snprintf函数的使用（Windows和Linux下的区别）[转] - Something TO DO
snprintf函数的使用（Windows和Linux下的区别）[转]
在C语言中，printf表示把字符串格式化输出到终端。sprintf表示把字符串格式化输出到另一个字符串。而snprintf表示把字符串格式化之后，输出指定位数到新的字符串。
我们看下面一段代码。
#include <stdio.h>
#define COPY_LENGTH 5
int main() {
char buff[100] = {0};
int result = snprintf(buff, COPY_LENGTH, "ABCDEF");
printf("%d, %s\n", result, buff);
}
我在Windows环境编译执行（我的编译环境是：Win7x64 ，gcc 4.8.3），执行结果为-1， ABCDE。
然后把这段代码放到Ubuntu里编译执行（我的编译环境是：Ubuntu Desktop 10.04，gcc 4.8.2），执行结果为6, ABCD。
在网上找了一下原因，在Linux C语言里其定义应该如下：
int snprintf(char *restrict buf, size_t n, const char * restrict format, ...);
函数说明:最多从源串中拷贝 n － 1 个字符到目标串中，然后再在后面加一个 0 。所以如果目标串的大小为 n 的话，将不会溢出。
函数返回值:若成功则返回欲写入的字符串长度，若出错则返回负值。
但在Windows平台，用snprintf只是引用_snprintf函数而已。_snprintf的定义如下：
int _snprintf(char *buffer, size_t count, const char *format [, argument] ...);
函数说明：
如果格式化后的字符串（源串）长度小于或者等于count，则拷贝源串到buffer，并返回源串长度。
如果格式化后的字符串（源串）长度大于count，则拷贝源串的前count个字符到buffer，并返回一个负数。
所以说，_snprintf函数最多可以比源串中拷贝 n 个字符到目标串中，有可能不会在后面加0。所以会出现上面的问题。如果想要使两者相同，可以在代码中直接把最后一位设置成0，这样两个平台就是一样的结果了。当然在实际应用中，可能根据要实现的功能，如何设置要自己判断。
比如，如果只是把最后一位设置为0，上面的代码可以改成：
#include <stdio.h>
#define COPY_LENGTH 5
int main() {
char buff[100] = {0};
snprintf(buff, COPY_LENGTH, "ABCDE");
buff[COPY_LENGTH - 1] = 0;
printf("%s\n", buff);
}
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