Question

How do I passively listen to stderr and obtain it as string for sending to callback? I have seen posts on reading stderr but I want to listen to it rather than actively reading it.

Background:
I have a cross-platform piece that uses 3rd party library (libcurl) which will output verbose info into stderr. This cross-platform piece is to be used by more than 1 non-cross-platform applications.

I would like to log these info, which I can do by providing FILE* to libcurl. But instead of doing that, I want to see if I can capture (passively listen to) the output in stderr as string, and send back to the calling main application via callback. This has the benefit of 1. main app can keep a single log using whatever logging tool it wants. 2. it will keep this piece cross-platform.

Answer 1

Doing this in a single process is a little tricky, but you can probably do it.

• 1: Using freopen() you can redirect your stderr to a named file. You can simultaneously open that file for reading on another handle. You might also need to call setvbuf() on stderr to turn off buffering on output to stderr so that you will be able to read it right away from the 2nd handle. Since it is being written to a file you can read it at anytime – when it is convenient. The unix function “select” is what you need if you want to be notified when the file changes. (see also fileno()).

• 2: More tricky would be to setup stderr as the write end of a pipe. Should be doable using dup3(), though this isn’t exactly cross-platform (to non-unixy OS’s). It would also require that a 2nd thread be reading from the pipe to prevent the writer from being blocked if they write very much.

Like:

FILE *stream = freopen("stderr.out", "w", stderr); // Added missing pointer
setvbuf(stream, 0, _IONBF, 0); // No Buffering
FILE *input = fopen("stderr.out", "r");
fprintf(stderr, "Output to stderr dude\n");
//fflush(stderr); // You can explicitly flush instead of setting no buffering.
char buffer[1024];
while (fgets(buffer, 512, input))
{
    printf(">>>%s\n", buffer);
}