大家熟知的C库函数printf函数就是一个可变参数函数，它是怎么实现的呢?不过他实现是有条件的，必须函数参数的入栈顺序为从右向左的顺序，也即函数的形参，在函数调用之前，必须是最右边的参数先入栈，并且参数都必须通过栈传递，以1个例子说明，如函数func(arg1, arg2,arg3),那么函数的堆栈应是：
ebp是帧指针寄存器，一般用来存取堆栈，有了堆栈结构，下面我们看看C可变参数的具体实现原理：
| #include <stdio.h> enum { ptChar, ptInt, ptFloat, ptDouble, }; void printSum(unsigned long paramFormat, ...) { /*高16位为可变参数类型，低16位为可变参数个数*/ int paramType = (paramFormat >> 16); int paramNum = paramFormat & 0xffff; /*¶mFormat = ebp + 8,第一个参数的地址*/ unsigned long *pArg = ¶mFormat; /*ebp + 0x0c, 第二个参数地址*/ pArg++; switch(paramType) { case ptChar: { int sum = 0; for (int i = 0; i < paramNum; i++) { char *pValue = (char *)pArg; sum += *pValue; pArg++; } printf("%d\n", sum); } break; case ptInt: { int sum = 0; for (int i = 0; i < paramNum; i++) { int *pValue = (int *)pArg; sum += *pValue; pArg++; } printf("%d\n", sum); } break; case ptFloat: { float sum = 0; /**/ pArg++; /*浮点参数，堆栈占8个字节，所以指针偏移为8*/ for (int i = 0; i < paramNum; i++) { float *pValue = (float *)pArg; sum += *pValue; pArg++; pArg++; } printf("%f\n", sum); } break; case ptDouble: { double sum = 0; /*双精度浮点参数，堆栈占8个字节，所以指针偏移为8*/ for (int i = 0; i < paramNum; i++) { double *pValue = (double *)pArg; sum += *pValue; pArg++; pArg++; } printf("%f\n", sum); } break; default: printf("unknowned type!\n"); break; } } void main() { unsigned long paramFormat = 3; char a = 1, b = 2, c = 3; printSum(paramFormat, a, b, c); paramFormat = ptInt << 16; paramFormat += 3; int ia = 1, ib = 2, ic = 3; printSum(paramFormat, ia, ib, ic); paramFormat = ptFloat << 16; paramFormat += 3; float fa = 1, fb = 2, fc = 3; printSum(paramFormat, fa, fb, fc); paramFormat = ptDouble << 16; paramFormat += 3; double da = 1, db = 2, dc = 3; printSum(paramFormat, da, db, dc); } |
