看過了高階的popen呼叫之後，我們來了解一下底層的pipe函式。popen函式用過啟動一個shell來解釋請求的命令。而pipe不需要啟動shell來解釋。

#include<unistd.h>
int pipe(int file_descriptor[2]);

pipe函式的引數是一個由兩個整數型別的檔案描述符組成的資料的指標。該函式是資料中填上兩個新的檔案描述符後返回0.如果失敗則返回-1並設定

errno來表明失敗的原因。這些錯誤有

1.EMFILE：程序使用的檔案描述符過多

2.ENFILE：系統的檔案表已滿

3.EFAULT：檔案描述符無效

兩個返回的檔案描述符以一種特殊的方式連線起來。寫到file_descriptor[1]的所有資料都可以從file_descriptor[0]讀回來。資料基於先進先出的原則

（通常簡寫為FIFO）程序處理，這意味著如果你把位元組1，2，3寫入到dile_descriptor[1]，從file_descriptor[0]讀出來的資料是1，2，3.

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

int main()
{
    int data_processed;
    int file_pipes[2];
    const char some_data[] = "123";
    char buffer[BUFSIZ + 1];

    memset(buffer, '\0', sizeof(buffer));

    if (pipe(file_pipes) == 0) {
        data_processed = write(file_pipes[1], some_data, strlen(some_data));
        printf("Wrote %d bytes\n", data_processed);
        data_processed = read(file_pipes[0], buffer, BUFSIZ);
        printf("Read %d bytes: %s\n", data_processed, buffer);
        exit(EXIT_SUCCESS);
    }
    exit(EXIT_FAILURE);
}
 

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main()
{
    int data_processed;
    int file_pipes[2];
    const char some_data[] = "123";
    char buffer[BUFSIZ + 1];
    pid_t fork_result;

    memset(buffer, '\0', sizeof(buffer));

    if (pipe(file_pipes) == 0) {
        fork_result = fork();
        if (fork_result == -1) {
            fprintf(stderr, "Fork failure");
            exit(EXIT_FAILURE);
        }
// We've made sure the fork worked, so if fork_result equals zero, we're in the child process.
        if (fork_result == 0) {
            data_processed = read(file_pipes[0], buffer, BUFSIZ);
            printf("Read %d bytes: %s\n", data_processed, buffer);
            exit(EXIT_SUCCESS);
        }
// Otherwise, we must be the parent process.

        else {
            data_processed = write(file_pipes[1], some_data,
                                   strlen(some_data));
            printf("Wrote %d bytes\n", data_processed);
        }
    }
    exit(EXIT_SUCCESS);
}
 

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

int main()
{
    int data_processed;
    int file_pipes[2];
    const char some_data[] = "123";
    char buffer[BUFSIZ + 1];
    pid_t fork_result;

    memset(buffer, '\0', sizeof(buffer));

    if (pipe(file_pipes) == 0) {
        fork_result = fork();
        if (fork_result == (pid_t)-1) {
            fprintf(stderr, "Fork failure");
            exit(EXIT_FAILURE);
        }

        if (fork_result == 0) {
            sprintf(buffer, "%d", file_pipes[0]);
            (void)execl("pipe4", "pipe4", buffer, (char *)0);
            exit(EXIT_FAILURE);
        }
        else {
            data_processed = write(file_pipes[1], some_data,
                                   strlen(some_data));
            printf("%d - wrote %d bytes\n", getpid(), data_processed);
        }
    }
    exit(EXIT_SUCCESS);
}

// The 'consumer' program, pipe4.c, that reads the data is much simpler.
//pipe4.c
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
    int data_processed;
    char buffer[BUFSIZ + 1];
    int file_descriptor;

    memset(buffer, '\0', sizeof(buffer));
    sscanf(argv[1], "%d", &file_descriptor);
    data_processed = read(file_descriptor, buffer, BUFSIZ);

    printf("%d - read %d bytes: %s\n", getpid(), data_processed, buffer);
    exit(EXIT_SUCCESS);
}