How I can add assembly code in C project? - c

Question

I would like to compile a simple C project that has some externals functions defined in a ASM file. My main file is a C++ that calls some "extern "C"" functions that are defined in a assembly file.
When I run task "g++ build active file", I receive some warnings about the "extern" and some errors about functions defined in asm file telling "reference to my_funcions not defined".
My C++ file contains a "extern" like this:
[...]
extern "C" {
// Subrutines en ASM
void posCurScreenP1();
void moveCursorP1();
void openP1();
void getMoveP1();
void movContinuoP1();
void openContinuousP1();
void printChar_C(char c);
int clearscreen_C();
int printMenu_C();
int gotoxy_C(int row_num, int col_num);
char getch_C();
int printBoard_C(int tries);
void continue_C();
}
[...]
and my asm file contains this:
.586
.MODEL FLAT, C
; Funcions definides en C
printChar_C PROTO C, value:SDWORD
printInt_C PROTO C, value:SDWORD
clearscreen_C PROTO C
clearArea_C PROTO C, value:SDWORD, value1: SDWORD
printMenu_C PROTO C
gotoxy_C PROTO C, value:SDWORD, value1: SDWORD
getch_C PROTO C
printBoard_C PROTO C, value: DWORD
initialPosition_C PROTO C
.code
[...]
Sure I'm doing some things wrong. Could you help me?
Thanks.

Answer 1

hmm
so.c
extern "C" int fun ( void );
int x;
int main()
{
x=fun();
return x;
}
fun.c
int fun ( void )
{
return(5);
}
build
gcc fun.c -O2 -c -o fun.o
g++ -O2 so.cpp fun.o -o so
No errors
00000000004003e0 <main>:
4003e0: 48 83 ec 08 sub $0x8,%rsp
4003e4: e8 17 01 00 00 callq 400500 <fun>
4003e9: 89 05 45 0c 20 00 mov %eax,0x200c45(%rip) # 601034 <x>
4003ef: 48 83 c4 08 add $0x8,%rsp
4003f3: c3 retq
0000000000400500 <fun>:
400500: b8 05 00 00 00 mov $0x5,%eax
400505: c3 retq
okay so
morefun.s
.globl fun
fun:
mov $0x5,%eax
retq
build
as morefun.s -o morefun.o
g++ -O2 so.cpp morefun.o -o so
no errors,
examine
00000000004003e0 <main>:
4003e0: 48 83 ec 08 sub $0x8,%rsp
4003e4: e8 0d 01 00 00 callq 4004f6 <fun>
4003e9: 89 05 45 0c 20 00 mov %eax,0x200c45(%rip) # 601034 <x>
4003ef: 48 83 c4 08 add $0x8,%rsp
4003f3: c3 retq
0000000004004f6 <fun>:
4004f6: b8 05 00 00 00 mov $0x5,%eax
4004fb: c3 retq
Still looks good, no problem adding assembly to a C++ project by making it look like a C function.
Other path
int fun ( void )
{
return(5);
}
gnu and most other sane compilers compile to asm then call the assembler so you can just see how they do it and repeat that
gcc -O2 -S fun.c -o fun.s
as fun.s -o fun.o
cat fun.s
.file "fun.c"
.section .text.unlikely,"ax",#progbits
.LCOLDB0:
.text
.LHOTB0:
.p2align 4,,15
.globl fun
.type fun, #function
fun:
.LFB0:
.cfi_startproc
movl $5, %eax
ret
.cfi_endproc
.LFE0:
.size fun, .-fun
.section .text.unlikely
.LCOLDE0:
.text
.LHOTE0:
.ident "GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.11) 5.4.0 20160609"
.section .note.GNU-stack,"",#progbits
or use save-temps.
gcc -O2 -c -save-temps fun.c -o fun.o
to see the asm generated by the compiler
It is generally more painful to try to use compiled assembly as a starting point as is, cutting and pasting sure but there is a lot of overhead and the machine generated labels that you would want to clean up rather than start from scratch. (I prefer to disassemble and work from that than to use the compiler output directly)