64bit - Program in x64 assembly modifying array passed from a C++ procedure in Linux does not work, though analogous solution worked for x86 -

i wrote program in x64 assembly replace string's lower-case letters stars. assembly procedure called c++ program , receives array of chars. similar logic applied x86 worked (the difference being registers used etc.), string remains unaltered when built x64. use debian linux , nasm.

section .text global  func  func:     push    rbp     mov rbp, rsp  ; zadanie jako takie     mov rax, qword [rbp+8]     loop:         cmp byte [rax], 97         jl increment         cmp byte [rax], 122         jg increment         mov byte [rax], 42         increment:         add rax, 1         cmp byte [rax], 0         jne loop       exit:     mov rax, 0          ;return 0     mov rsp, rbp     pop rbp     ret 

it called following c++ program:

#include <stdio.h> #define length 1024  extern "c" int func(char *a);  int main(void) {   int result;    char text[length];   printf( "write string\n" );   fgets( text, length -1, stdin );     printf("string: %s\n", text);   result=func(text);   printf("string: %s\n", text);    return 0; } 

if necessary, here's makefile:

cc=gcc asmbin=nasm  : asm cc link asm :      $(asmbin) -o func.o -f elf64 -l func.lst func.asm cc :     $(cc) -m64 -c -g -o0 main.cc link :     $(cc) -m64 -o test -lstdc++ main.o func.o clean :     rm *.o     rm test     rm errors.txt        rm func.lst 

additionally, resources porting x86 x64 appreciated.

the reason program not work different calling convention in x64. see: link reference. address of string array wasn't passed on stack, case of x86, stored in rdi register. therefore solution change instruction loading array address into:

mov rax, rdi