C：不定长参数相关操作va_list，va_start，va_arg，va_end

 

不定长参数传递问题

void kprintf(char *fmt, ...) 
{
  char* p;

  va_list vl;
  
  va_start(vl, fmt);
  //_asm lea ax,word ptr [bp+6];
  //_asm mov di,ax;

  for (p = fmt; *p; p++)
  {
	char c = *p;
	if (c == '$') break;
    if (c == '%')
    {
	  //char v;

      char v = va_arg(vl, char);
	  //_asm add di,2;
	  //_asm mov al,byte ptr ss:[di-2];

	  //_asm mov v, al;
	  _int10_0e(v);
    }
	else 
	{
	  _int10_0e(c);
	}
  }
  
  va_end(vl);
}

 对应汇编代码：

000302ab: (                    ): push bp                   ; 55
000302ac: (                    ): mov bp, sp                ; 8bec
000302ae: (                    ): sub sp, 0x0002            ; 83ec02
000302b1: (                    ): push si                   ; 56
000302b2: (                    ): push di                   ; 57
000302b3: (                    ): lea ax, word ptr ss:[bp+6] ; 8d4606
000302b6: (                    ): mov di, ax                ; 8bf8
000302b8: (                    ): mov si, word ptr ss:[bp+4] ; 8b7604
000302bb: (                    ): jmp .+53                  ; eb35
000302bd: (                    ): mov al, byte ptr ds:[si]  ; 8a04
000302bf: (                    ): mov byte ptr ss:[bp-1], al ; 8846ff
000302c2: (                    ): cmp byte ptr ss:[bp-1], 0x24 ; 807eff24
000302c6: (                    ): jnz .+2                   ; 7502
000302c8: (                    ): jmp .+45                  ; eb2d
000302ca: (                    ): cmp byte ptr ss:[bp-1], 0x25 ; 807eff25
000302ce: (                    ): jnz .+23                  ; 7517
000302d0: (                    ): add di, 0x0002            ; 83c702
000302d3: (                    ): mov bx, di                ; 8bdf
000302d5: (                    ): mov al, byte ptr ds:[bx-2] ; 8a47fe
000302d8: (                    ): mov byte ptr ss:[bp-2], al ; 8846fe
000302db: (                    ): mov ah, 0x0e              ; b40e
000302dd: (                    ): mov al, byte ptr ss:[bp-2] ; 8a46fe
000302e0: (                    ): mov bx, 0x000f            ; bb0f00
000302e3: (                    ): int 0x10                  ; cd10
000302e5: (                    ): jmp .+10                  ; eb0a
000302e7: (                    ): mov ah, 0x0e              ; b40e
000302e9: (                    ): mov al, byte ptr ss:[bp-1] ; 8a46ff
000302ec: (                    ): mov bx, 0x000f            ; bb0f00
000302ef: (                    ): int 0x10                  ; cd10
000302f1: (                    ): inc si                    ; 46
000302f2: (                    ): cmp byte ptr ds:[si], 0x00 ; 803c00
000302f5: (                    ): jnz .-58                  ; 75c6
000302f7: (                    ): pop di                    ; 5f
000302f8: (                    ): pop si                    ; 5e
000302f9: (                    ): mov sp, bp                ; 8be5
000302fb: (                    ): pop bp                    ; 5d
000302fc: (                    ): ret                       ; c3

 另一个例子：

void args(char *prefix, ...)
{
  va_list vl;
  char c;

  va_start(vl, prefix);
  c = va_arg(vl, char);

  _asm mov ah, 0eh;
  _asm mov al, c;
  _asm mov bx, 0fh;
  _asm int 10h;

  va_end(vl);
}

 对应汇编代码：

0E3B:0291 55            PUSH    BP
0E3B:0292 8BEC          MOV     BP,SP
0E3B:0294 83EC02        SUB     SP,+02
0E3B:0297 56            PUSH    SI
0E3B:0298 8D4606        LEA     AX,[BP+06]
0E3B:029B 8BF0          MOV     SI,AX
0E3B:029D 83C602        ADD     SI,+02
0E3B:02A0 8A44FE        MOV     AL,[SI-02]
0E3B:02A3 8846FF        MOV     [BP-01],AL
0E3B:02A6 B40E          MOV     AH,0E
0E3B:02A8 8A46FF        MOV     AL,[BP-01]
0E3B:02AB BB0F00        MOV     BX,000F
0E3B:02AE CD10          INT     10
0E3B:02B0 5E            POP     SI
0E3B:02B1 8BE5          MOV     SP,BP
0E3B:02B3 5D            POP     BP
0E3B:02B4 C3            RET