重定向输入与输出

大多数的IDE环境都具备这样一种功能，加载编译器编译好程序并执行，执行结果显示于IDE环境中的结果信息窗口，那么它是如何捕获程序运行结果的呢？其实简单地用dup/dup2重定位输出并通过管道获取该输出就可以达到这种效果。

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>

int
main(int argc, char **argv)
{
    int fd[2];
    pid_t childpid;

    if (pipe(fd) == -1)
    {
        printf("%s: %d -- Create pipe error: %s\n", __FILE__, __LINE__, strerror(errno));
        exit(errno);
    }

    childpid = fork();

    if (childpid == -1)
    {
        printf("%s: %d -- fork error: %s\n", __FILE__, __LINE__, strerror(errno));
        exit(errno);
    }

    if (childpid == 0)
    {
        close(fd[0]);

        if (dup2(fd[1], STDOUT_FILENO) == -1)
        {
            printf("%s: %d -- dup2 error: %s\n", __FILE__, __LINE__, strerror(errno));
            exit(errno);
        }

        if (execlp("ls", "ls", "/", NULL) == -1)
        {
            printf("%s: %d -- execlp error: %s\n", __FILE__, __LINE__, strerror(errno));
            exit(errno);
        }

        exit(0);
    }
    else
    {
        char buff[4096];

        close(fd[1]);
        memset(buff, sizeof(buff), 0);

        printf("read %d bytes\n", read(fd[0], buff, sizeof(buff)));
        printf("output: %s\n", buff);

        exit(0);
    }
}

以下代码则相反，重定向输入：

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>

int
main(int argc, char **argv)
{
    int fd[2];
    pid_t childpid;

    if (pipe(fd) == -1)
    {
        printf("%s: %d -- Create pipe error: %s\n", __FILE__, __LINE__, strerror(errno));
        exit(errno);
    }

    childpid = fork();

    if (childpid == -1)
    {
        printf("%s: %d -- fork error: %s\n", __FILE__, __LINE__, strerror(errno));
        exit(errno);
    }

    if (childpid == 0)
    {
        close(fd[1]);
        if (dup2(fd[0], STDIN_FILENO) == -1)
        {
            printf("%s: %d -- dup2 error: %s\n", __FILE__, __LINE__, strerror(errno));
            exit(errno);
        }

        if (execlp("xargs", "xargs", "ls", NULL) == -1)
        {
            printf("%s: %d -- execlp error: %s\n", __FILE__, __LINE__, strerror(errno));
            exit(errno);
        }

        exit(0);
    }
    else
    {
        int status;

        close(fd[0]);
        write(fd[1], "/", 1);

        printf("waitpid result: %d\n", waitpid(childpid, &status, WNOHANG));
        printf("status value: %d\n", status);

        if (WIFEXITED(status))
            printf("child process was terminated normally by calling exit or _exit\n");
        else if (WEXITSTATUS(status))
            printf("returns the exit status of the child by exit or _exit\n");
        else if (WIFSIGNALED(status))
            printf("child process was terminated by a signal\n");
        else if (WTERMSIG(status))
            printf("returns the number of the signal that caused the child process to terminate\n");
        else if (WCOREDUMP(status))
            printf("child produced a core dump\n");
        else if (WIFSTOPPED(status))
            printf("child process was stopped by delivery of a signal\n");
        else if (WSTOPSIG(status))
            printf("retuns the number of the signal which caused the child to stop\n");
        else if (WIFCONTINUED(status))
            printf("child process was resumed by delivery of SIGCONT\n");
        exit(0);
    }
}

原文转自：http://www.ltesting.net