calling assembly function from c

Question

I'm trying to call an assembly function from c,but i keep getting errors.

    .text
    .globl integrate
    .type integrate, @function
integrate:
    push %ebp
    mov %esp, %ebp
    mov $0,%edi
start_loop:                
    cmp %edi,1024           
    je loop_exit
    mov 8(%ebp),%eax          
    mov 12(%ebp),%ecx          
    sub %eax,%ecx              
    add %edi,%ecx
    incl %edi                
    jmp start_loop             
loop_exit:                 
    movl %ebp, %esp
    popl %ebp
    ret   

This is my assembly function,file called integrate.s.

#include <stdio.h>

extern int integrate(int from,int to);

void main()
{
    printf("%d",integrate(1,10));
}

Heres my c code.

function.c:5:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
/tmp/cciR63og.o: In function `main':
function.c:(.text+0x19): undefined reference to `integrate'
collect2: ld returned 1 exit status

Whenever i try to compile my code with gcc -Wall function.c -o function,it gives the 'undefined reference to integrate' error.I also tried adding link to the integrate.s file from c,like

#include<(file path)/integrate.s>

but it didnt work as well.Btw what assembly code is doing is not important,for now im just trying to call the function from c successfully.Can anyone help me about solving this problem ?

Answer 1

I see the following problems with the code:

• calling convention mandates you must preserve the value of edi
• cmp %edi,1024 is using 1024 as address and will probably fault. You want cmp $1024,%edi for comparing with an immediate number
• you are reloading eax and ecx from the arguments each iteration so the calculation you perform has no effect
• you don't seem to put any sensible return value into eax (it will return the value of from that was passed in)

The first two points apply even if "what assembly code is doing is not important".

Answer 2

warning: return type of ‘main’ is not ‘int’

means that the return type of ‘main’ is not ‘int’... Change it to int, then:

int main()
{
}

Also, to solve the linker error, invoke GCC as

gcc -o myprog main.c integrate.s

and that should work.

Answer 3

Not sure if you have solved this or not, but here is how I have done this.

When compiling make sure to add both files: $gcc main.c print_msg.s -o main

To run the assembler file on its own: $as print_msg.s -o print_msg.o followed by $ld print_msg.o -e print -o print_msg. Note that this is not required if you only want to run it from your C file.

The assembler file: print_msg.s

# A program to be called from a C program
# Declaring data that doesn't change
.section .data
    string: .ascii  "Hello from assembler\n"
    length: .quad   . - string

# The actual code
.section .text
.global print
.type print, @function              #<-Important

print:
    mov     $0x1,%rax               # Move 1(write) into rax
    mov     $0x1,%rdi               # Move 1(fd stdOut) into rdi.
    mov     $string,%rsi            # Move the _location_ of the string into rsi
    mov     length,%rdx             # Move the _length_ of the string into rdx
    syscall                         # Call the kernel

    mov     %rax,%rdi               # Move the number of bytes written to rdi
    mov     $0x3c,%rax              # Move 60(sys_exit) into rax
    syscall                         # Call the kernel

then the C file: main.c

extern void print(void);

int main(void)
{
    print();
    return 0;
}