Question

Maybe the title does not word the question so precisely: I know that when I run gcc foo.c GCC calls other sub-programs that do all the work for it, making the main gcc program just an interface. But how exactly is this done?

Does it use system or exec or some other function? The reason I want to know this because I want to build a web crawler based on a similar system, where there would be a interface program and several other sub-programs like crawl and download.

I’m sorry if this question has already been asked but I didn’t find it using search or the “Questions with similar titles”.

Thank you in advance.

Answer 1

While your question is really more general (and only using gcc as an example), my first idea would be to use strace to figure out what it’s doing. On my system (Ubuntu 11.10/x64), I just ran strace, like so:

strace -F -o sout gcc -Wall -o test test.c

This shows system calls for the gcc process, while following forks (-F) and sending the output of the trace to sout. Doing this, I can see that gcc here calls vfork(), and then execve() in the child, though the actual program’s source might just do a simple fork()/exec().

The relevant output from sout is:

26264 stat("/usr/lib/gcc/x86_64-linux-gnu/4.6.1/cc1", {st_mode=S_IFREG|0755, st_size=11248824, ...}) = 0
26264 access("/usr/lib/gcc/x86_64-linux-gnu/4.6.1/cc1", X_OK) = 0
26264 vfork()                           = 26265
26264 wait4(26265,  <unfinished ...>
26265 execve("/usr/lib/gcc/x86_64-linux-gnu/4.6.1/cc1", ["/usr/lib/gcc/x86_64-linux-gnu/4."..., "-quiet", "-imultilib", ".", "-imultiarch", "x86_64-linux-gnu", "test.c", "-quiet", "-dumpbase", "test.c", "-mtune=generic", "-march=x86-64", "-auxbase", "test", "-Wall", "-fstack-protector", ...], [/* 46 vars */]) = 0

At the begnning of each line is the pid of the process running. So the primary process calls stat() to find cc1, then forks, and the child executes it.

That being said, I could have answered your question without the citation; fork/exec is a common way to call sub-processes from your program.