Properly getting output from a pipe in C / C++

Question

I am writing C / C++ code to run a shell command and capture the output:

static const int BUFFER_SIZE=512;

// (...)

FILE* pipe;
char buffer[BUFFER_SIZE];

// (...)

if (!(pipe = popen(cmd.c_str(), "r"))) {
  out("\nProblem executing command: " + cmd + "\n");
  return 1;
}

while (!feof(pipe)) {
  int read = fread(buffer, sizeof(char), sizeof buffer, pipe);
  int pos = -1;
  int i;

  for (i = 0; i < strlen(buffer); i++) {
    if (buffer[i] == '\n') {
      pos = i;
    }
  }

  // ... get a substring from buffer var from 0 to pos,
  // and set a "reminder" var with pos+1 to end of string
}

It's working with a glitch: I see that the buffer contain some non-ascii chars at the end that are messing up every other operation I do with the string afterwards (the substring I mentioned on the comment).

Here's an example of what I am seeing:

Since I am a C / C++ beginner, I'd like to ask advice on how can I can capture the process output and stream it somewhere else, splitting it when necessary. The code can be in C or C++.

Answer 1

fread does not NUL terminate the buffer, so strlen is not appropriate.

Use the result read to limit your loop.