父子進程計時

Question

我寫了這個小程序來幫助我更好地理解流程：

#include <stdio.h>
#include <unistd.h>

int main()
{
    int pid;
    int i;
    if ((pid = fork()) == 0)
    {
        for (i = 0; i < 100; i++)
        {
            printf("child, pid = %d, i = %d\n", getpid(), i);
        }
    }
    else if (pid > 0)
    {
        for (i = 0; i > -100; i--)
        {
            printf("parent, pid = %d, i = %d\n", getpid(), i);
        }
    }
    else
    {
        perror("failed fork!\n");
        exit(1);
    }
    return 0;
}

當我實際運行程序時，我驚訝地看到輸出。 父進程首先開始，這也可能反過來發生，對嗎？ 在我的機器中，它似乎總是選擇父級。 但令我驚訝的是，一旦開始，該過程無法完成輸出所有 i 值。 為什么是這樣？ 給一個進程的時間那么小嗎？ 然后更奇怪的是，父母和孩子開始輪流使用 printfs，每人只輸出一行，直到父母完成循環，因為它有一個先機，然后孩子必須打印其余的 i 值。 再次如何解釋這種行為？

這是我得到的輸出：

parent, pid = 3246, i = 0
parent, pid = 3246, i = -1
parent, pid = 3246, i = -2
parent, pid = 3246, i = -3
parent, pid = 3246, i = -4
parent, pid = 3246, i = -5
parent, pid = 3246, i = -6
parent, pid = 3246, i = -7
parent, pid = 3246, i = -8
parent, pid = 3246, i = -9
parent, pid = 3246, i = -10
parent, pid = 3246, i = -11
parent, pid = 3246, i = -12
parent, pid = 3246, i = -13
parent, pid = 3246, i = -14
parent, pid = 3246, i = -15
parent, pid = 3246, i = -16
parent, pid = 3246, i = -17
parent, pid = 3246, i = -18
parent, pid = 3246, i = -19
parent, pid = 3246, i = -20
parent, pid = 3246, i = -21
parent, pid = 3246, i = -22
parent, pid = 3246, i = -23
parent, pid = 3246, i = -24
parent, pid = 3246, i = -25
parent, pid = 3246, i = -26
parent, pid = 3246, i = -27
parent, pid = 3246, i = -28
parent, pid = 3246, i = -29
parent, pid = 3246, i = -30
parent, pid = 3246, i = -31
parent, pid = 3246, i = -32
parent, pid = 3246, i = -33
parent, pid = 3246, i = -34
parent, pid = 3246, i = -35
parent, pid = 3246, i = -36
parent, pid = 3246, i = -37
parent, pid = 3246, i = -38
parent, pid = 3246, i = -39
parent, pid = 3246, i = -40
parent, pid = 3246, i = -41
parent, pid = 3246, i = -42
parent, pid = 3246, i = -43
parent, pid = 3246, i = -44
parent, pid = 3246, i = -45
parent, pid = 3246, i = -46
parent, pid = 3246, i = -47
parent, pid = 3246, i = -48
parent, pid = 3246, i = -49
parent, pid = 3246, i = -50
child, pid = 3247, i = 0
parent, pid = 3246, i = -51
parent, pid = 3246, i = -52
child, pid = 3247, i = 1
parent, pid = 3246, i = -53
child, pid = 3247, i = 2
parent, pid = 3246, i = -54
child, pid = 3247, i = 3
parent, pid = 3246, i = -55
child, pid = 3247, i = 4
parent, pid = 3246, i = -56
child, pid = 3247, i = 5
parent, pid = 3246, i = -57
child, pid = 3247, i = 6
parent, pid = 3246, i = -58
child, pid = 3247, i = 7
parent, pid = 3246, i = -59
child, pid = 3247, i = 8
parent, pid = 3246, i = -60
child, pid = 3247, i = 9
parent, pid = 3246, i = -61
child, pid = 3247, i = 10
parent, pid = 3246, i = -62
child, pid = 3247, i = 11
parent, pid = 3246, i = -63
child, pid = 3247, i = 12
parent, pid = 3246, i = -64
child, pid = 3247, i = 13
parent, pid = 3246, i = -65
child, pid = 3247, i = 14
parent, pid = 3246, i = -66
child, pid = 3247, i = 15
parent, pid = 3246, i = -67
child, pid = 3247, i = 16
parent, pid = 3246, i = -68
child, pid = 3247, i = 17
parent, pid = 3246, i = -69
child, pid = 3247, i = 18
parent, pid = 3246, i = -70
child, pid = 3247, i = 19
parent, pid = 3246, i = -71
child, pid = 3247, i = 20
