close pipe from parent and child process - c

Question

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/wait.h>
int main() {
int p[2];
pipe(p);
if (fork() == 0) {
// child
/*#0*/ close(p[1]);
int received = -1;
while (read(p[0], &received, 4) != 0) {
printf("receive integer: %d\n", received);
received = -1;
}
printf("child exit\n");
exit(0);
} else {
// parent
/*#1*/ close(p[0]);
int sent = 42;
write(p[1], &sent, 4);
/*#2*/ close(p[1]);
printf("wait for child\n");
wait(0);
}
printf("finished\n");
}
I'm trying to understand fork and pipe in C. This program fork a child process, which receive an integer from parent process then exit when pipe closed. When executing, it prints
wait for child
receive integer: 42
child exit
finished
Yet the while loop got stuck after close(p[1]); at position #0 removed: that read would infinitely wait for an incoming variable from the pipe and never detect the pipe closed.
Can someone explain to me why p[1] has to be closed by both parent (position #2) and child (position #0) process?

Answer 1

Here is the code (from Linux manual page) with comments at the bottom of the code.
https://man7.org/linux/man-pages/man2/pipe.2.html
At /#2/ close(pipefd[1]), the comment states that "Reader will see EOF". It means there is nothing to read into child process anymore and then the statement "read(p[0], &received, 4)" will return 0. In the Linux manaul page https://man7.org/linux/man-pages/man2/read.2.html
states that "On success, the number of bytes read is returned (zero indicates end of file)"
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int
main(int argc, char *argv[])
{
int pipefd[2];
pid_t cpid;
char buf;
if (argc != 2) {
fprintf(stderr, "Usage: %s <string>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (pipe(pipefd) == -1) {
perror("pipe");
exit(EXIT_FAILURE);
}
cpid = fork();
if (cpid == -1) {
perror("fork");
exit(EXIT_FAILURE);
}
if (cpid == 0) { /* Child reads from pipe */
close(pipefd[1]); /* Close unused write end */
while (read(pipefd[0], &buf, 1) > 0)
write(STDOUT_FILENO, &buf, 1);
write(STDOUT_FILENO, "\n", 1);
close(pipefd[0]);
_exit(EXIT_SUCCESS);
} else {/* Parent writes argv[1] to pipe */
close(pipefd[0]); /* Close unused read end */
write(pipefd[1], argv[1], strlen(argv[1]));
/*#2*/ close(pipefd[1]); /* Reader will see EOF */
wait(NULL); /* Wait for child */
exit(EXIT_SUCCESS);
}
}