Get function assembly code from GCC

Now I encounter problem. My project works on assembly-level. So I need assembly-level programming but the project scale is too large to do work on assembly-level only. Because of this issue, I determine get binary code(of assembly-code) from c-source-file made by gcc. but how? How can I get binary code(of assembly-code) from gcc?

I have a idea,

write(fd, (char *)function_name, sizeof(function_name))

but It's not good way to solve this problem, It would make me need to handle many file when I need many function as target.

Is there other good way to solve this problem? I think ideal solution that need only function name(and output file name if needed) and works on command-line. Is ideal solution which I think impossible?