linux - GCC: putchar(char) in inline assembly

Question

Overflow,

how can I implement the putchar(char) procedure using inline assembly only? I would like to do this in x86-64 assembly. The reason for me doing this is to implement my own standard-lib (or at least part of it). Here is what I have so far:

void putchar(char c)
{   
    /* your code here: print character c on stdout */
    asm(...);
}   

void _start()
{   
    /* exit system call */
    asm("mov $1,%rax;"
        "xor %rbx,%rbx;"
        "int  $0x80"
    );  
}

I am compiling with:

gcc -nostdlib -o putchar putchar.c

Thanks for helping me!

Answer 1

Here's an example my_putchar in GCC x86-64 inline assembly (in Intel syntax, converting to AT&T should be trivial).

Compiles with:

gcc -ggdb -masm=intel -o gcc_asm_putchar gcc_asm_putchar.c

Edit: rdi was missing from clobbered registers. Fixed.

Here's the code:

int main(void)
{
    char my_char;

    for (my_char = 'a'; my_char <= 'z'; my_char++)
            my_putchar(my_char);

    my_char = '
';
    my_putchar(my_char);
    return 0;
}

void my_putchar(char my_char)
{
    int dword_char;
    dword_char = (int)my_char;
    asm volatile(
                    ".intel_syntax noprefix;"
                    "mov r10,rsp;"   // save rsp.
                    "sub rsp,8;"     // space for buffer, align by 8.
                    "mov [rsp],al;"  // store the character into buffer.
                    "mov edi,1;"     // STDOUT.
                    "mov rsi,rsp;"   // pointer to buffer.
                    "mov edx,1;"     // string length in bytes.
                    "mov eax,1;"     // WRITE.
                    "syscall;"       // clobbers rcx & r11.
                    "mov rsp,r10;"   // restore rsp.
                    ".att_syntax prefix;"
                    /* outputs */
                    :
                    /* inputs: eax */
                    : "a"(dword_char)
                    /* clobbered regs */
                    : "rcx", "rdx", "rsi", "rdi", "r10", "r11"
                );
}