C: popen() 函数执行的 Linux 命令不显示结果
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C: Linux command executed by popen() function not showing results
提问by sven
I have the code below that I refer the thread on hereto use the popenfunction
我有下面的代码，我在这里引用线程来使用该popen函数
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;
}
It outputs:
它输出：
21 Apr 03:03:03 ntpdate[4393]: no server can be used, exiting
buffer is:
why printfdoes not output anything? If I use lsas a command, then printf outputs the ls output. what am I doing wrong ntpdateexecuting?
为什么printf不输出任何东西？如果我ls用作命令，则 printf 输出 ls 输出。我在做什么错ntpdate执行？
If I execute the code below (referring the webpage)
如果我执行下面的代码（参考网页）
#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;
}
I get
我得到
21 Apr 03:15:45 ntpdate[5334]: no servers can be used, exiting
-?2}?????"|?4#|?-
Error: Failed to close command stream
what are wrongs on the codes above?
上面的代码有什么问题？
采纳答案by Shafik Yaghmour
Since the output is going to stderryou need to redirect stderrlike so:
由于输出将要发送给stderr您，stderr因此您需要像这样重定向：
FILE* file = popen("ntpdate 2>&1", "r");
this will redirect stderrto stdoutand so you will see output from both. Second issue fscanfwill stop at the first space so you can replace with fgets:
这将重定向stderr到stdout，因此您将看到两者的输出。第二个问题fscanf将在第一个空格处停止，因此您可以替换为fgets：
fgets(buffer, 100, file);
回答by Jonathan Leffler
As Shafik Yaghmourcorrectly diagnosed, the output you see from ntpdateis written (correctly) to its standard error, which is the same as your programs standard error.
正如Shafik Yaghmour正确诊断的那样，您看到的输出ntpdate被（正确）写入其标准错误，这与您的程序标准错误相同。
To get the error messages sent down the pipe, use:
要获取通过管道发送的错误消息，请使用：
FILE *file = popen("ntpdate 2>&1", "r");
That sends the standard error output from ntpdateto the standard output of the command, which is the pipe you're reading from.
ntpdate它将标准错误输出发送到命令的标准输出，即您正在读取的管道。
Of course, it looks like using ntpdateisn't going to work well until you've configured something.
当然，ntpdate在您配置某些内容之前，看起来使用不会很好地工作。
