C库函数 int vfprintf(FILE *stream, const char *format, va_list arg)  发送格式化输出到一个流使用传递给它的参数列表。

声明

以下是vfprintf()函数的声明。

int vfprintf(FILE *stream, const char *format, va_list arg)

参数

• stream -- 这是一个文件对象的标识流的指针。

• format -- 这是C字符串，其中包含的文字写入流，它可以包含嵌入的格式在随后的附加参数指定的值所取代的标签和格式化的要求。格式标签原型： %[flags][width][.precision][length]specifier, 详细说明如下：

specifier	输出
c	Character.
d or i	Signed decimal integer
e	Scientific notation (mantissa/exponent) using e character
E	Scientific notation (mantissa/exponent) using E character
f	Decimal floating yiibai
g	Use the shorter of %e or %f.
G	Use the shorter of %E or %f
o	Signed octal
s	String of characters
u	Unsigned decimal integer
x	Unsigned hexadecimal integer
X	Unsigned hexadecimal integer (capital letters)
p	Yiibaier address
n	Nothing printed.
%	Character.

flags	描述
-	Left-justify within the given field width; Right justification is the default (see width sub-specifier).
+	Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign..
(space)	If no sign is going to be written, a blank space is inserted before the value.
#	Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero. Used with e, E and f, it forces the written output to contain a decimal yiibai even if no digits would follow. By default, if no digits follow, no decimal yiibai is written. Used with g or G the result is the same as with e or E but trailing zeros are not removed.
0	Left-pads the number with zeroes (0) instead of spaces, where padding is specified (see width sub-specifier).

width	描述
(number)	Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*	The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.

.precision	描述
.number	For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For e, E and f specifiers: this is the number of digits to be printed after de decimal yiibai. For g and G specifiers: This is the maximum number of significant digits to be printed. For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. For c type: it has no effect. When no precision is specified, the default is 1. If the period is specified without an explicit value for precision, 0 is assumed.
.*	The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.

length	描述
h	The argument is interpreted as a short int or unsigned short int (only applies to integer specifiers: i, d, o, u, x and X).
l	The argument is interpreted as a long int or unsigned long int for integer specifiers (i, d, o, u, x and X), and as a wide character or wide character string for specifiers c and s.
L	The argument is interpreted as a long double (only applies to floating yiibai specifiers: e, E, f, g and G).

• arg -- 可变参数列表中的对象。这应该va_start宏定义初始化在<stdarg>。

返回值

如果成功，返回写入的字符的总数，否则则返回一个负数。

例子

下面的例子演示了如何使用 vfprintf() 函数。

#include <stdio.h>
#include <stdarg.h>

void WriteFrmtd(FILE *stream, char *format, ...)
{
   va_list args;

   va_start(args, format);
   vfprintf(stream, format, args);
   va_end(args);
}

int main ()
{
   FILE *fp;

   fp = fopen("file.txt","w");

   WriteFrmtd(fp, "This is just one argument %d 
", 10);

   fclose(fp);
   
   return(0);
}

让我们编译和运行上面的程序，这将打开一个文件 file.txt 写在当前目录，并且将其写入以下内容：

This is just one argument 10