I need to access an argument from a C function
void printk(char* msg);
and store it into ESI. However I can not do
mov $msg, %esi
or
mov $_msg, %esi
Both return a linker error (Saying that the symbol msg or _msg is undefined. I am pretty sure I could access msg by reading from the stack but it would be easier just to access the symbol msg in my assembly code. Is this even possible in GCC?
You can't refer to variables like that in GCC inline assembly. I think you want something like:
asm("mov %[msg], %%esi",
: /* no outputs */
: [msg] "r" (msg)
: "esi");
Related
I want to get the value of EIP from the following code, but the compilation does not pass
Command :
gcc -o xxx x86_inline_asm.c -m32 && ./xxx
file contetn x86_inline_asm.c:
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
unsigned int eip_val;
__asm__("mov %0,%%eip":"=r"(eip_val));
return 0;
}
How to use the inline assembly to get the value of EIP, and it can be compiled successfully under x86.
How to modify the code and use the command to complete it?
This sounds unlikely to be useful (vs. just taking the address of the whole function like void *tmp = main), but it is possible.
Just get a label address, or use . (the address of the current line), and let the linker worry about getting the right immediate into the machine code. So you're not architecturally reading EIP, just reading the value it currently has from an immediate.
asm volatile("mov $., %0" : "=r"(address_of_mov_instruction) );
AT&T syntax is mov src, dst, so what you wrote would be a jump if it assembled.
(Architecturally, EIP = the end of an instruction while it's executing, so arguably you should do
asm volatile(
"mov $1f, %0 \n\t" // reference label 1 forward
"1:" // GAS local label
"=r"(address_after_mov)
);
I'm using asm volatile in case this asm statement gets duplicated multiple times inside the same function by inlining or something. If you want each case to get a different address, it has to be volatile. Otherwise the compiler can assume that all instances of this asm statement produce the same output. Normally that will be fine.
Architecturally in 32-bit mode you don't have RIP-relative addressing for LEA so the only good way to actually read EIP is call / pop. Reading program counter directly. It's not a general-purpose register so you can't just use it as the source or destination of a mov or any other instruction.
But really you don't need inline asm for this at all.
Is it possible to store the address of a label in a variable and use goto to jump to it? shows how to use the GNU C extension where &&label takes its address.
int foo;
void *addr_inside_function() {
foo++;
lab1: ; // labels only go on statements, not declarations
void *tmp = &&lab1;
foo++;
return tmp;
}
There's nothing you can safely do with this address outside the function; I returned it just as an example to make the compiler put a label in the asm and see what happens. Without a goto to that label, it can still optimize the function pretty aggressively, but you might find it useful as an input for an asm goto(...) somewhere else in the function.
But anyway, it compiles on Godbolt to this asm
# gcc -O3 -m32
addr_inside_function:
.L2:
addl $2, foo
movl $.L2, %eax
ret
#clang -O3 -m32
addr_inside_function:
movl foo, %eax
leal 1(%eax), %ecx
movl %ecx, foo
.Ltmp0: # Block address taken
addl $2, %eax
movl %eax, foo
movl $.Ltmp0, %eax # retval = label address
retl
So clang loads the global, computes foo+1 and stores it, then after the label computes foo+2 and stores that. (Instead of loading twice). So you still can't usefully jump to the label from anywhere, because it depends on having foo's old value in eax, and on the desired behaviour being to store foo+2
I don't know gcc inline assembly syntax for this, but for masm:
call next0
next0: pop eax ;eax = eip for this line
In the case of Masm, $ represents the current location, and since call is a 5 byte instruction, an alternative syntax without a label would be:
call $+5
pop eax
I write a boot loader in asm and want to add some compiled C code in my project.
I created a test function here:
test.c
__asm__(".code16\n");
void print_str() {
__asm__ __volatile__("mov $'A' , %al\n");
__asm__ __volatile__("mov $0x0e, %ah\n");
__asm__ __volatile__("int $0x10\n");
}
And here is the asm code (the boot loader):
hw.asm
[org 0x7C00]
[BITS 16]
[extern print_str] ;nasm tip
start:
mov ax, 0
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7C00
mov si, name
call print_string
mov al, ' '
int 10h
mov si, version
call print_string
mov si, line_return
call print_string
call print_str ;call function
mov si, welcome
call print_string
jmp mainloop
mainloop:
mov si, prompt
call print_string
mov di, buffer
call get_str
mov si, buffer
cmp byte [si], 0
je mainloop
mov si, buffer
;call print_string
mov di, cmd_version
call strcmp
jc .version
jmp mainloop
.version:
mov si, name
call print_string
mov al, ' '
int 10h
mov si, version
call print_string
mov si, line_return
call print_string
jmp mainloop
name db 'MOS', 0
version db 'v0.1', 0
welcome db 'Developped by Marius Van Nieuwenhuyse', 0x0D, 0x0A, 0
prompt db '>', 0
line_return db 0x0D, 0x0A, 0
buffer times 64 db 0
cmd_version db 'version', 0
%include "functions/print.asm"
%include "functions/getstr.asm"
%include "functions/strcmp.asm"
times 510 - ($-$$) db 0
dw 0xaa55
I need to call the c function like a simple asm function
Without the extern and the call print_str, the asm script boot in VMWare.
