Bind the pipe to stdin, there is a blocking problem - c

Question

I want to associate the standard output of the parent process with the standard input of the child process through an anonymous pipe. I did achieve my needs, but I also encountered a problem: the program blocks when calling scanf, even if stdin is set to none Buffer, replace scanf with read to run normally! I know this has something to do with the standard I/O function buffer, but I don't know the internal reason, I hope I can get help, thank you!
Code:
int main(void)
{
int fd[2];
int cpid;
pipe(fd);
if((cpid = fork()) < 0)
{
perror("fork error");
exit(-1);
}
else if(cpid == 0)
{
sleep(1);
char buf[100] = {0};
close(fd[1]);
if(dup2(fd[0], 0) < 0)
{
perror("parent - dup2 error");
exit(-1);
}
setbuf(stdin, NULL);
scanf("%s", buf);
//read(0, buf, 100);
printf("child: \n\t%s", buf);
return 0;
}
sleep(1);
close(fd[0]);
if(dup2(fd[1], 1) < 0)
{
perror("parent - dup2 error");
exit(-1);
}
setbuf(stdout, NULL);
printf("123");
wait(NULL);
return 0;
}

Answer 1

Problem is that to scanf to terminates its reading, necessitate that the data is ended in some way.
You are reading a string, but how scanf can know that there is nothing after 123? So it is waiting for something, either the end of communication, space, newlines, whatever can tell it that a string is fully available.
Try adding a simple spacing, a new line in the writing part:
printf("123 ");
or
printf("123\n");
Note that as is, you can not use the closing solution, just because you forgot to close the original descriptors after the dup.
Rewrite the code like this:
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
int main(void) {
int fd[2];
int cpid;
pipe(fd);
if((cpid = fork()) < 0) {
perror("fork error");
exit(-1);
}
else if(cpid == 0) {
sleep(1);
char buf[100] = {0};
close(fd[1]);
if(dup2(fd[0], 0) < 0) {
perror("parent - dup2 error");
exit(-1);
}
close(fd[0]); // close not useful original desriptor
setbuf(stdin, NULL);
scanf("%s", buf);
printf("child: \n\t%s", buf);
return 0;
}
sleep(1);
close(fd[0]);
if(dup2(fd[1], 1) < 0) {
perror("parent - dup2 error");
exit(-1);
}
close(fd[1]); // close not useful original desriptor
setbuf(stdout, NULL);
printf("123");
close(1); // close the "connexion" (ie the last open for writing descriptor on the pipe)
wait(NULL);
return 0;
}