parent, pid = 3246, i = -72
child, pid = 3247, i = 21
parent, pid = 3246, i = -73
child, pid = 3247, i = 22
parent, pid = 3246, i = -74
child, pid = 3247, i = 23
parent, pid = 3246, i = -75
child, pid = 3247, i = 24
parent, pid = 3246, i = -76
child, pid = 3247, i = 25
parent, pid = 3246, i = -77
child, pid = 3247, i = 26
parent, pid = 3246, i = -78
child, pid = 3247, i = 27
parent, pid = 3246, i = -79
child, pid = 3247, i = 28
parent, pid = 3246, i = -80
child, pid = 3247, i = 29
parent, pid = 3246, i = -81
child, pid = 3247, i = 30
parent, pid = 3246, i = -82
child, pid = 3247, i = 31
parent, pid = 3246, i = -83
child, pid = 3247, i = 32
parent, pid = 3246, i = -84
child, pid = 3247, i = 33
parent, pid = 3246, i = -85
child, pid = 3247, i = 34
parent, pid = 3246, i = -86
child, pid = 3247, i = 35
parent, pid = 3246, i = -87
child, pid = 3247, i = 36
parent, pid = 3246, i = -88
child, pid = 3247, i = 37
parent, pid = 3246, i = -89
child, pid = 3247, i = 38
parent, pid = 3246, i = -90
child, pid = 3247, i = 39
parent, pid = 3246, i = -91
child, pid = 3247, i = 40
parent, pid = 3246, i = -92
child, pid = 3247, i = 41
parent, pid = 3246, i = -93
child, pid = 3247, i = 42
parent, pid = 3246, i = -94
child, pid = 3247, i = 43
parent, pid = 3246, i = -95
child, pid = 3247, i = 44
parent, pid = 3246, i = -96
child, pid = 3247, i = 45
parent, pid = 3246, i = -97
child, pid = 3247, i = 46
parent, pid = 3246, i = -98
child, pid = 3247, i = 47
parent, pid = 3246, i = -99
child, pid = 3247, i = 48
child, pid = 3247, i = 49
child, pid = 3247, i = 50
child, pid = 3247, i = 51
child, pid = 3247, i = 52
child, pid = 3247, i = 53
child, pid = 3247, i = 54
child, pid = 3247, i = 55
child, pid = 3247, i = 56
child, pid = 3247, i = 57
child, pid = 3247, i = 58
child, pid = 3247, i = 59
child, pid = 3247, i = 60
child, pid = 3247, i = 61
child, pid = 3247, i = 62
child, pid = 3247, i = 63
child, pid = 3247, i = 64
child, pid = 3247, i = 65
child, pid = 3247, i = 66
child, pid = 3247, i = 67
child, pid = 3247, i = 68
child, pid = 3247, i = 69
child, pid = 3247, i = 70
child, pid = 3247, i = 71
child, pid = 3247, i = 72
child, pid = 3247, i = 73
child, pid = 3247, i = 74
child, pid = 3247, i = 75
child, pid = 3247, i = 76
child, pid = 3247, i = 77
child, pid = 3247, i = 78
child, pid = 3247, i = 79
child, pid = 3247, i = 80
child, pid = 3247, i = 81
child, pid = 3247, i = 82
child, pid = 3247, i = 83
child, pid = 3247, i = 84
child, pid = 3247, i = 85
child, pid = 3247, i = 86
child, pid = 3247, i = 87
child, pid = 3247, i = 88
child, pid = 3247, i = 89
child, pid = 3247, i = 90
child, pid = 3247, i = 91
child, pid = 3247, i = 92
child, pid = 3247, i = 93
child, pid = 3247, i = 94
child, pid = 3247, i = 95
child, pid = 3247, i = 96
child, pid = 3247, i = 97
child, pid = 3247, i = 98
child, pid = 3247, i = 99

Answer 1

父進程首先啟動。

這里的重點是您正在運行多個“並行”運行的進程。 現在，在現實生活中，它們可能會或可能不會並行運行。 在只有一個內核的舊處理器中，並行執行意味着操作系統在可運行進程周圍快速循環，並從每個進程運行一點。 這樣的“位”稱為時間片，通常在10ms左右。

如今隨着多核 cpu 時代的到來，您可能會看到真正並行運行的事物。

所以你可以看到：

• 首先只有父進程運行一個時間片
• 然后父母和孩子都運行一個時間片
• 最后只有 clild 運行。

進程之間的實際調度（使用一個或多個內核，何時在進程之間切換）取決於您使用的操作系統。

Answer 2

您應該注意到，您的進程執行了數十萬條指令代碼，到目前為止，其中大部分都在保護 stdout 流不被多個線程訪問的鎖中。 您實際輸入並顯示在您的問題中的代碼幾乎為零。

進程的行為 99.9999% 完全由 stdout 上的鎖管理，因此“父和子開始輪流”-完全可以預料到它們在鎖上循環。