Strange output using pipes for thread communication - c

Question

I have three threads - the first one reads a string, the second counts characters, and the third displays it. I'm using pipes for communication.
However, after running it nothing happens, and when I type in something, let's say "asd", I get:
asd
asd
Enter the message: Enter the message:
or
asd
asd
Enter the message:
What's wrong?
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
#include <linux/stat.h>
#include <pthread.h>
#include <string.h>
int first[2];
int second[2];
void *input(void *ptr)
{
char str[100];
int length;
while(1)
{
printf("Enter the message: ");
length = read(STDIN_FILENO, str, sizeof(str));
if(length <= 0)
{
if(length == -1)
perror("read");
close(first[1]);
exit(2);
}
if(write(first[1], str, length) != length)
{
perror("write");
exit(2);
}
}
}
void *countChars(void *ptr)
{
char str[100];
int length, count = 0;
while(1)
{
length = read(first[0], str, sizeof(str));
if(length <= 0)
{
if(length == -1)
perror("read");
close(first[0]);
close(second[1]);
exit(2);
}
if(write(STDOUT_FILENO, str, length) != length)
{
perror("write");
exit(2);
}
while(str[count] != '\n') count++;
write(second[1], &count, sizeof(count));
count = 0;
}
}
void *output(void *ptr)
{
int length, count = 0;
while(1)
{
length = read(second[0], &count, sizeof(count));
if(length <= sizeof(count))
{
close(second[0]);
exit(2);
}
printf("Number of characters: %d\n", count);
}
}
int main()
{
pthread_t t1, t2, t3;
if(pipe(first) == -1)
{
printf("First pipe error");
exit(1);
}
if(pipe(second) == -1)
{
printf("Second pipe error");
exit(1);
}
pthread_create(&t1, NULL, input, NULL);
pthread_create(&t2, NULL, countChars, NULL);
pthread_create(&t3, NULL, output, NULL);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
pthread_join(t3, NULL);
return 0;
}

Answer 1

You have a logic problem in your code. In output:
if (length < sizeof (count)) { // not <=
length will always equal sizeof (count) on successful write of an integer.
Also, wrapping everything function in while (1) {...} is not the safest. Remove the while (1) loops and replace them with a return at the end of the function. i.e. return ptr; For example:
void *
output (void *ptr) {
int length, count = 0;
printf ("\noutput:\n\n");
// while (1) {
length = read (second[0], &count, sizeof (count));
printf ("count: %d\n", count);
if (length < sizeof (count)) { // not <=
printf ("closing second[0] and exiting\n");
close (second[0]);
exit (2);
}
printf ("Number of characters: %d\n", count);
// }
return ptr;
}

Answer 2

That's because printf only writes content to buffer. The content will be actually sent to stdout once it receives a line feed ('\n') or fflush(stdout) is called.
You can try to add flush(stdout); next to the printf("Enter the message: ");, and you should see what you expect.