Link and compile an assembly function

Question

I have an assembly function like so

rfact: pushl %ebp movl %esp, %ebp pushl %ebx subl $4, %esp movl 8(%ebp), %ebx movl $1, %eax cmpl $1, %ebx jle .L53 leal -1(%ebx), %eax movl %eax, (%esp) call rfact imull %ebx, %eax .L53: addl $4, %esp popl %ebx popl %ebp ret

I understand I can't just save this as rfact.s and compile it. There has to be certain items (such as .text) appended to the top of the assembly. What are these for a linux system? And I'd like to call this function from a main function written in normal c file called rfactmain.c

Sorry about that. I didn't realized it got mangled when I pasted it. I have fixed it now! — Aman
– Aman, Commented Feb 13, 2014 at 23:44

Brett Hale · Accepted Answer · 2014-02-14 01:21:46Z

Here's a 'minimal' prefix of directives - for ELF/SysV i386, and GNU as:

.text .p2align 4 .globl rfact .type rfact, @function

I'd also recommend appending a function size directive at the end:

.size rfact, .-rfact

The easiest way to compile is with: gcc [-m32] -c -P rfact.S
With the -P option, you can use C-style comments and not have to worry about line number output, etc. This results in an object file you can link with. The -m32 flag is required if gcc targets x86-64 by default.

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