Piping between multiple child processes - c

Question

I'm trying to process input through different child process from one parent. I can make it though the first 3 children, but after that, I can't seem to get any input into or anything of out sort.
Here is my code.
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
int main(int argc, char** argv)
{
pid_t pid;
int pipes_1[2];
int pipes_2[2];
pipe(pipes_1);
pipe(pipes_2);
switch(pid=fork())
{
case 0:
dup2(pipes_1[0], 0);//copy stdin onto pipe 1 read
dup2(pipes_2[1], 1);//copy stdout onto pipe 2 write
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
execlp("sed", "sed","s/[^a-zA-Z]/ /g", "test.txt", (char*)NULL);
break;
default:
break;
}
switch(pid = fork())
{
case 0:
dup2(pipes_2[0],0); //copy std onto pipes 2 read
dup2(pipes_1[1],1);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
execlp("tr", "tr", "[A-Z]", "[a-z]", (char*)NULL);
break;
default:
break;
}
switch(pid=fork())
{
case 0:
dup2(pipes_1[0], 0);
dup2(pipes_2[1], 1);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
execlp("awk", "awk", "{for(i = 1; i <= NF; i++) {print $i;}}", (char*)NULL);
break;
default:
break;
}
switch(pid=fork())
{
case 0:
dup2(pipes_2[0], 0);
//dup2(pipes_1[1], 1);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
execlp("sort", "sort", (char*)NULL);
default:
break;
}
wait();
return 0;
}
I know the formatting is bad. But can you see what I'm doing wrong? I think it may have something to do with only using two pipes.
EDIT: Updated code with 4 pipes.
int main(int argc, char** argv)
{
pid_t pid;
int pipes_1[2];
int pipes_2[2];
int pipes_3[2];
int pipes_4[2];
pipe(pipes_1);
pipe(pipes_2);
pipe(pipes_3);
pipe(pipes_4);
switch(pid=fork())
{
case 0:
dup2(pipes_1[0], 0);//copy stdin onto pipe 1 read
dup2(pipes_2[1], 1);//copy stdout onto pipe 2 write
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
execlp("sed", "sed","s/[^a-zA-Z]/ /g", "test.txt", (char*)NULL);
break;
default:
break;
}
switch(pid = fork())
{
case 0:
dup2(pipes_2[0],0); //copy std onto pipes 2 read
dup2(pipes_3[1],1);
close(pipes_3[0]);
close(pipes_3[1]);
close(pipes_2[0]);
close(pipes_2[1]);
execlp("tr", "tr", "[A-Z]", "[a-z]", (char*)NULL);
break;
default:
break;
}
switch(pid=fork())
{
case 0:
dup2(pipes_3[0], 0);
dup2(pipes_4[1], 1);
close(pipes_3[0]);
close(pipes_3[1]);
close(pipes_4[0]);
close(pipes_4[1]);
execlp("awk", "awk", "{for(i = 1; i <= NF; i++) {print $i;}}", (char*)NULL);
break;
default:
break;
}*/
switch(pid=fork())
{
case 0:
dup2(pipes_4[0], 0);
//dup2(pipes_1[1], 1);
close(pipes_4[0]);
close(pipes_4[1]);
//close(pipes_2[0]);
//close(pipes_2[1]);
execlp("sort", "sort", (char*)NULL);
default:
break;
}
wait();
return 0;
}

Answer 1

You seem to have a pipeline:
sed … | tr … | awk … | sort
You create just two pipes, where three are needed. Create the third pipe and handle it correctly, and you'll be OK.
Adapting second code. Note that only three pipes are needed for four processes (and in general N-1 pipes are needed for N processes).
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
static inline void error(const char *msg)
{
perror(msg);
exit(EXIT_FAILURE);
}
int main(void)
{
int pid;
int pipes_1[2];
int pipes_2[2];
int pipes_3[2];
pipe(pipes_1);
pipe(pipes_2);
pipe(pipes_3);
if ((pid = fork()) == 0)
{
dup2(pipes_1[1], 1);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
close(pipes_3[0]);
close(pipes_3[1]);
execlp("sed", "sed", "s/[^a-zA-Z]/ /g", "test.txt", (char *)NULL);
error("Failed to exec sed");
}
printf("sed: %d\n", pid);
if ((pid = fork()) == 0)
{
dup2(pipes_1[0], 0);
dup2(pipes_2[1], 1);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
close(pipes_3[0]);
close(pipes_3[1]);
execlp("tr", "tr", "[A-Z]", "[a-z]", (char *)NULL);
error("Failed to exec tr");
}
printf("tr: %d\n", pid);
if ((pid = fork()) == 0)
{
dup2(pipes_2[0], 0);
dup2(pipes_3[1], 1);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
close(pipes_3[0]);
close(pipes_3[1]);
execlp("awk", "awk", "{for(i = 1; i <= NF; i++) {print $i;}}", (char *)NULL);
error("Failed to exec awk");
}
printf("awk: %d\n", pid);
if ((pid = fork()) == 0)
{
dup2(pipes_3[0], 0);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
close(pipes_3[0]);
close(pipes_3[1]);
execlp("sort", "sort", (char *)NULL);
error("Failed to exec sort");
}
printf("sort: %d\n", pid);
close(pipes_1[0]);
close(pipes_1[1]);
close(pipes_2[0]);
close(pipes_2[1]);
close(pipes_3[0]);
close(pipes_3[1]);
int status;
int corpse;
while ((corpse = wait(&status)) > 0)
printf("PID %d died 0x%.4X\n", corpse, status);
return 0;
}
Sample input:
Happy Go Lucky!
PENULTIMATE DESTINY#
missing all upper-case=
What gives?
Digital 023123098 Diarrhea
Sample output:
sed: 74841
tr: 74842
awk: 74843
sort: 74844
PID 74841 died 0x0000
PID 74842 died 0x0000
PID 74843 died 0x0000
all
case
destiny
diarrhea
digital
gives
go
happy
lucky
missing
penultimate
upper
what
PID 74844 died 0x0000
The process ID information is primarily diagnostic. If you pipe the output of the program to a filter, you will get different output (because of buffering, etc), but the difference is in the sequence in which the sorted data appears vs the diagnostics. Print the diagnostics to standard error, or add a fflush(stdout) after each printf() and you'll get something similar to the shown output more routinely.