Linux-Function-truncate;

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#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函數由參數path命名或者由參數fd指向的有規律的檔案截取到參數length指定大小的位元組*/

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‘).

/*如果原檔案比指定大小要大，則超出的資料會丟失。如果原檔案如果小些，則將被擴大，擴大的部分讀取為空白字元*/

The file offset is not changed.

/*檔案指標不會改變*/

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.

/*如果大小改變，檔案的st_ctime和st_time成員將更新，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。失敗返回-1並且設定適當的錯誤號碼errno*/

With ftruncate(), the file must be open for writing; with truncate. the file must be writable.

/*要可寫，檔案的open模式必須包含寫標記*/

Simple example:

int main()

{

　　struct stat st;

　　int fd = open("b.txt",O_CREAT|O_RDWR);

　　//或者O_WRONLY,此時新檔案大小為0

　　ftruncate(fd,10);

　　//將檔案擴充為十個位元組

　　/*此處可以添加寫入代碼*/

　　fstat(fd,&st);

　　printf("Size of file is %d", st.st_size);

　　close(fd);

　　truncate("b.txt",5);

　　//將檔案截取為5個位元組大小，多餘的5個位元組將被捨棄

　　stat("b.txt",&st);

　　printf("Size of file is %d", st.st_size);

　　return 0;

}

Linux-Function-truncate;