在 C 語言中使用 pthread_join 函式
本文將介紹幾種如何在 C 語言中使用 pthread_join 函式的方法。
使用 pthread_join 函式等待執行緒終止
程式使用 pthread_create 函式建立執行緒，通常，它等待它們通過 pthread_join 函式終止。pthread_join 僅接受兩個引數：用於指定等待執行緒的執行緒 ID 和指向可以儲存指定執行緒的退出狀態的 void*的指標。如果使用者不想檢索等待的執行緒的退出程式碼，則應將 NULL 值作為第二個引數傳遞。在下面的示例中，我們演示了建立 8 個執行緒並在每個執行緒中執行 printHello 函式的程式。然後，呼叫執行緒在迴圈中等待帶有 pthread_join 呼叫的每個執行緒。注意，我們還將執行緒的退出狀態程式碼儲存在 retval 變數中，並通過將其強制轉換為 int 來列印其值。請注意，如果執行緒被取消，則將 PTHREAD_CANCELED 值放置在 retval 地址中。
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <errno.h>
#ifndef NUM_THREADS
#define NUM_THREADS 8
#endif
void *printHello(void *threadid) {
long tid;
tid = (long)threadid;
printf("Hello from thread %ld, pthread ID - %lu\n", tid, pthread_self());
return NULL;
}
int main(int argc, char const *argv[]) {
pthread_t threads[NUM_THREADS];
int rc;
long t;
for (t = 0; t < NUM_THREADS; t++) {
rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
if (rc) {
printf("ERORR; return code from pthread_create() is %d\n", rc);
exit(EXIT_FAILURE);
}
}
int ret;
for (t = 0; t < NUM_THREADS; t++) {
void *retval;
ret = pthread_join(threads[t], &retval);
if (retval == PTHREAD_CANCELED)
printf("The thread was canceled - ");
else
printf("Returned value %d - ", (int)retval);
}
pthread_exit(NULL);
}
輸出：
Hello from thread 0, pthread ID - 140716669929216
Hello from thread 1, pthread ID - 140716661536512
Hello from thread 2, pthread ID - 140716653143808
Hello from thread 3, pthread ID - 140716644751104
Hello from thread 5, pthread ID - 140716627965696
Hello from thread 4, pthread ID - 140716636358400
Hello from thread 6, pthread ID - 140716550387456
Hello from thread 7, pthread ID - 140716541994752
使用 pthread_join 函式返回值檢查錯誤
與設定 errno 全域性變數的函式相比，pthread_join 函式返回的整數值還指示不同的錯誤程式碼。如果呼叫成功，則返回值為 0，這可以確保給定執行緒已終止。如果返回的整數等於 EDEADLK，則報告檢測到死鎖。如果返回 EINVAL 值，則給定執行緒不可連線，如果該值等於 ESRCH，則表明找不到給定執行緒 ID。在這種情況下，我們實現 switch 語句以檢查每種情況，並將相應的訊息列印到 stdout。
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <errno.h>
#ifndef NUM_THREADS
#define NUM_THREADS 8
#endif
void *printHello(void *threadid) {
long tid;
tid = (long)threadid;
printf("Hello from thread %ld, pthread ID - %lu\n", tid, pthread_self());
return NULL;
}
int main(int argc, char const *argv[]) {
pthread_t threads[NUM_THREADS];
int rc;
long t;
for (t = 0; t < NUM_THREADS; t++) {
rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
if (rc) {
printf("ERORR; return code from pthread_create() is %d\n", rc);
exit(EXIT_FAILURE);
}
}
int ret;
for (t = 0; t < NUM_THREADS; t++) {
void *retval;
ret = pthread_join(threads[t], &retval);
if (retval == PTHREAD_CANCELED)
printf("The thread was canceled - ");
else
printf("Returned value %d - ", (int)retval);
switch (ret) {
case 0:
printf("The thread joined successfully\n");
break;
case EDEADLK:
printf("Deadlock detected\n");
break;
case EINVAL:
printf("The thread is not joinable\n");
break;
case ESRCH:
printf("No thread with given ID is found\n");
break;
default:
printf("Error occurred when joining the thread\n");
}
}
pthread_exit(NULL);
}
輸出：
Hello from thread 0, pthread ID - 140082577512192
Hello from thread 1, pthread ID - 140082569119488
Hello from thread 3, pthread ID - 140082552334080
Hello from thread 5, pthread ID - 140082535548672
Hello from thread 6, pthread ID - 140082527155968
Returned value 0 - The thread joined successfully
Hello from thread 4, pthread ID - 140082543941376
Hello from thread 2, pthread ID - 140082560726784
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Hello from thread 7, pthread ID - 140082518763264
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.
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