使用POSIX線程的文件操作

Question

我正在學習多線程的概念，並且遇到了使用信號量互斥鎖的問題。

這裏是我的代碼片段：

void *thread1_funct(void * myfileptr) 
{ 
static int count; 
printf("\nThread1 ID:%u\n",(unsigned int) pthread_self()); 
printf("\nThread1 function, file pointer received:0x%x\n", (FILE*)myfileptr); 

for (;;) 
{ 
count++; 
sem_wait(&mutex); 
fprintf(myfileptr, "%d\n", count); 
sem_post(&mutex); 
} 

return NULL; 
} 


void *thread2_funct(void *myfileptr) 
{ 
static int count=0; 

printf("\nThread2 ID:%u\n",(unsigned int) pthread_self()); 
printf("\nThread2 function, file pointer received:0x%x\n", (FILE*)myfileptr); 

for (;;) 
{sem_wait(&mutex); 
fscanf(myfileptr, "%d\n", &count); 
printf("\n......%d......\n", count); 
sem_post(&mutex); 
} 

return NULL; 
} 

兩個線程我已經建立。一個將dfata寫入一個文件，另一個將讀取最新的數據。

這是我的主要方法：

int main(int argc, char **argv) 
{ 
FILE *fileptr = fopen(*(argv+1), "a+"); 

sem_init(&mutex, 0x00, 0x01); 

if ((thread1_ret = pthread_create(&thread1, NULL, thread1_funct, (void*)fileptr)) == 0) 
printf("\nThread1 created successfully....\n"); 
else 
printf("\nFailed to create Thread1\n"); 

if ((thread2_ret = pthread_create(&thread2, NULL, thread2_funct, (void*)fileptr)) == 0) 
printf("\nThread2 created successfully....\n"); 
else 
printf("\nFailed to create Thread2\n"); 

pthread_join(thread1, NULL);         
pthread_join(thread2, NULL);         

fclose(fileptr); 

sem_destroy(&mutex); 

pthread_exit(NULL); 
return 0; 
} 

預期輸出是： ........ 1 ........ ........ 2 ........ ........ 3 ........

等......直到程序手動中斷。

但我的輸出全部爲0： ........ 0 ....... ........ 0 ....... .... .... 0 ....... ........ 0 .......

等等....

請幫助我。我哪裏錯了？

Answer 1

線程1寫入文件，並將文件指針前進 - 文件末尾。線程2從指向文件末尾的文件指針中讀取，所以你什麼也得不到。

您可以使用fseek，或者在線程2中回滾以獲取數據。