lseek与truncate
1.lseek function declaration
1 #include <sys/types.h>
2 #include <unistd.h>
3
4 off_t lseek(int fd, off_t offset, int whence);
Function usage instructions:
Parameter 1: File descriptor
Parameter 2: The position where the cursor needs to move, positive numbers move backward, negative numbers move forward
Parameter 3: Corresponding to three macros
1、SEEK_CUR == 0
2、SEEK_CUR == 1
3、SEEK_END == 2
Return value: The current cursor position is returned successfully, and -1 is returned if it fails, and the failure information will be saved in errno.
File extension function implementation code:
1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <sys/stat.h>
4 #include <fcntl.h>
5 #include <stdlib.h>
6 #include <string.h>
7 #include <unistd.h>
8
9 int main(void)
10 {
11 int fd = open("buf.txt", O_RDWR);
12 if (fd == -1)
13 {
14 perror("open");
15 exit(- 1 );
16 }
17
18 int len = lseek(fd, 10 , SEEK_END);
19
20 // If a write operation is not performed, the expansion will not succeed
21 write(fd, " 1 " , 1 );
22
23 return 0 ;
24 }
2. truncate function declaration
1 #include <unistd.h>
2 #include <sys/types.h>
3
4 int truncate(const char *path, off_t length);
5 int ftruncate(int fd, off_t length);
Function usage instructions:
truncate：
Parameter 1: file name
Parameter 2: The final size of the file
ftruncate
Parameter 1: file descriptor
Parameter 2: The final size of the file
Return value: The current cursor position is returned successfully, and -1 is returned if it fails, and the failure information will be saved in errno.
File extension function implementation code:
1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <stdlib.h>
4 #include <unistd.h>
5
6 int main(void)
7 {
8 int fd = open("buf.txt", O_RDWR);
9 if (fd == -1)
10 {
11 perror("open");
12 exit(-1);
13 }
14
15 ftruncate(fd, 100);
16 //truncate("buf.txt", 100);
17
18 return 0;
19 }