I tried to compile with:
nasm -f elf32
But i can't call org 0x7C00
Compiling & Linking NASM and GCC Code
This question has a more complex answer than one might believe, although it is possible. Can the first stage of a bootloader (the original 512 bytes that get loaded at physical address 0x07c00) make a call into a C function? Yes, but it requires rethinking how you build your project.
For this to work you can no longer us -f bin with NASM. This also means you can't use the org 0x7c00 to tell the assembler what address the code expects to start from. You'll need to do this through a linker (either us LD directly or GCC for linking). Since the linker will lay things out in memory we can't rely on placing the boot sector signature 0xaa55 in our output file. We can get the linker to do that for us.
The first problem you will discover is that the default linker scripts used internally by GCC don't lay things out the way we want. We'll need to create our own. Such a linker script will have to set the origin point (Virtual Memory Address aka VMA) to 0x7c00, place the code from your assembly file before the data and place the boot signature at offset 510 in the file. I'm not going to write a tutorial on Linker scripts. The Binutils Documentation contains almost everything you need to know about linker scripts.
OUTPUT_FORMAT("elf32-i386");
/* We define an entry point to keep the linker quiet. This entry point
* has no meaning with a bootloader in the binary image we will eventually
* generate. Bootloader will start executing at whatever is at 0x07c00 */
ENTRY(start);
SECTIONS
{
. = 0x7C00;
.text : {
/* Place the code in hw.o before all other code */
hw.o(.text);
*(.text);
}
/* Place the data after the code */
.data : SUBALIGN(2) {
*(.data);
*(.rodata*);
}
/* Place the boot signature at LMA/VMA 0x7DFE */
.sig 0x7DFE : {
SHORT(0xaa55);
}
/* Place the uninitialised data in the area after our bootloader
* The BIOS only reads the 512 bytes before this into memory */
.bss : SUBALIGN(4) {
__bss_start = .;
*(COMMON);
*(.bss)
. = ALIGN(4);
__bss_end = .;
}
__bss_sizeb = SIZEOF(.bss);
/* Remove sections that won't be relevant to us */
/DISCARD/ : {
*(.eh_frame);
*(.comment);
}
}
This script should create an ELF executable that can be converted to a flat binary file with OBJCOPY. We could have output as a binary file directly but I separate the two processes out in the event I want to include debug information in the ELF version for debug purposes.
Now that we have a linker script we must remove the ORG 0x7c00 and the boot signature. For simplicity sake we'll try to get the following code (hw.asm) to work:
extern print_str
global start
bits 16
section .text
start:
xor ax, ax ; AX = 0
mov ds, ax
mov es, ax
mov ss, ax
mov sp, 0x7C00
call print_str ; call function
/* Halt the processor so we don't keep executing code beyond this point */
cli
hlt
You can include all your other code, but this sample will still demonstrate the basics of calling into a C function.
Assume the code above you can now generate the ELF object from hw.asm producing hw.o using this command:
nasm -f elf32 hw.asm -o hw.o
You compile each C file with something like:
gcc -ffreestanding -c kmain.c -o kmain.o
I placed the C code you had into a file called kmain.c . The command above will generate kmain.o. I noticed you aren't using a cross compiler so you'll want to use -fno-PIE to ensure we don't generate relocatable code. -ffreestanding tells GCC the C standard library may not exist, and main may not be the program entry point. You'd compile each C file in the same way.
To link this code to a final executable and then produce a flat binary file that can be booted we do this:
ld -melf_i386 --build-id=none -T link.ld kmain.o hw.o -o kernel.elf
objcopy -O binary kernel.elf kernel.bin
You specify all the object files to link with the LD command. The LD command above will produce a 32-bit ELF executable called kernel.elf. This file can be useful in the future for debugging purposes. Here we use OBJCOPY to convert kernel.elf to a binary file called kernel.bin. kernel.bin can be used as a bootloader image.
