考慮下面的C程序:如何在Win64上使用varargs和C中的函數指針?
#include <stdio.h>
#include <stdarg.h>
typedef void (callptr)();
static void fixed(void *something, double val)
{
printf("%f\n", val);
}
static void dynamic(void *something, ...)
{
va_list args;
va_start(args, something);
double arg = va_arg(args, double);
printf("%f\n", arg);
}
int main()
{
double x = 1337.1337;
callptr *dynamic_func = (callptr *) &dynamic;
dynamic_func(NULL, x);
callptr *fixed_func = (callptr *) &fixed;
fixed_func(NULL, x);
printf("%f\n", x);
}
基本上,這個想法是存儲與可變參數的函數在一個「通用」函數指針。作爲比較,我還包含了另一個帶有固定參數列表的函數。現在看到在x86的Linux,AMD64的Linux,Win32和Win64中運行這個時候發生了什麼:
$ gcc -m32 -o test test.c
$ file test
test: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.9, not stripped
$ ./test
1337.133700
1337.133700
1337.133700
$ gcc -o test test.c
$ file test
test: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.9, not stripped
$ ./test
1337.133700
1337.133700
1337.133700
C:\>gcc -o test.exe test.c
C:\>file test.exe
test.exe: PE32 executable for MS Windows (console) Intel 80386 32-bit
C:\>test.exe
1337.133700
1337.133700
1337.133700
C:\>x86_64-w64-mingw32-gcc -o test.exe test.c
C:\>file test.exe
test.exe: PE32+ executable for MS Windows (console) Mono/.Net assembly
C:\>test.exe
0.000000
1337.133700
1337.133700
爲什麼動態功能得到Win64上的變量參數列表零值,而不是在任何其他配置的?這樣的事情甚至合法嗎?我認爲這是因爲編譯器沒有抱怨。
我敢肯定,這是不合法的;函數指針不能像這樣轉換。 –