Calling assembly routines from C source code
assembly, c, linker-errors, linux, nasm
Solution
As far as I can see from your question, you overwrite your object file that came from the assembler `s.o` by the C program. So you don't have the assembler routine any more.
You should probably write
Generate the `s.o` object file
nasm s.asm -f elf -o s.o
Generate the `test.o` (your command created another s.o)
gcc test.c -c
Link the app
gcc s.o test.o -o testapp
(I chose testapp as output binary because `test` is often a very bad name for a program, it collides with the Unix command `test`)
Problem
I have this simple C source code : ``` #include <stdio.h> extern int Sum(int,int); int main() { int a,b,s; a=1 , b=2; s = Sum(a,b); return 0; } ``` and i have this s.asm which defines the function _Sum : ``` global _Sum _Sum: push ebp ; create stack frame mov ebp, esp mov eax, [ebp+8] ; grab the first argument mov ecx, [ebp+12] ; grab the second argument add eax, ecx ; sum the arguments pop ebp ; restore the base pointer ret ``` now , i compiled the .asm using : ``` nasm s.asm -f elf -o s.o ``` and compiled and linked the .c file using : ``` gcc s.o test.o -o testapp ``` this is the outcome : ``` /tmp/ccpwYHDQ.o: In function `main': test.c:(.text+0x29): undefined reference to `Sum' collect2: ld returned 1 exit status ``` So what is the problem ? I'm using Ubuntu-Linux Any help would be greatly appreciated , Thanks [SOLVED] : i checked with nm the test.o file and it expected to find the symbol 'Sum' not '_Sum' so changing that solved the problem.