Thread: Communication through pipes

Hello everyone,
I'm trying to communicate between father and son processes using pipes.

Wouldn't this be the proper way for the son to communicate with his father ?

Code:

 else if(pid==0) {
        /* son */
        dup2(fd[1],1);
        close(fd[1]);
        write(fd[1],buffer,1000);
        int a = execlp(cmd,cmd,arg,NULL);
        _exit(pid); // son dies
 }
 
 wait(&pid); 
 read(fd[0],buffer,strlen(arg));

Tried that.
After

Code:

read(fd[0],buffer,strlen(arg));

I have this printf :

Code:

printf("-----> %s \n",buffer);

And it prints all NULL.

Why ain't it reading properly ? :/

Here it is Salem :

Code:

    void map(char* cmd, char *arg){    int i;
    int pid;
    int fd[2];
    char* buffer;
    char* novo;
    int status;
    FILE *stream;
    pipe(fd);
    pid=fork();
    stream = fopen("merdas.txt","a+");


    if(pid==-1) /* erro */
    {
        perror("impossivel de criar um filho") ;
        exit(-1) ;
    }
    else if(pid==0) {
        /* filho */
        dup2(fd[1],1);
        close(fd[1]);
       // read(1,buffer,1000);
        printf("li %s\n",&buffer);
        //write(1,buffer,1000);
        printf("buffer:a%sa\n",&buffer);
        printf("Resultado do execlp:\n");
        int a = execlp(cmd,cmd,arg,NULL); //o comando é o primeiro argumento da main e o argumento é a linha/ficheiro...
        
        _exit(pid); //filho morra
        //sleep(1);
    }
    printf("adoro merda antes do wait \n");
    wait(&pid); //esperar que o filho termine
    printf("ainda mais depois \n");
    /*dup2(fd[0],fd[1]);
    close(fd[0]);*/
    //fprintf(stream,"%s\n",buffer);
    read(0,buffer,strlen(arg));
    printf("-----> %s \n",buffer);
}


int main(int argc, char** argv){
   // FILE *stream;
    char *str;
    //if((stream = freopen("merdas.txt", "w", stdout)) == NULL)
        //exit(-1);
    //stream = fopen("merdas.txt","a+");
    char *aux;
    size_t tam=0;
    int line=0;
    int i;
    //  init();
   // printf("iq\n");
    while(line=getline(&str,&tam,stdin)!=-1){
        tam=strlen(str);
        str[tam-1]='\0';
        map(argv[1],str);
      //  fprintf(stream,"%s\n",str);
    }
    //fprintf(stream,"END");
    //freopen ("/dev/tty", "a", stdout);
    //map2();
    return(1);
 
}

Read the manual page for dup2. What does this function do? What is its return value? Are you using this function properly?

Originally Posted by Salem

> char* buffer;
Where does this point?

Having char* buffer and char buffer[] is the same thing I guess.

> printf("li %s\n",&buffer);
You use buffer and &buffer like they were interchangeable.
Whist this is true (value wise at least, the types are always different) for a true array, you can't to the same thing with pointers.

> stream = fopen("merdas.txt","a+");
Do this before the fork, not after.

Doing this after means one process will fail, because two processes can't write to the same file.

I've done this, thanks.

> _exit(pid); //filho morra
exit takes a small integer result, not a process ID.
Typically, _exit(EXIT_FAILURE) when it's all gone wrong.

Done, thanks.

> while(line=getline(&str,&tam,stdin)!=-1)
You should initialise str to NULL before calling this for the first time, otherwise it will just try to do something stupid with the garbage pointer.

Done aswell, thanks for the tip.

Originally Posted by Salem

> Having char* buffer and char buffer[] is the same thing I guess.
Guess again.

Try with
char buffer[1001];

> stream = fopen("merdas.txt","a");
I know I said before the fork, but having looked at it again, it might be better if you only did this in the parent.

Thanks for the kind replies and explanation.
I found out what's happening, I just do not know how to solve it.

Thing is my wait function isn't working, this :

Code:

else
        waitpid(-1, &status, 0);     

For some reason waitpid isn't working and my function is going straight to the parent process, not going to the son and therefor not doing exec .

Any idea on why this is happening ? :/

Originally Posted by Salem

> int *status;
...
> waitpid(-1, &status, 0);
Please tell me you're getting compiler warnings for this.

Fix ALL warnings before trying to run the code.

I would go so far as to suggest you add -Werror to the compiler command line to stop you from running code with warnings still present.

cc1: warnings being treated as errors
soluis2.c: In function ‘map2’:
soluis2.c:93: warning: format ‘%s’ expects type ‘char *’, but argument 4 has type ‘char (*)[1000]’
soluis2.c:93: warning: format ‘%s’ expects type ‘char *’, but argument 4 has type ‘char (*)[1000]’
soluis2.c: In function ‘map’:
soluis2.c:143: warning: passing argument 2 of ‘waitpid’ from incompatible pointer type

Thanks, I'll fix these warnings and see what I get.

Doing instead

Code:

waitpid(-1, status, 0);

Managed to cut the error out but still it won't work >_>

Code:

    if(pipe(fd)==-1){perror("demo");exit(1);}
    if ((pid = fork()) == -1)   {perror("demo"); exit(1);}
    else
        waitpid(-1, &status, 0);
    
    if(pid==0) // filho
    {
        dup2(fd[1],1);

That doesn't look right. Assuming the fork() succeeds, then the child will have pid 0 and the parent will have its pid. So having done the fork, both the parent and the child will go to waitpid() because neither pid is -1.

You need to sort out your types and make sure you're not passing garbage pointers all over the place.

Code:

 int *status; // this is an uninitialised int pointer. 
 char* buffer; // again uninitialised pointer, trying to write to this could crash your program or cause weird behaviour
 char buffer[1000]; // this has statically allocated 1000 bytes of space. So you can actually write to this
 int status2;  // Just an int. You can use &status to pass the address where an int* is required, and just read it as a normal int here.

If you pass status to waitpid correctly you can check it and find out why it didn't stop.