C vfprintf函數不能與字符串參數一起使用

Question

我試圖用以下方式使用vfprintf
vfprintf（fp，buffer，arg）;在沒有找到vfprintf來源：我創建一個使用運行的日期和時間，緩衝區和ARGS

fp = fopen(filename, "a"); 
va_start(arg, message); 
vprintf(message, arg); // print stdout 
vfprintf(fp, buffer, arg); // print to file 
va_end(arg); 
fclose(fp); 

它完全用數字和它的錯誤可怕的死亡其餘 FP日誌文件。 ...

有沒有人有任何想法我做錯了任何機會？

Answer 1

您可能正在使用64位體系結構，其中可變長度arg只能傳遞一次。爲了讓更多的傳球複製arg使用va_copy()：

va_list arg, arg2; 
va_start(arg, message); 
va_copy(arg2, arg); 
vprintf(message, arg); // print stdout 
va_end(arg); 
vfprintf(fp, buffer, arg2); // print to file 
va_end(arg2); 

從man va_arg：

The va_arg() macro expands to an expression that has the type and value of the next argument in the call. The argument ap is the va_list ap initialized by va_start(). Each call to va_arg() modifies ap so that the next call returns the next argument. The argument type is a type name specified so that the type of a pointer to an object that has the specified type can be obtained simply by adding a * to type.

...

If ap is passed to a function that uses va_arg(ap,type) then the value of ap is undefined after the return of that function.