You should be able to run it with QEMU using this command:
qemu-system-i386 -fda kernel.bin
When run it may look like:
You'll notice the letter A appears on the last line. This is what we'd expect from the print_str code.
GCC Inline Assembly is Hard to Get Right
If we take your example code in the question:
__asm__ __volatile__("mov $'A' , %al\n");
__asm__ __volatile__("mov $0x0e, %ah\n");
__asm__ __volatile__("int $0x10\n");
The compiler is free to reorder these __asm__ statements if it wanted to. The int $0x10 could appear before the MOV instructions. If you want these 3 lines to be output in this exact order you can combine them into one like this:
__asm__ __volatile__("mov $'A' , %al\n\t"
"mov $0x0e, %ah\n\t"
"int $0x10");
These are basic assembly statements. It's not required to specify __volatile__on them as they are already implicitly volatile, so it has no effect. From the original poster's answer it is clear they want to eventually use variables in __asm__ blocks. This is doable with extended inline assembly (the instruction string is followed by a colon : followed by constraints.):
With extended asm you can read and write C variables from assembler and perform jumps from assembler code to C labels. Extended asm syntax uses colons (‘:’) to delimit the operand parameters after the assembler template:
asm [volatile] ( AssemblerTemplate
: OutputOperands
[ : InputOperands
[ : Clobbers ] ])
This answer isn't a tutorial on inline assembly. The general rule of thumb is that one should not use inline assembly unless you have to. Inline assembly done wrong can create hard to track bugs or have unusual side effects. Unfortunately doing 16-bit interrupts in C pretty much requires it, or you write the entire function in assembly (ie: NASM).
This is an example of a print_chr function that take a nul terminated string and prints each character out one by one using Int 10h/ah=0ah:
#include <stdint.h>
__asm__(".code16gcc\n");
void print_str(char *str) {
while (*str) {
/* AH=0x0e, AL=char to print, BH=page, BL=fg color */
__asm__ __volatile__ ("int $0x10"
:
: "a" ((0x0e<<8) | *str++),
"b" (0x0000));
}
}
hw.asm would be modified to look like this:
push welcome
call print_str ;call function
The idea when this is assembled/compiled (using the commands in the first section of this answer) and run is that it print out the welcome message. Unfortunately it will almost never work, and may even crash some emulators like QEMU.
code16 is Almost Useless and Should Not be Used
In the last section we learn that a simple function that takes a parameter ends up not working and may even crash an emulator like QEMU. The main problem is that the __asm__(".code16\n"); statement really doesn't work well with the code generated by GCC. The Binutils AS documentation says:
‘.code16gcc’ provides experimental support for generating 16-bit code from gcc, and differs from ‘.code16’ in that ‘call’, ‘ret’, ‘enter’, ‘leave’, ‘push’, ‘pop’, ‘pusha’, ‘popa’, ‘pushf’, and ‘popf’ instructions default to 32-bit size. This is so that the stack pointer is manipulated in the same way over function calls, allowing access to function parameters at the same stack offsets as in 32-bit mode. ‘.code16gcc’ also automatically adds address size prefixes where necessary to use the 32-bit addressing modes that gcc generates.
.code16gcc is what you really need to be using, not .code16. This force GNU assembler on the back end to emit address and operand prefixes on certain instructions so that the addresses and operands are treated as 4 bytes wide, and not 2 bytes.
The hand written code in NASM doesn't know it will be calling C instructions, nor does NASM have a directive like .code16gcc. You'll need to modify the assembly code to push 32-bit values on to the stack in real mode. You will also need to override the call instruction so that the return address needs to be treated as a 32-bit value, not 16-bit. This code:
push welcome
call print_str ;call function
Should be:
jmp 0x0000:setcs
setcs:
cld
push dword welcome
call dword print_str ;call function
GCC has a requirement that the direction flag be cleared before calling any C function. I added the CLD instruction to the top of the assembly code to make sure this is the case. GCC code also needs to have CS to 0x0000 to work properly. The FAR JMP does just that.
You can also drop the __asm__(".code16gcc\n"); on modern GCC that supports the -m16 option. -m16 automatically places a .code16gcc into the file that is being compiled.
Since GCC also uses the full 32-bit stack pointer it is a good idea to initialize ESP with 0x7c00, not just SP. Change mov sp, 0x7C00 to mov esp, 0x7C00. This ensures the full 32-bit stack pointer is 0x7c00.
