invalid output constraint in asm

I am trying to make an operating system in C. I get these errors when i compile the scripts:

gcc -m32 -nostdlib -nodefaultlibs -Wall -Wextra -ffreestanding -O2 -Iinclude -c src/io.c -o src/io.osrc/io.c:8:44: error: invalid input constraint 'a' in asm    __asm__ __volatile__("outb %0, %1" : : "a"(value), "Nd"(port));                                           ^src/io.c:13:41: error: invalid output constraint '=a' in asm    __asm__ __volatile__("inb %1, %0" : "=a"(ret) : "Nd"(port));                                        ^2 errors generated.make: *** [src/io.o] Error 1

This is my full code for it:

#include "io.h"#include <stdarg.h>// Function declarationvoid itoa(char* buf, int value, int base);void outb(unsigned short port, unsigned char value) {    __asm__ __volatile__("outb %0, %1" : : "a"(value), "Nd"(port));}unsigned char inb(unsigned short port) {    unsigned char ret;    __asm__ __volatile__("inb %1, %0" : "=a"(ret) : "Nd"(port));    return ret;}void printf(const char* format, ...) {    char** arg = (char**)&format;    int c;    char buf[20];    arg++;    while ((c = *format++) != 0) {        if (c != '%') {            outb(0xE9, c);        } else {            char* p;            c = *format++;            switch (c) {            case 'd':            case 'u':            case 'x':                itoa(buf, *((int*)arg++), c == 'd' ? 10 : (c == 'x' ? 16 : 10));                p = buf;                goto string;                break;            case 's':                p = *arg++;                if (!p) {                    p = "(null)";                }string:                while (*p) {                    outb(0xE9, *p++);                }                break;            default:                outb(0xE9, '%');                outb(0xE9, c);                break;            }        }    }}void gets(char* buffer) {    char* buf_ptr = buffer;    while (1) {        char c = inb(0x60);  // Read keyboard input        if (c == '\n') {            *buf_ptr = '\0';            return;        } else {            *buf_ptr++ = c;            outb(0xE9, c); // Echo character        }    }}// Definition of itoavoid itoa(char* buf, int value, int base) {    char* ptr = buf;    char* ptr1 = buf;    char tmp_char;    int tmp_value;    if (base < 2 || base > 36) {        *buf = '\0';        return;    }    do {        tmp_value = value;        value /= base;        *ptr++ = "zyxwvutsrqponmlkjihgfedcba9876543210123456789abcdefghijklmnopqrstuvwxyz"                 [35 + (tmp_value - value * base)];    } while (value);    if (tmp_value < 0 && base == 10) *ptr++ = '-';    *ptr-- = '\0';    while (ptr1 < ptr) {        tmp_char = *ptr;        *ptr-- = *ptr1;        *ptr1++ = tmp_char;    }}

I tried multiple diffrent constraints but it didn't work. I am making the kernel here and compiling it to kernel.bin

I have an makefile which runs the commands when i run "make":

C_SOURCES = $(wildcard src/*.c)OBJS = ${C_SOURCES:.c=.o}CC = gccCFLAGS = -m32 -nostdlib -nodefaultlibs -Wall -Wextra -ffreestanding -O2 -Iincludeall: kernel.binkernel.bin: $(OBJS)    ld -m elf_i386 -T linker.ld -o kernel.bin $(OBJS)%.o: %.c    $(CC) $(CFLAGS) -c $< -o $@clean:    rm -f src/*.o kernel.bin