Troubles with execlp's output and pipes - c

Question

I'm writing a program that has an interface like:
myprog file1 file2 c
This program creates two children and P2 with an execlp opens file2, makes a grep -c on this file for founding c and gives the results to his brother P1 (I have to do it closing the STDOUT's FD and dupping the pipe p2p1 between them). P1 receives this from p2p1 and sends this value to P0. Moreover this also makes the same thing with file1 and gives the result to P0, which will print them.
The problem is: the father P0 reads something, but it is wrong.
What I'm supposed to do? Here's the code and thanks for your attention.
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#define MAX_STRING_LENGTH 128
/**************************/
/* DICHIARAZIONE FUNZIONI */
/**************************/
void wait_child();
void processo_p2(char *inputfile, char *c);
void processo_p1(char *inputfile, char *c);
/*********************/
/* VARIABILI GLOBALI */
/*********************/
int p1p0[2], p2p1[2];
int main(int argc, char* argv[])
{
int pid[2], i, value, count=0;
char *c, buf[10];
if (argc !=4)
{
fprintf(stderr, "Numero di argomenti errato\n");
fprintf(stderr, "Usage: %s file1 file2 C\n", argv[0]);
exit(EXIT_FAILURE);
}
c=argv[3];
/* Init */
pipe(p1p0);
pipe(p2p1);
for (i=0; i<2; i++)
{
pid[i] = fork();
if (pid[i] < 0)
{
fprintf(stderr, "P0: Errore nella fork");
exit(EXIT_FAILURE);
}
else if (pid[i] == 0)
{
if (i==0) /*P1*/
{
close(p1p0[0]);
close(p2p1[1]);
sleep(1);
processo_p1(argv[1], c);
close(p2p1[0]);
close(p1p0[1]);
exit(EXIT_SUCCESS);
}
else if (i==1)
{
close(p2p1[0]);
close(p1p0[0]);
close(p1p0[1]);
processo_p2(argv[2],c);
close(p2p1[1]);
exit(EXIT_SUCCESS);
}
}
else
{
printf("P0: created child P%d with PID %d\n", i+1, pid[i]);
close(p2p1[0]);
close(p2p1[1]);
close(p1p0[1]);
}
}
i=0;
int nread;
while ( (nread = read(p2p1[0], &buf[i], sizeof(char)) ) > 0 ) {
i++;
buf[i] = '\0';
printf("%s\n",buf);
for(i=0;i<2;i++)
{
wait_child();
}
return 0;
}
void processo_p2(char *inputfile, char *c)
{
int fd, nread, i=0, found=0;
char temp, row[100];
close(1);
dup(p2p1[1]);
execlp("grep", "grep", "-c", c, inputfile, (char *)0);
perror("P2: errorr in exec");
close(1);
}
void processo_p1(char *inputfile, char *c)
{
int fd, nrw, sk, nread, p2=0, i=0;
int value=1;
char temp, row[100], buf[10];
//RECEIVING DATA FROM P2 AND SENDING TO P0
while ( (nread = read(p2p1[0], &buf[i], sizeof(char)) ) > 0 ) {
i++;
}
buf[i] = '\0';
printf("from p2: %s\n",buf); //NOTHING STAMPED
write(p1p0[1],&buf,strlen(buf)+1);
close(1);
dup(p1p0[1]);
execlp("grep", "grep", "-c", c, inputfile, (char *)0);
perror("P1: errore in exec");
close(p1p0[1]);
}
void wait_child() {
int pid_terminated,status;
pid_terminated=wait(&status);
if (pid_terminated < 0)
{
fprintf(stderr, "%d\n", getpid());
perror("P0: errore in wait");
exit(EXIT_FAILURE);
}
if(WIFEXITED(status))
{
printf("P0: terminazione volontaria del figlio %d con stato %d\n",
pid_terminated, WEXITSTATUS(status));
if (WEXITSTATUS(status) == EXIT_FAILURE)
{
fprintf(stderr, "P0: errore nella terminazione del figlio pid_terminated\n");
exit(EXIT_FAILURE);
}
}
else if(WIFSIGNALED(status))
{
fprintf(stderr, "P0: terminazione involontaria del figlio %d a causa del segnale %d\n",
pid_terminated,WTERMSIG(status));
exit(EXIT_FAILURE);
}
}

Answer 1

Trivia (but it stops the code compiling): The code shown originally has a close brace in the wrong place — the functions are all apparently embedded inside main(). The last brace should be moved up the file considerably.
In the loop that launches the child processs, you have the parent process execute:
printf("P0: created child P%d with PID %d\n", i+1, pid[i]);
close(p2p1[0]);
close(p2p1[1]);
close(p1p0[1]);
In the parent process, immediately after the loop, you have:
int nread;
while ( (nread = read(p2p1[0], &buf[i], sizeof(char)) ) > 0 ) {
i++;
buf[i] = '\0';
printf("%s\n",buf);
for(i=0;i<2;i++)
{
wait_child();
}
return 0;
}
'Tis a pity that you closed the file descriptor you're trying to read from. The parent should be reading from p1p0[0], which you did leave open.
Fixes:
Move the set of three closes out of the loop.
Read from the correct file descriptor.
Insert a missing close brace for the while loop shown, or remove its open brace.
The question in the comments has, as usual for comments, inscrutable code. The problem though is that you have too big a loop.
This is what the tail of my fixed version of your main() function looks like:
…rest of loop to launch children…
else
{
printf("P0: created child P%d with PID %d\n", i + 1, pid[i]);
}
}
close(p2p1[0]);
close(p2p1[1]);
close(p1p0[1]);
i = 0;
int nread;
while ((nread = read(p1p0[0], &buf[i], sizeof(char)) ) > 0)
i++;
buf[i] = '\0';
printf("%s\n", buf);
for (i = 0; i < 2; i++)
{
wait_child();
}
return 0;
}
Given that I saved the source code in a file pip.c and created the program pip from it, a sample run produced:
$ ./pip pip.c pip.c c
P0: created child P1 with PID 75458
P0: created child P2 with PID 75459
from p2: 49
49
P0: terminazione volontaria del figlio 75459 con stato 0
P0: terminazione volontaria del figlio 75458 con stato 0
$
I'm not entirely happy with all the newlines, but at least the same answer is given twice, as it obviously should be.