how to explain the thread3 from the Bengining Linux Programming 3rd edition.

baumann.Pan_at_gmail.com
Date: 05/17/05 

Date: 17 May 2005 05:41:07 -0700

hi all,
in this book charpter 12, Semaphore, author listed a thread3.c , after
that, he changes some about the thread3.c , below is the codes after
changed.

but I don't know why the output is :
nput some text. Enter 'end' to finish
asdfasdfa
You input 9 characters:asdfasdfa

FAST
You input 8 characters:Wheeee...
You input 8 characters:Wheeee...
You input 8 characters:Wheeee...
end

Waiting for thread to finish...
Thread joined

after input FAST before input end
why there are 3 lines output like that?

the author gives an explaination to that. but I don't understand how
can semaphore get increased since work_area value has changed, sem_post
would not get executed?

below is the code get that output.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <semaphore.h>
void *thread_function(void *arg);
sem_t bin_sem;

#define WORK_SIZE 1024
char work_area[WORK_SIZE];
int main() {
int res;
pthread_t a_thread;
void *thread_result;
res = sem_init(&bin_sem, 0, 0);
if (res != 0) {
perror("Semaphore initialization failed");
exit(EXIT_FAILURE);
}
res = pthread_create(&a_thread, NULL, thread_function, NULL);
if (res != 0) {
perror("Thread creation failed");
exit(EXIT_FAILURE);
}
printf("Input some text. Enter 'end' to finish\n");
while(strncmp("end", work_area, 3) != 0) {
fgets(work_area, WORK_SIZE, stdin);
sem_post(&bin_sem);
}
printf("\nWaiting for thread to finish...\n");
res = pthread_join(a_thread, &thread_result);
if (res != 0) {
perror("Thread join failed");
exit(EXIT_FAILURE);
}
printf("Thread joined\n");
sem_destroy(&bin_sem);
exit(EXIT_SUCCESS);
}
void *thread_function(void *arg) {
sem_wait(&bin_sem);
while(strncmp("end", work_area, 3) != 0) {
printf("You input %d characters\n", strlen(work_area) -1);
sem_wait(&bin_sem);
}
pthread_exit(NULL);
}

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