linux命令由popen这基于C code执行 [英] linux command executing by popen on C code

问题描述

我有以下的code我指的<一个线程href=\"http://stackoverflow.com/questions/671461/how-can-i-execute-external-commands-in-c-linux\">here使用的popen 函数

I have the code below that I refer the thread on here to use the popen function

int main(int argc,char *argv[]){    
    FILE* file = popen("ntpdate", "r");
    char buffer[100];
    fscanf(file, "%100s", buffer);
    pclose(file);
    printf("buffer is :%s\n", buffer);
    return 0;
}

它输出：

21 Apr 03:03:03 ntpdate[4393]: no server can be used, exiting
buffer is:

为什么的printf 不输出什么？如果我使用 LS 作为一个命令，然后printf的输出LS输出。我究竟做错了的ntpdate 执行？

why printf does not output anything? If I use ls as a command, then printf outputs the ls output. what am I doing wrong ntpdate executing?

如果我执行以下（指的网页上的code ）

If I execute the code below (referring the webpage)

#define COMMAND_LEN 8
#define DATA_SIZE 512

int main(int argc,char *argv[]){


    FILE *pf;
       char command[COMMAND_LEN];
       char data[DATA_SIZE];

       // Execute a process listing
       sprintf(command, "ntpdate");

       // Setup our pipe for reading and execute our command.
       pf = popen(command,"r");

       if(!pf){
         fprintf(stderr, "Could not open pipe for output.\n");
         return;
       }

       // Grab data from process execution
       fgets(data, DATA_SIZE , pf);

       // Print grabbed data to the screen.
       fprintf(stdout, "-%s-\n",data);

       if (pclose(pf) != 0)
           fprintf(stderr," Error: Failed to close command stream \n");

       return 0;
}

我得到

21 Apr 03:15:45 ntpdate[5334]: no servers can be used, exiting
-�2}�����"|�4#|�-
 Error: Failed to close command stream 

什么是在codeS上面的过错？

what are wrongs on the codes above?

推荐答案

由于输出将会标准错误您需要重定向标准错误像这样：

Since the output is going to stderr you need to redirect stderr like so:

FILE* file = popen("ntpdate 2>&1", "r");

这将标准错误重定向到标准输出键，所以你会看到两个输出。第二个问题的fscanf 将在第一次太空停止，所以你可以用替换与fgets ：

this will redirect stderr to stdout and so you will see output from both. Second issue fscanf will stop at the first space so you can replace with fgets:

fgets(buffer, 100, file);

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