C language lseek() function: move the read and write location of the file

Correlation function ：dup, open, fseek

The header file ：#include <sys/types.h>    #include <unistd.h>

Defined function ：off_t lseek(int fildes, off_t offset, int whence);

Function description ：
Every open file has a read / write location , When a file is opened, its read / write position usually points to the beginning of the file , If you open the file as an attachment ( Such as O_APPEND), Then the read / write position will point to the end of the file . When read() or write() when , The reading and writing positions will increase ,lseek() It is used to control the reading and writing position of the file . Parameters fildes Describe a word for an open file , Parameters offset Is based on the parameter whence To move the displacement of the read-write position .

Parameters whence It's one of the following :

• SEEK_SET Parameters offset It's the new read-write location .
• SEEK_CUR Increase from the current reading and writing position offset Displacement .
• SEEK_END Point the read / write position to the end of the file and add offset Displacement . When whence The value is SEEK_CUR or SEEK_END when , Parameters offet Allow negative values .

Here are some special ways to use :

1. To move the read / write position to the beginning of the file :lseek(int fildes, 0, SEEK_SET);
2. To move the read / write position to the end of the file :lseek(int fildes, 0, SEEK_END);
3. When you want to get the current file location :lseek(int fildes, 0, SEEK_CUR);

Return value ： When the call is successful, the current read / write position will be returned , That is, how many bytes from the beginning of the file . If there is an error, return -1, errno Will store error codes .

Additional notes ：Linux System not allowed lseek() Yes tty Device function , This action will make lseek() return ESPIPE.