Question

运行以下代码时，我收到“grep :(标准输入）：错误的文件描述符”。知道为什么吗？该程序应该打印出您的主目录。谢谢！

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

int main(void) {
pid_t childpid;
int fd[2];

if(pipe(fd) == -1) { /* setup a pipe */
    perror("Failed to setup pipeline");
    return 1;
}
if((childpid = fork()) == -1){ /* fork a child */
    perror("Failed to fork a child");
    return 1;
}
if(childpid == 0){ /* env is the child */

    if(dup2(fd[1],STDOUT_FILENO)==-1)
        perror("Failed to redirect stdout of env");
    else if(close(fd[0] == -1)) /* close unused file descriptor */
        perror("Failed to close extra pipe descriptors on env");
    else {
        execl("/usr/bin/env", "env", NULL); /* execute env */
        perror("Failed to exec env");
    }
    return 1;
}
if(dup2(fd[0],STDIN_FILENO)==-1) 
/*grep is the parent*/
    perror("Failed to redirect stdin of grep");
else if(close(fd[1]==-1))
    perror("Failed to close extra pipe file descriptors on grep");
else {
    execl("/bin/grep", "grep", "HOME", NULL); /*execute "grep HOME"*/
    perror("Failed to exec grep");
}
return 1;
}

Answer 1

错误在第2行：

else if(close(fd[0] == -1))
...
else if(close(fd[1]==-1))

应该是：

else if(close(fd[0]) == -1)
...
else if(close(fd[1]) == -1)

否则，无需执行外部程序即可实现HOME env：

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

int main(int argc, char **argv, char** envp)
{
    char** env;
    for (env = envp; *env != 0; env++)
    {
        char* thisEnv = *env;
        if (strncmp(thisEnv, "HOME=", 5) == 0)
        {
            printf("%s\n", thisEnv + 5);
            break;
        }
    }
    return(0);
}

C程序打印主目录显示“grep :(标准输入）：错误的文件描述符”

1 个答案: