c - 当使用 NASM extern 语句访问 printf 时，GCC 输出错误 "undefined reference to ` printf'"

Question

我正在学习 NASM 并试图编译这段代码(我找到了 here)。它使用这个 NASM 命令进行组装:

nasm -f coff -l printf.lst  printf1.asm

到 printf.o 但是这个 gcc 链接命令:

gcc -o printf1  printf1.o

因错误而失败:

printf1.o:printf1.asm:(.text+0x1a): undefined reference to `printf'
collect2: ld returned 1 exit status

我做错了什么？提前致谢。 (编辑:我在 Windows 7 上)；

; printf1.asm   print an integer from storage and from a register
; Assemble: nasm -f coff -l printf.lst  printf1.asm
; Link:     gcc -o printf1  printf1.o
; Run:      printf1
; Output:   a=5, eax=7

; Equivalent C code
; /* printf1.c  print an int and an expression */
; #include 
; int main()
; {
;   int a=5;
;   printf("a=%d, eax=%d\n", a, a+2);
;   return 0;
; }

; Declare some external functions
;
        extern  printf      ; the C function, to be called

section .data           ; Data section, initialized variables

        a: dd   5           ; int a=5;
        fmt: db "a=%d, eax=%d", 10, 0 ; The printf format, "\n",'0'


section .text               ; Code section.

        global _main        ; the standard gcc entry point
_main:              ; the program label for the entry point
        push    ebp     ; set up stack frame
        mov     ebp,esp

    mov eax, [a]    ; put a from store into register
    add eax, 2      ; a+2
    push    eax     ; value of a+2
        push    dword [a]   ; value of variable a
        push    dword fmt   ; address of ctrl string
        call    printf      ; Call C function
        add     esp, 12     ; pop stack 3 push times 4 bytes

        mov     esp, ebp    ; takedown stack frame
        pop     ebp     ; same as "leave" op

    mov eax,0       ;  normal, no error, return value
    ret         ; return

Answer 1

我相信您需要使 _printf 符合 C 调用约定(就像您使用 _main 而不是 main 一样)。