truncate and ftruncate Functions

truncate, ftruncate - truncate a file to a specified length

Sometimes we would like to truncate a file by chopping off(砍去) data at the end of the file.
Emptying a file, which we can do with the O_TRUNC flag to open, is a special case of truncation.

函数原型如下：

#include <unistd.h>
#include <sys/types.h>

int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
//Both Returns： 0 if OK， -1 on error

将会将文件截断到length字节。如果length大于先前的长度，则两长度之间的数据无法访问。如果先前长度小于length，则会将文件尺寸扩大到length，在old end 和 new end之间的数据在read时作为0.

We use ftruncate in the program shown in Figure 13.6 when we need to empty a file after obtaining a lock on the file.