#include <unistd.h>
#include <sys/types>
int truncate(const char *path, off_t length);
int ftruncate(int fd,off_t length);
The truncate and ftruncate function cause the regular file named by path or referenced by fd to be truncated to a size of precisely
length bytes.
/*truncate 和 ftruncate函数由参数path命名或者由参数fd指向的有规律的文件截取到参数length指定大小的字节*/
If the file previously was larger then this size , the extra data is lost . If the file previously was shorter , it is extended, and the extended
part reads as null bytes(‘\0‘).
/*如果原文件比指定大小要大，则超出的数据会丢失。如果原文件如果小些，则将被扩大，扩大的部分读取为空字符*/
The file offset is not changed.
/*文件指针不会改变*/
If the size changed , then the st_ctime and st_time fields for the file are update ,and the set-usr-ID and set-group-ID permission bits may
be cleared.
/*如果大小改变，文件的st_ctime和st_time成员将更新，set-usr-ID和set-group-ID位可能被清除*/
On success zero is returned , On error -1 is returned and errno is set appropriately.
/*成功返回0。失败返回-1并且设置适当的错误号errno*/
With ftruncate(), the file must be open for writing; with truncate. the file must be writable.
/*要可写，文件的open模式必须包含写标记*/
Simple example:
int main()
{
struct stat st;
int fd = open("b.txt",O_CREAT|O_RDWR);
//或者O_WRONLY,此时新文件大小为0
ftruncate(fd,10);
//将文件扩展为十个字节
/*此处可以添加写入代码*/
fstat(fd,&st);
printf("Size of file is %d", st.st_size);
close(fd);
truncate("b.txt",5);
//将文件截取为5个字节大小，多余的5个字节将被舍弃
stat("b.txt",&st);
printf("Size of file is %d", st.st_size);
return 0;
}