LinuxQuestions.org - popen() doesn't behave as expected

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popen() doesn't behave as expected

Hi!
I'm writing a small program that allow the user to compile a C++ program, but the problem is that popen() works on the commented code

Code:

//char *command="ls -l";

but not

Code:

char *command="g++ /home/kim/Programming/QPort/hello.cpp -o /home/kim/Programming/QPort/hello";

Why doesn't it show compiler output?
Here is my code so far:

Code:

void Simple::fileCompile()

{

  textOutput->insertPlainText("compiling...\n");



  FILE *fpipe;

  //char *command="ls -l";

  char *command="g++ /home/kim/Programming/QPort/hello.cpp -o /home/kim/Programming/QPort/hello";

  char line[256];

  QString stored_result;



  if ( !(fpipe = (FILE*)popen(command,"r")) )

  {  // If fpipe is NULL

      perror("Problems with pipe");

      exit(1);

  }



  while ( fgets( line, sizeof line, fpipe))

  {

    printf("%s", line);

    stored_result = line;

    textOutput->insertPlainText(stored_result);

  }

  pclose(fpipe);



  textOutput->insertPlainText("done!\n");

}

My program is using Qt4, but I doesn't think that have anything to do with the problem.

I tested whether your use of popen() was correct. Here's what I did:

Code:

wally:~/friday$ cat g.cpp

#include <stdio.h>

#include <stdlib.h>



int main(void)

{

  FILE *fpipe;



  //char *command="ls -l";

  char *command1="g++ /home/kim/Programming/QPort/hello.cpp -o /home/kim/Programming/QPort/hello";

  char *command2="g++ h.cpp -o h";

  char line[256];



  if ( !(fpipe = (FILE*)popen(command1,"r")) )

  {  // If fpipe is NULL

      perror("Problems with pipe");

      exit(1);

  }



  while ( fgets( line, sizeof line, fpipe))

  {

    printf("%s", line);

  }

  pclose(fpipe);



  if ( !(fpipe = (FILE*)popen(command2,"r")) )

  {  // If fpipe is NULL

      perror("Problems with pipe");

      exit(1);

  }



  while ( fgets( line, sizeof line, fpipe))

  {

    printf("%s", line);

  }

  pclose(fpipe);

}

wally:~/friday$ g++ g.cpp -o g

wally:~/friday$ ls

f.cpp  g  g.cpp  h.cpp  typescript

wally:~/friday$ g

g++: /home/kim/Programming/QPort/hello.cpp: No such file or directory

g++: no input files

wally:~/friday$ ls

f.cpp  g  g.cpp  h  h.cpp  typescript

wally:~/friday$ # Note that the file "h" has appeared.

wally:~/friday$

Everything worked. It ain't popen().

Edit:

Owe. Weight. You wanted "compiler output" perhaps to the screen?

If the compile goes perfectly well, the compiler doesn't send anything to standard output or standard error. That's why the file h "silently" appeared after running the above compile.

Hope this helps.

I have narrowed the problem to this line:

Code:

printf("%s", line);

It only works for "ls -l" but not "g++ myapp.cpp -o myapp"
Why is the variable "line" empty even if the compiler failed? :scratch:

Hi

It's because the error messages are sent to stderr, not stdout. And popen only read stdout. To get stderr when using popen, append 2>&1 to the command:

char *command1="g++ /home/kim/Programming/QPort/hello.cpp -o /home/kim/Programming/QPort/hello 2>&1";

Thanks Guttorm! This solved my little problem. :)