The modified kmain.c code should now look like:
#include <stdint.h>
void print_str(char *str) {
while (*str) {
/* AH=0x0e, AL=char to print, BH=page, BL=fg color */
__asm__ __volatile__ ("int $0x10"
:
: "a" ((0x0e<<8) | *str++),
"b" (0x0000));
}
}
and hw.asm:
extern print_str
global start
bits 16
section .text
start:
xor ax, ax ; AX = 0
mov ds, ax
mov es, ax
mov ss, ax
mov esp, 0x7C00
jmp 0x0000:setcs ; Set CS to 0
setcs:
cld ; GCC code requires direction flag to be cleared
push dword welcome
call dword print_str ; call function
cli
hlt
section .data
welcome db 'Developped by Marius Van Nieuwenhuyse', 0x0D, 0x0A, 0
These commands can be build the bootloader with:
gcc -fno-PIC -ffreestanding -m16 -c kmain.c -o kmain.o
ld -melf_i386 --build-id=none -T link.ld kmain.o hw.o -o kernel.elf
objcopy -O binary kernel.elf kernel.bin
When run with qemu-system-i386 -fda kernel.bin it should look simialr to:
In Most Cases GCC Produces Code that Requires 80386+
There are number of disadvantages to GCC generated code using .code16gcc:
ES=DS=CS=SS must be 0
Code must fit in the first 64kb
GCC code has no understanding of 20-bit segment:offset addressing.
For anything but the most trivial C code, GCC doesn't generate code that can run on a 286/186/8086. It runs in real mode but it uses 32-bit operands and addressing not available on processors earlier than 80386.
If you want to access memory locations above the first 64kb then you need to be in Unreal Mode(big) before calling into C code.
If you want to produce real 16-bit code from a more modern C compiler I recommend OpenWatcom C
The inline assembly is not as powerful as GCC
The inline assembly syntax is different but it is easier to use and less error prone than GCC's inline assembly.
Can generate code that will run on antiquated 8086/8088 processors.
Understands 20-bit segment:offset real mode addressing and supports the concept of far and huge pointers.
wlink the Watcom linker can produce basic flat binary files usable as a bootloader.
Zero Fill the BSS Section
The BIOS boot sequence doesn't guarantee that memory is actually zero. This causes a potential problem for the zero initialized region BSS. Before calling into C code for the first time the region should be zero filled by our assembly code. The linker script I originally wrote defines a symbol __bss_start that is the offset of the BSS memory and __bss_sizeb is the size in bytes. Using this info you can use the STOSB instruction to easily zero fill it. At the top of hw.asm you can add:
extern __bss_sizeb
extern __bss_start
And after the CLD instruction and before calling any C code you can do the zero fill this way:
; Zero fill the BSS section
mov cx, __bss_sizeb ; Size of BSS computed in linker script
mov di, __bss_start ; Start of BSS defined in linker script
rep stosb ; AL still zero, Fill memory with zero
Other Suggestions
To reduce the bloat of the code generated by the compiler it can be useful to use -fomit-frame-pointer. Compiling with -Os can optimize for space (rather than speed). We have limited space (512 bytes) for the initial code loaded by the BIOS so these optimizations can be beneficial. The command line for compiling could appear as:
gcc -fno-PIC -fomit-frame-pointer -ffreestanding -m16 -Os -c kmain.c -o kmain.o
I write a boot loader in asm and want to add some compiled C code in my project.
Then you need to use a 16-bit x86 compiler, such as OpenWatcom.
GCC cannot safely build real-mode code, as it is unaware of some important features of the platform, including memory segmentation. Inserting the .code16 directive will make the compiler generate incorrect output. Despite appearing in many tutorials, this piece of advice is simply incorrect, and should not be used.
First i want to express how to link C compiled code with assembled file.
I put together some Q/A in SO and reach to this.
