linux c截断文件

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
int main(int argc, char **argv)
{
        if ( argc != 4 )
        {
                printf("Usage %s file_name offset length\n", argv[0]);
                return -1;
        }
        off_t offset = atoi(argv[2]);
        off_t length = atoi(argv[3]);
        if ( offset < 0 || length < 0 )
        {
                return -1;
        }
        int fd = open(argv[1], O_RDWR);
        if ( fd < 0 )
        {
                perror("open");
                return -1;
        }
        off_t file_len = lseek(fd, 0, SEEK_END);
        lseek(fd, offset + length, SEEK_SET);
        char buff[1024];
        int n;
        while ( (n = read(fd, buff, 1024)) != 0 )
        {
                if ( n < 0 )
                {
                        if ( errno == EINTR )
                                continue ;
                        else
                                break;
                }
                lseek(fd, - (length + n), SEEK_CUR);
                write(fd, buff, n);
                lseek(fd, (length), SEEK_CUR);
        }
        ftruncate(fd, file_len - length);
        close(fd);
}