waits for the previous one to have completed all it's commands before starting it's own.

I get something like is as an output:

Code:

COMMAND_A on file1
COMMAND_B on file1
...
COMMAND_Z on file1
COMMAND_A on file2
COMMAND_B on file2
...
COMMAND_Z on file2
COMMAND_A on file3
COMMAND_B on file3
...
COMMAND_Z on file3

sounds like you need a memory barrier

Only the outout let you think that the childs runs one after one, but this is actually the buffering of each process.
Write after all printf-commands in the child the line:

Code:

fflush(stdout)

With this, the childs flushes there buffer and print out immediately.

And how Salem wrote, look at the wait-commands.
You should have one wait for every child that you forked.