在 C 語言中使用 thread_local 變數
本文將介紹幾種如何在 C 語言中使用 thread_local 變數的方法。
使用 _Thread_local 型別宣告帶有執行緒儲存持續時間的變數
C 語言為不同的儲存類別定義了多個關鍵字，例如 auto，static，register，extern。從 C11 標準的規範開始，新增了 _Thread_local 指定符。_Thread_local 儲存持續時間始於執行緒建立時，並終止於執行緒終止。啟動執行緒時，將初始化儲存在 _Thread_local 物件中的值，並線上程終止時對其進行清理。通常，執行緒區域性物件是避免共享資源中競爭條件的另一種選擇。也就是說，我們隱式地將執行緒之間的資料分開，因為 _Thread_local 合格的物件為每個執行緒具有單獨的例項。
#include <stdio.h>
#include <stdlib.h>
#include <threads.h>
#include <stdatomic.h>
#ifndef NUM_THREADS
#define NUM_THREADS 4
#endif
_Thread_local int counter = 0;
enum {MAX_ITER = 10000};
void *incrementCounter(void *thr_id) {
long tid;
tid = (long)thr_id;
printf("thread %ld started incrementing ID - %lu\n", tid, thrd_current());
for (int i = 0; i < MAX_ITER; ++i) {
counter += 1;
}
return (void *)counter;
}
int main(int argc, char const *argv[]) {
thrd_t threads[NUM_THREADS];
int rc, sum = 0;
for (int i = 0; i < NUM_THREADS; ++i) {
rc = thrd_create(&threads[i], (thrd_start_t) incrementCounter, (void *)i);
if (rc == thrd_error) {
printf("ERORR; thrd_create() call failed\n");
exit(EXIT_FAILURE);
}
}
int retval;
for (int i = 0; i < NUM_THREADS; ++i) {
thrd_join(threads[i], &retval);
sum += retval;
}
printf("count = %d\n", sum);
thrd_exit(EXIT_SUCCESS);
}
輸出：
thread 1 started incrementing ID - 140162648991488
thread 0 started incrementing ID - 140162657384192
thread 2 started incrementing ID - 140162640598784
thread 3 started incrementing ID - 140162632206080
count = 40000
使用 thread_local 型別宣告具有執行緒儲存持續時間的變數
另外，C 語言定義了一個巨集表示式 thread_local，以將說明符表示為 _Thread_local。注意，應該在檔案作用域中宣告 thread_local 變數，以使其對所有執行緒可見，否則使用者也可以顯式新增 static 說明符，以將其作用域擴充套件到檔案級。由於執行緒應將 thread_local 物件的值傳達給主執行緒，因此需要修改程式結構。應該修改實現簡單計數器程式的先前示例程式碼，以將遞增的計數器值傳遞迴呼叫執行緒。因此，我們需要利用 thrd_join 函式及其第二個引數來儲存執行緒例程的返回值。請注意，所有遞增的值都應在主執行緒中求和，這也負責將最終結果列印到 stdout。
#include <stdio.h>
#include <stdlib.h>
#include <threads.h>
#include <stdatomic.h>
#ifndef NUM_THREADS
#define NUM_THREADS 4
#endif
thread_local int counter = 0;
enum {MAX_ITER = 10000};
void *incrementCounter(void *thr_id) {
long tid;
tid = (long)thr_id;
printf("thread %ld started incrementing ID - %lu\n", tid, thrd_current());
for (int i = 0; i < MAX_ITER; ++i) {
counter += 1;
}
return (void *)counter;
}
int main(int argc, char const *argv[]) {
thrd_t threads[NUM_THREADS];
int rc, sum = 0;
for (int i = 0; i < NUM_THREADS; ++i) {
rc = thrd_create(&threads[i], (thrd_start_t) incrementCounter, (void *)i);
if (rc == thrd_error) {
printf("ERORR; thrd_create() call failed\n");
exit(EXIT_FAILURE);
}
}
int retval;
for (int i = 0; i < NUM_THREADS; ++i) {
thrd_join(threads[i], &retval);
sum += retval;
}
printf("count = %d\n", sum);
thrd_exit(EXIT_SUCCESS);
}
輸出：
thread 1 started incrementing ID - 140162648991488
thread 2 started incrementing ID - 140162640598784
thread 0 started incrementing ID - 140162657384192
thread 3 started incrementing ID - 140162632206080
count = 40000
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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