Check if unix pipe closed without writing anything? - c

Question

Basically I have a parent process that forks a child and feeds it it's stdin through a pipe. The child process can terminate in one of two cases:
the write end of the pipe is closed by the parent, meaning it reached the end of stdin thus receiving an EOF,
or it receives a certain input through the pipe(-1 in this case) and exits
My parent code looks roughly like this:
close(pi[0]); // close input end
signal(SIGPIPE, SIG_IGN); // do not handle SIGPIPE
char buffer;
int ok = 1;
while(ok && read(STDIN_FILENO, &buffer, 1) > 0) {
int b_written = write(pi[1], &buffer, 1);
if(b_written == -1) {
if(errno == EPIPE) ok = 0;
else perror("pipe write"); // some other error
}
}
As you can see, I check whether the read end of a pipe is closed by checking for errno == EPIPE. However this means that the read loop does one extra iteration before closing. How could I possibly poll to see if the pipe is closed without necessarily writing something to it?

Answer 1

This snippet will check if the other end of a writable pipe is closed using poll(2). This works on Linux -- I'm not sure about other OSes or what POSIX says.
#include <poll.h>
#include <stdbool.h>
#include <stdio.h>
#include <unistd.h>
bool is_pipe_closed(int fd) {
struct pollfd pfd = {
.fd = fd,
.events = POLLOUT,
};
if (poll(&pfd, 1, 1) < 0) {
return false;
}
return pfd.revents & POLLERR;
}

Answer 2

The child could send a signal, such as SIGUSR1 when it detects it has finished. Parent could set a flag to when it receives SIGUSR1 signal, and check this flag before trying to read input. But I am not absolutely sure SIGUSR1 could not be received after checking the flag ans before reading input from stdin). So I prefer to use a control pipe, each time child know it will be able to read one more data it write a 1 in this control pipe. The result could be something like that:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>
#define STOP_VALUE 100
#define SIZE_STDIN_BUFFER 1024
static char can_read_more = 1;
static int handle_child(int *p_child_input_stream, int *p_control_stream)
{
int pipefd[2][2];
pid_t fk;
if (pipe(pipefd[0]) < 0) // Pipe to read input from
{
perror("pipe");
return -1;
}
if (pipe(pipefd[1]) < 0) // Pipe to notifiate parent input can be processed
{
perror("pipe");
close(pipefd[0][0]);
close(pipefd[0][1]);
return -1;
}
if ((fk = fork()) < 0)
{
perror("fork");
close(pipefd[0][0]);
close(pipefd[0][1]);
close(pipefd[1][0]);
close(pipefd[1][1]);
return -1;
}
if (fk == 0)
{
close(pipefd[0][1]);
close(pipefd[1][0]);
write(pipefd[1][1], &can_read_more, sizeof(char)); // sizeof(char) == 1
ssize_t nb_read = 0;
char buffer;
while (nb_read >= 0)
{
nb_read = read(pipefd[0][0], &buffer, sizeof(char));
if (nb_read > 0)
{
printf("0x%02x\n", (unsigned int) buffer);
if (buffer == STOP_VALUE)
{
nb_read = -1;
}
else
{
write(pipefd[1][1], &can_read_more, sizeof(char));
}
}
}
close(pipefd[0][0]);
close(pipefd[1][1]);
exit(0);
}
close(pipefd[0][0]);
close(pipefd[1][1]);
*p_child_input_stream = pipefd[0][1];
*p_control_stream = pipefd[1][0];
return 0;
}
int main()
{
int child_input_stream;
int control_stream;
if (handle_child(&child_input_stream, &control_stream) < 0)
{
return 1;
}
char stdin_buffer[SIZE_STDIN_BUFFER];
char buffer;
int ok = 1;
int child_available_input = 0;
while(ok)
{
while (child_available_input <= 0 && ok)
{
ssize_t nb_control = read(control_stream, &buffer, sizeof(char));
if (nb_control > 0)
{
child_available_input += buffer;
}
else
{
fprintf(stderr, "End of child reading its input detected.\n");
ok = 0;
}
}
if (ok)
{
if (fgets(stdin_buffer, SIZE_STDIN_BUFFER, stdin) == NULL)
{
ok = 0;
}
else
{
if (stdin_buffer[strlen(stdin_buffer) - 1] == '\n')
{
stdin_buffer[strlen(stdin_buffer) - 1] = '\0';
}
char dummy;
int input;
if (sscanf(stdin_buffer, "%d%c", &input, &dummy) == 1)
{
buffer = (char) input;
write(child_input_stream, &buffer, sizeof(char));
child_available_input--;
}
}
}
}
return 0;
}