C code:
func.c
//__asm__(".code16gcc\n");when we use eax, 32 bit reg we cant use this as truncate
//problem
#include <stdio.h>
int x = 0;
int madd(int a, int b)
{
return a + b;
}
void mexit(){
__asm__ __volatile__("mov $0, %ebx\n");
__asm__ __volatile__("mov $1, %eax \n");
__asm__ __volatile__("int $0x80\n");
}
char* tmp;
///how to direct use of arguments in asm command
void print_str(int a, char* s){
x = a;
__asm__("mov x, %edx\n");// ;third argument: message length
tmp = s;
__asm__("mov tmp, %ecx\n");// ;second argument: pointer to message to write
__asm__("mov $1, %ebx\n");//first argument: file handle (stdout)
__asm__("mov $4, %eax\n");//system call number (sys_write)
__asm__ __volatile__("int $0x80\n");//call kernel
}
void mtest(){
printf("%s\n", "Hi");
//putchar('a');//why not work
}
///gcc -c func.c -o func
Assembly code:
hello.asm
extern mtest
extern printf
extern putchar
extern print_str
extern mexit
extern madd
section .text ;section declaration
;we must export the entry point to the ELF linker or
global _start ;loader. They conventionally recognize _start as their
;entry point. Use ld -e foo to override the default.
_start:
;write our string to stdout
push msg
push len
call print_str;
call mtest ;print "Hi"; call printf inside a void function
; use add inside func.c
push 5
push 10
call madd;
;direct call of <stdio.h> printf()
push eax
push format
call printf; ;printf(format, eax)
call mexit; ;exit to OS
section .data ;section declaration
format db "%d", 10, 0
msg db "Hello, world!",0xa ;our dear string
len equ $ - msg ;length of our dear string
; nasm -f elf32 hello.asm -o hello
;Link two files
;ld hello func -o hl -lc -I /lib/ld-linux.so.2
; ./hl run code
;chain to assemble, compile, Run
;; gcc -c func.c -o func && nasm -f elf32 hello.asm -o hello && ld hello func -o hl -lc -I /lib/ld-linux.so.2 && echo &&./hl
Chain commands for assemble, compile and Run
gcc -c func.c -o func && nasm -f elf32 hello.asm -o hello && ld hello func -o hl -lc -I /lib/ld-linux.so.2 && echo && ./hl
Edit[toDO]
Write boot loader code instead of this version
Some explanation on how ld, gcc, nasm works.
I have the following code in a GNU C program:
void *segment = malloc(1024);
asm volatile("mov $%0, %%rax" : : "r" (segment));
And I get the following error:
Error: illegal immediate register operand %rax
What is wrong with %rax?
While FrankH's points are valid, strictly speaking cause of this error is the dollar sign. Dollar signs in assembler are used to denote constants. So "mov $1, %%eax" would work. However, your code generates:
mov $%rax, %rax
$%rax is meaningless and generates a error. This will resolve the error:
void *segment = malloc(1024);
asm volatile("mov %0, %%rax" : : "r" (segment));
Since malloc will return its value in rax, this will (most likely) generate "mov %rax, %rax".
In other words, it will still be meaningless, unsafe and inefficient, but it will compile without error.
Assuming this code is intended to be more than an experiment to teach you something about using asm, you will need to provide more details to get a more useful answer.
GCC inline assembly error: Error: junk `(%esp)' after expression
I'm studying gcc inline assembly. My environment is Win 7 32bit, mingw-gcc 4.6.1.
I have got a problem about the 'm' constraint. Here is my c function code:
static int asm_test(int a, int b)
{
int c = 0;
__asm__ __volatile__(".intel_syntax\n"
"mov eax, %1\n" //error
"mov edx, %2\n" //error
"add eax, edx\n"
"mov %0, eax\n" //error
".att_syntax"
:"=m"(c)\
:"m"(a),"m"(b)\
:"eax","edx"
);
return c;
}
For at&t code, it is like this:
static int asm_test(int a, int b)
{
int c = 0;
__asm__ __volatile__(
"movl %1, $eax\n" //error
"movl %2, $edx\n" //error
"addl $edx, $eax\n"
"movl $eax, %0\n" //error
:"=m"(c)\
:"m"(a),"m"(b)\
:"eax","edx"
);
return c;
}
For each of the three lines which operate input/output operands, gcc generate an error when compiling, read like this:
C:\Users\farta\AppData\Local\Temp\cc99HxYj.s:22: Error: junk `(%esp)' after expression
If i use 'r' for input/output constraint, the code will work. But I cannot understand why it works and what the error stands for. Can anyone tell me? As far as I know 'm' is just telling gcc not to allocate registers but directly access them in memory if inline asm code try to access input/output operands. Is this correct?
Thanks a lot.
The problem here is the GCC generates AT&T syntax construct for %0, %1 and %2. If you look at the generated assembly, it looks like:
.intel_syntax
mov eax, 8(%ebp)
mov edx, 12(%ebp)
add eax, edx
mov -4(%ebp), eax
which is not a valid Intel syntax.
