Can't read from a pipe after dup2() and fork. C - c

Question

I'm writing a code that echo a string and sed it two times. My output is correct, but when I try to place that string on an array it blocks on read and goes on with the other calls.
Here's the code:
#include <unistd.h>
#include <sys/types.h>
#include <dirent.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
char **sendout=NULL;
int send_i=0;
void sender2(char* str_) {
int fd[2];
int fd1[2];
int fd2[2];
int pid;
char* echo[] = {"echo", str_, NULL};
char* sed[] = {"sed", "regex1", NULL};
char* sed2[] = {"sed", "regex2", NULL};
int status;
if (pipe(fd) < 0) {
exit(100);
}
pid = fork();
if (pid == 0) {
close(fd[0]);
dup2(fd[1], 1);
close(fd[1]);
execvp(echo[0], echo);
printf("Error in execvp1\n");
}
if (pipe(fd1) < 0) {
exit(100);
}
pid = fork();
if (pid == 0) {
close(fd[1]);
close(fd1[0]);
dup2(fd[0], 0);
dup2(fd1[1], 1);
dup2(fd1[1], 2);
close(fd[0]);
close(fd1[1]);
execvp(sed2[0], sed2);
printf("Error in execvp2\n");
}
if (pipe(fd2) < 0) {
exit(100);
}
pid = fork();
if (pid == 0) {
close(fd1[1]);
close(fd2[0]);
dup2(fd1[0], 0);
dup2(fd2[1], 1);
dup2(fd2[1], 2);
close(fd2[1]);
close(fd1[0]);
execvp(sed[0], sed);
}
pid = fork();
if (pid == 0) {
close(fd2[1]);
char* line = NULL;
size_t len = 0;
ssize_t read_;
FILE* f_pipe;
f_pipe = fdopen(fd2[0], "r");
printf("1\n");
while ((read_ = getline(&line, &len, f_pipe)) != -1) {
printf("2\n");
sendout = realloc(sendout, sizeof(char*) * (send_i + 1));
sendout[send_i] = strdup(line);
send_i++;
printf("%s\n", line);
}
fclose(f_pipe);
close(fd2[0]);
return;
}
close(fd[1]);
close(fd[0]);
close(fd1[1]);
close(fd1[0]);
close(fd2[1]);
close(fd2[0]);
if (pid != 0) {
wait(&status);
}
}
int main() {
sender2("hello");
}
Like I said it all worked until the read. If I pass 3 string to the function the output is like:
1
1
1
If I don't dup to the last pipe it prints pretty well what I need, I also used return in the last fork because it's the only child process that isn't killed from execvp. But it doesn't even reach the first print. I even tried opening the pipe as a file or with a classic open, so it goes that I tried open and also fopen, as you can see. I'm failing because it can't read anything. That would be a time problem.

Answer 1

Fork and File Descriptors
When you fork a process, copies of all file descriptors are inherited. Since those are copies, the descriptors must be closed in both the child and the parent. You should always close them as soon as possible. This is especially true if you fork several times.
It's very easy to miss something here. It is therefore best to check very carefully that all file descriptors have been closed.
Minimum Amount of Changes
So the minimum number of changes for your code to get a result would be as follows.
If the first fork in line 41 is successful then in the parent you need to close the pipe file descriptors fd[0] and fd[1], e.g. in line 56.
pid = fork();
if (pid == 0) {
...
}
close(fd[0]); //<-- add these two lines
close(fd[1]);
if (pipe(fd2) < 0) {
...
Likewise you need to do the same after the second fork for fd1, so:
pid = fork();
if (pid == 0) {
...
}
close(fd1[0]); //<-- add these two lines
close(fd1[1]);
pid = fork();
When you now run your code you would already get as output:
1
2
hello
Better Test Case
This would not yet verify that both sed commands would run correctly. For a test case change the call in main to:
sender2("hello mars");
and change your sed commands to:
char* sed[] = {"sed", "s/moon/world/", NULL};
char* sed2[] = {"sed", "s/mars/moon/", NULL};
(sed2 command is executed before sed in your code, it would make the code a bit easier to understand if sed is executed before sed2)
This gives as output then:
1
2
hello world
So both sed commands are executed.
Additional Remarks
Below are some remarks in no particular order, mainly concerning error handling.
A call to fork returns pid_t and not int. So you should change your definition of the variable pid to: pid_t pid;.
If execvp fails one should print the error cause and exit with an error status, e.g. something like this:
perror("execvp of command xyz failed");
exit(EXIT_FAILURE);
If opening a pipe fails, also print a descriptive message on stderr.
Also fork calls can fail, this should also be handled. In this case fork returns -1. Same as above, print error message on stderr and return an error status.
In main you should return a success or failure state (e.g. return EXIT_SUCCESS;).
You don't use the the variable read_. Then the variable can be removed.
If fdopen fails it returns NULL. This error case should be handled.
The memory allocated with realloc is never released.