Capturing program output.

Question

i am making a small library that will basically capture the standard outputs of a program (such as printf()) into a separate process/thread...this process should then perform certain tasks (lets say write these captured outputs to a file)...i am just beginning to do serious C programming so i am still learning.

i wanted to know what is the best way to do this, i mean using process or a thread...how do i capture these printf() statements...also this library must handle any child process if spawned by the programs...the general assumption is the program that uses it is a threaded one so may be what sort of approach should i take.

Answer 1

The easiest way to capture the STDOUT from another program is to simply pipe it into the STDIN of your program (via the command-line ">" or "|" operator). So basically, in your C library, you should just read from STDIN with scanf, or gets, or whatever STDIN function you're using.

This is a pretty standard convention in the Unix/Linux world - programs read from STDIN and write to STDOUT in some well-formatted way, so that you can pipeline different programs together by simply adding pipes to the command line, e.g.:

grep "somestring" file1 file2 file3 | cut -d, -f1 | sort | uniq

Answer 2

If you want your program or library to launch the program and capture its output, look at popen(3). It will give you a FILE pointer where you can read the output from the program.