How do I get a full assembly code from C file?

Question

I'm currently trying to figure out the way to produce equivalent assembly code from corresponding C source file.

I've been using the C language for several years, but have little experience with assembly language.

I was able to output the assembly code using the -S option in gcc. However, the resulting assembly code contained call instructions which in turn make a jump to another function like _exp. This is not what I wanted, I needed a fully functional assembly code in a single file, with no dependency to other code.

Is it possible to achieve what I'm looking for?

To better describe the problem, I'm showing you my code here:

#include <math.h>
float sigmoid(float i){
    return 1/(1+exp(-i));
}

The platform I am working on is Windows 10 64-bit, the compiler I'm using is cl.exe from MSbuild.

My initial objective was to see, at a lowest level possible, how computers calculate mathematical functions. The level where I decided to observe the calculation process is assembly code, and the mathematical function I've chosen was sigmoid defined as above.

Answer 1

_exp is the standard math library function double exp(double); apparently you're on a platform that prepends a leading underscore to C symbol names.

Given a .s that calls some library functions, build it the same way you would a .c file that calls library functions:

gcc foo.S -o foo  -lm

You'll get a dynamic executable by default.

---

But if you really want all the code in one file with no external dependencies, you can link your .c into a static executable and disassemble that.

gcc -O3 -march=native foo.c -o foo -static -lm
objdump -drwC -Mintel foo > foo.s

There's no guarantee that the _exp implementation in libm.a (static library) is identical to the one you'd get in libm.so or libm.dll or whatever, because it's a different file. This is especially true for a function like memcpy where dynamic-linker tricks are often used to select an optimal version (for your CPU) at run-time.