Generally, you don't need to include in the inline assembly explicit load/store operation - just specify register constraint and the compiler will generate loads/stores by itself. This has the advantage that even if your variables (parameters, locals) do not reside in memory at all, but are in registers your code will still be correct - unlike in the case if you explicitly put memory load/stores there.
For your example, try the following code, look at the assembly (gcc -S) and notice how the compiler will perform moves from argument area (e.g. stack on x86) all by itself.
int asm_test(int a, int b)
{
__asm__ __volatile__ (
".intel_syntax\n"
"add %0, %1 \n"
".att_syntax \n"
:"+r"(a)
:"r"(b));
return a;
}
I am writing a bit of 16-bit (pun intended) code in C++, compiling it with G++. More on the context I'm compiling in here: Force GCC to push arguments on the stack before calling function (using PUSH instruction)
The problem I am facing now is regarding an error LD throws when trying to link my object files. Specifically, here's a code situation:
asm(".code16gcc\n");
void f(const char*);
int main(){
f("A constant string put in section .rodata at link-time");
}
void f(const char* s){ }
In assembly code, with -S and -mno-accumulate-outgoing-args options G++ would translate this to (only relevant parts of the assembly written):
/APP
.code16gcc
.section .rodata
.LC0:
.string "A constant string put in section .rodata at link-time"
main:
.LFB0:
/* here would be main's prologue, not put because it ain't relevant */
// THIS IS THE CALL f("A constant string put in section .rodata at link-time");
push OFFSET FLAT:.LC0
call _Z1fPKc
This application is part of an OS I'm developing. Specifically, the bootloader loads this code at address 0x70D00 in BIOS memory. That makes .rodata's address be bigger than 0x70D00. Since GCC does not have built-in support for pure 16-bit code, it doesn't know that executing the 'push OFFSET FLAT:.LC0' would mean pushing a WORD UNDER PURE 16-BIT circumstances. Which means that, if the address of .rodata is - say - 0x70DAA, the instruction would be 'push 0x70DAA'. That's why the linker throws the error:
In function main': relocation truncated to fit: R_386_16 against.rodata'
-- because the linker knows that 0x70DAA DOES NOT FIT IN A WORD. What would solve the problem is asking GCC to MOV the arguments IN A REGISTER BEFORE PUSHING THEM. Something like:
/APP
.code16gcc
.section .rodata
.LC0:
.string "A constant string put in section .rodata at link-time"
main:
.LFB0:
/* here would be main's prologue, not put because it ain't relevant */
// THIS IS THE CALL f("A constant string put in section .rodata at link-time"); , now using EAX before pushing the string literal's offset in .rodata
mov eax, OFFSET FLAT:.LC0 // move in eax instead
push eax // and push eax!
call _Z1fPKc
This is what MSVC does to optimize in some situations. I was wondering if there's a way to force GCC to do the same thing...one alternative that apparently would work is associating the attribute((regparm(N))) to function f. But this is not really a good alternative, since it DOESN'T REALLY PUSH the registers on the stack, rather than using them directly in f - and can't do this for any function. You can find out more on this by doing a short google search and if needed I'll post exactly what this option does here and why it would't really work, but this question-post starts to get too long.
In short, my question is:
Can I ask GCC to MOV the arguments passed to functions IN A REGISTER BEFORE PUSHING THEM?
Thanks in advance!
I have thought of a work-around for this problem, although I would have prefered a MOV-to-REG-and-PUSH sort-of method. What I've thought of is that this only happens for addresses that the compiler can calculate at compile time, like the address of the string which was put in .rodata.
Knowing that, I have created a local variable in main and used that as the passed argument instead, like this:
asm(".code16gcc\n");
void f(const char*);
int main(){
const char* s = "A constant string put in section .rodata at link-time";
// Now use 's' as the argument instead of the string literal
f(s);
}
void f(const char* s){ }
This effectively changes the generated assembly code to:
/APP
.code16gcc
.section .rodata
.LC0:
.string "A constant string put in section .rodata at link-time"
main:
.LFB0:
/* here would be main's prologue, not put because it ain't relevant */
// THIS IS THE CALL f(s);
mov DWORD PTR [ebp-12], OFFSET FLAT:.LC0 // now specifically loaded in the DWORD 's'
sub esp, 12
push DWORD PTR [ebp-12]
call _Z1fPKc
As it can be seen, the local variable is used now instead, the address to the string literal (in .rodata) being transferred specifically in a DWORD. This effectively avoids the linker error, although it uses some neglijable extra stack space.