Pipes between parent and two children in C - c

Question

I am trying to write a C program that uses pipes to send information between the parent and two children. The goal of the program is to achieve something similar to merge sort, for strings. I read the number of strings and then the Strings. The strings get divided between the 2 children, recursively until each child has only one string. I have to redirect the stdin of the child to read from the stdout of the parent.
For some reason none of the children read more than the first string.
How could I solve this problem?
int main(int argc, char * argv[]) {
int nrrows = 0;
char * buffer = NULL;
size_t n = 0;
getline(&buffer, &n, stdin);
char * endptr;
nrrows = strtol(buffer, &endptr, 10);
char rows[nrrows][MAX_LEN];
int i = 0;
n = 0;
while(i < nrrows) {
char * row = NULL;
getline(&row, &n, stdin);
strcpy(rows[i], row);
i++;
}
if(nrrows == 1) {
fprintf(stderr, "%s", rows[0]);
return 0;
}
int fdcp1[2];
int fdcp2[2];
if(pipe(fdcp1) < 0) {
fprintf(stderr, "pipe unsuccessfull\n");
return EXIT_FAILURE;
}
if(pipe(fdcp2) < 0) {
fprintf(stderr, "pipe unsuccessfull\n");
return EXIT_FAILURE;
}
pid_t chpid1 = fork();
if(chpid1 < 0) {
fprintf(stderr, "fork unsuccessfull\n");
return EXIT_FAILURE;
}
else if(chpid1 == 0) {
close(fdcp2[0]);
close(fdcp2[1]);
close(fdcp1[1]);
dup2(fdcp1[0], STDIN_FILENO);
execlp("./forksort", "child1", NULL);
}else {
close(fdcp1[0]);
dup2(fdcp1[1], STDOUT_FILENO);
double half = (nrrows / 2);
int h = half;
char b[2];
b[0] = '0' + h;
b[1] = '\n';
write(fdcp1[1], b, sizeof(b));
for(i = 0; i < h; i ++) {
rows[i][strlen(rows[i])] = '\0';
write(fdcp1[1], rows[i], sizeof(rows[i]));
}
pid_t chpid2 = fork();
if(chpid2 < 0) {
fprintf(stderr, "fork unsuccessfull\n");
return EXIT_FAILURE;
}else if(chpid2 == 0) {
close(fdcp1[0]);
close(fdcp1[1]);
close(fdcp2[1]);
dup2(fdcp2[0], STDIN_FILENO);
execlp("./forksort", "child2", NULL);
}else {
close(fdcp2[0]);
dup2(fdcp2[1], STDOUT_FILENO);
half = (nrrows / 2);
h = half;
char b[2];
b[0] = '0' + (nrrows - h);
b[1] = '\n';
write(fdcp2[1], b, sizeof(b));
for(i = h; i < nrrows; i ++) {
rows[i][strlen(rows[i])] = '\0';
write(fdcp2[1], rows[i], sizeof(rows[i]));
}
}
}
return 0;
}

