How to convert string to int in assembly MSVC - c++

Question

So, I have a task that I need to convert from string for example string "542215" to int 542215 or with any other ASCII symbols. I am kindof new to assembly programming so, I have almost no clue to what I am doing with it, but I wrote my experimental code.
int main(int argc, char** argv)
{
int Letters;
char* argv1 = argv[1];
if (argc < 2) {
printf("Nepateiktas parametras*/\n");
return(0);
}
__asm {
push eax
push ebx
push ecx
xor ecx, ecx
mov eax, argv1
NextChar:
movzx eax, byte ptr [esi + ecx]
test eax, eax
jz Done
push ecx
sub eax, 48
push eax
call PrintIt
pop ecx
inc ecx
jmp NextChar
Done:
pop ecx
pop ebx
pop eax
PrintIt:
mov[Letters], eax
pop ecx
pop ebx
pop eax
};
printf("Count of letters in string %s is %d\n", argv[1], Letters);
return(0);
}
and I am getting error that "Run-time check failure #0 - The value of ESP was not properly saved across a function call. This is usually a result of calling a function declared with one calling convention with a function pointer declared with a different calling convention.
This error basicly gives me 0 ideas what is going on, or what even ESP is, so any help would be appreciated with my code.

Answer 1

you don't need to push/pop registers around your whole asm statement; MSVC already notices which registers you modify.
You're popping the registers twice at the end, when execution falls through from Done: to PrintIt. Or even worse, on strings of non-zero length, you're leaving the asm statement via a path of execution that does 3 pushes at the start, push ecx, push eax, call PrintInt, and then 3 pops. So one of your register-restores is actually reloading the return address pushed by call, and leaving 3 pushed values on the stack.
If you don't know that ESP is the stack pointer, one of x86's 8 general-purpose integer registers, you need to continue reading an intro to basic x86 asm before using stack instructions like push and call.
Also something about the fact that labels aren't the same thing as functions, and how execution continues to the next instruction.
And BTW, int->string will involve a multiply by 10. See NASM Assembly convert input to integer? for the standard algorithm and an asm implementation.