Problem with pipe IPC

[cifz]

I'm currently studying IPC, I have a first program

A: Do an exec for B and wait

B: Receive through a fifo a string from a third program "C" and have to resend it to A

I was thinking to open a pipe in A before the exec, then passing fd[1] to B as an argument

if(pipe(fd)==-1){
    perror("Pipe Failed");
    myExit(EXIT_FAILURE);
}

close(fd[1]);   
sprintf(fdString,"%d",fd[1]);

    .......

if((int)(pid=fork())>0)

    waiting(status);    

else if(pid==0){    

    close(fd[0]);   

    execl("./B","B",fdString,(char*)0);
    perror("Exec failed");
    myExit(EXIT_FAILURE);

}

Then in B:

int fd=atoi(argv[1]);

    //Receive string from C 

    len=strlen(string)+1;

if(write(fd,&len,sizeof(int))==-1){
    perror("Error on writing length");
    exit(EXIT_FAILURE); 
}

if(write(fd,&string,len)==-1){
    perror("Error on writing string");
    exit(EXIT_FAILURE);     
}

My problem now is reading this string in A. I was thinking to send a SIGUSR1 to A as soon as the string is written by B on the pipe and having in A something like:

signal(SIGUSR1, signalHandler);
    ........
    static void signalHandler(int signo){
    switch(signo){
    case SIGUSR1:
        listen();
        break;
    default: break;

    }
}
    ........
static void listen(){

    int len;

    if(read(fd[0],&len,sizeof(int))<sizeof(int)){
        perror("Error reading len");
        myExit(EXIT_FAILURE);   
    }

    char string[len];

    if(read(fd[0],string,len)<len){
        perror("Error reading string");
        myExit(EXIT_FAILURE);   
    }
    printf("String: %s with length %d\n", string, len);
}

However what I get is "Error reading len: Success" , what's wrong ?

Sorry if my English is bad, any help is appreciated, thanks!

[oogabooga]

In the first code block on line 6 you close the output side of the pipe, but you haven't forked yet, so the output will also be closed for B. Fork first, then close the unneeded side of the pipe.

Instead of waiting in the parent, why not just do the read? It will block until it gets something.

[anduril462]

Along with what oogabooga said, may I suggest this fine tutorial on *nix IPCs: Beej's Guide to Unix IPC.

[cifz]

In the first code block on line 6 you close the output side of the pipe, but you haven't forked yet, so the output will also be closed for B. Fork first, then close the unneeded side of the pipe.

Instead of waiting in the parent, why not just do the read? It will block until it gets something.

Ooh, thank you a lot! I definitely need more exercise!

I need to do extra work, not just what I've explained here, so I need to use wait syscall

Along with what oogabooga said, may I suggest this fine tutorial on *nix IPCs: Beej's Guide to Unix IPC.

Thanks, I will!