從文件中讀取最後20個字節

Question

int fd, read_byte; 
char *c; 

fd = open("foo.txt", O_RDONLY); 

read_byte = read(fd, c, 20); 

printf(""); 

如何從文件中讀取最後20個字節並將read_byte打印到屏幕。

Answer 1

使用lseek(2)

#include <stdio.h> 
#include <unistd.h> 
#include <sys/types.h> 
#include <sys/stat.h> 
#include <fcntl.h> 
#include <string.h> 

int main() 
{ 
    int fd, read_byte; 
    char c[21]; 

    fd = open("foo.txt", O_RDONLY); 
    if (fd == -1) { 
     printf("Error opening file\n"); 
     return -1; 
    } 

    // reposition fd to position `-20` from the end of file. 
    lseek(fd, -20L, SEEK_END); 
    read_byte = read(fd, c, 20); // Read 20 bytes 
    c[read_byte] = '\0'; 

    printf("%s\n", c); 

    close(fd); 

    return 0; 
}