Answer 1

It's bad news to modify a file descriptor that is associated with an open stream. I would account it highly likely to cause you trouble, and there is, moreover, no need to do that here. The parent should instead use fdopen() to open new streams on top of its ends of the pipes, and conduct I/O with its children via those instead of via the standard streams. In addition to being safer, that leaves the process's original standard streams available for it to communicate with its parent process.
With that approach, you could even stream the strings to be sorted back and forth among the processes, instead of redundantly buffering blocks of them in each process's memory. For instance, you might do something like this:
#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char * argv[]) {
char * buffer = NULL;
size_t buflen = 0;
int nrrows;
int fdpc1[2];
int fdcp1[2];
int fdpc2[2];
int fdcp2[2];
pid_t chpid1;
pid_t chpid2;
FILE *pipeout;
FILE *pipein1;
FILE *pipein2;
int half;
int i;
fprintf(stderr, "%s!!!!!!!!!!!!!!!!!\n", argv[0]);
getline(&buffer, &buflen, stdin);
fprintf(stderr, "number: %s from %s\n", buffer, argv[0]);
nrrows = strtol(buffer, NULL, 10);
if(nrrows <= 0) {
fprintf(stderr, "This is not a valid >0 number\n");
return EXIT_FAILURE;
} else if (nrrows == 1) {
/* ... read and echo back the one row ... */
getline(&buffer, &buflen, stdin);
fprintf(stderr, "%s", buffer);
return EXIT_SUCCESS;
}
/* There are at least two rows to sort */
if (pipe(fdcp1) < 0) {
fprintf(stderr, "pipe unsuccessfull\n");
return EXIT_FAILURE;
}
if (pipe(fdpc1) < 0) {
fprintf(stderr, "pipe unsuccessfull\n");
return EXIT_FAILURE;
}
chpid1 = fork();
if (chpid1 == 0) {
/* this is child process 1 */
close(fdcp1[1]);
close(fdpc1[0]);
dup2(fdcp1[0], STDIN_FILENO);
close(fdcp1[0]);
dup2(fdpc1[1], STDOUT_FILENO);
close(fdpc1[1]);
execlp("./forksort", "child1", NULL);
} else if (chpid1 < 0) {
fprintf(stderr, "fork unsuccessfull\n");
return EXIT_FAILURE;
}
/* this is the parent process */
close(fdcp1[0]);
close(fdpc1[1]);
if (pipe(fdcp2) < 0) {
fprintf(stderr, "pipe unsuccessfull\n");
return EXIT_FAILURE;
}
if (pipe(fdpc2) < 0) {
fprintf(stderr, "pipe unsuccessfull\n");
return EXIT_FAILURE;
}
chpid2 = fork();
if (chpid2 == 0) {
/* this is child process 2 */
close(fdcp1[1]);
close(fdpc1[0]);
close(fdcp2[1]);
close(fdpc2[0]);
dup2(fdcp2[0], STDIN_FILENO);
close(fdcp2[0]);
dup2(fdpc2[1], STDOUT_FILENO);
close(fdpc2[1]);
execlp("./forksort", "child2", NULL);
} else if (chpid2 < 0) {
fprintf(stderr, "fork unsuccessfull\n");
return EXIT_FAILURE;
}
/* this is the parent process */
close(fdcp2[0]);
close(fdpc2[1]);
/* copy the first half of the lines from input to child 1 */
pipeout = fdopen(fdcp1[1], "w");
if (pipeout == NULL) {
fprintf(stderr, "fdopen unsuccessful\n");
return EXIT_FAILURE;
}
half = nrrows / 2;
fprintf(pipeout, "%d\n", half);
for (i = 0; i < half; i += 1) {
getline(&buffer, &buflen, stdin);
fprintf(stderr,"row[%d] from %s: %s", i, argv[0], buffer);
fputs(buffer, pipeout);
}
fclose(pipeout);
/* copy the second half of the lines from input to child 2 */
pipeout = fdopen(fdcp2[1], "w");
if (pipeout == NULL) {
fprintf(stderr, "fdopen unsuccessful\n");
return EXIT_FAILURE;
}
fprintf(pipeout, "%d\n", nrrows - half);
for (; i < nrrows; i += 1) {
getline(&buffer, &buflen, stdin);
fprintf(stderr,"row[%d] from %s: %s", i, argv[0], buffer);
fputs(buffer, pipeout);
}
fclose(pipeout);
/* now read and merge sorted lines from the children */
pipein1 = fdopen(fdpc1[0], "r");
pipein2 = fdopen(fdpc2[0], "r");
if (pipein1 == NULL || pipein2 == NULL) {
fprintf(stderr, "fdopen unsuccessful\n");
return EXIT_FAILURE;
}
/* ... */
fclose(pipein1);
fclose(pipein2);
return 0;
}