linux c怎么实现从文件的最后一行一行向前读文件

下面的例子使用mmap读最后20行(假设最后20行不会超过1024字节)
/*-
* Copyright (C), 1988-2014, mymtom
*
* vi:set ts=4 sw=4:
*/
#ifndef lint
static const char rcsid[] = "$Id$";
#endif /* not lint */
/**
* @file last20.c
* @brief
*/
#include
#include
#include
#include
#include
#include
#include
char *memchrr(const void *v1, const char *v2, int c)
{
char *s1, *s2;
char *p;
s1 = (char *)v1;
s2 = (char *)v2;
for (p = s2; p >= s1; --p) {
if (*p == c)
return p;
}
return NULL;
}
#define READSIZE 1024
int main(int argc, char *argv[])
{
int ret;
FILE *fp;
char *addr;
size_t len;
int prot;
int flags;
int fd;
off_t off;
off_t rem;
long pagesize;
struct stat buf;
pagesize = sysconf(_SC_PAGESIZE);
fp = fopen("last20.c", "rb");
fd = fileno(fp);
ret = fstat(fd, &buf);
if (buf.st_size <= READSIZE || buf.st_size <= pagesize) {
off = 0;
len = buf.st_size;
} else {
off = buf.st_size - READSIZE;
rem = off % pagesize;
off = off - rem;
len = READSIZE + rem;
}
/*
printf("size=%d READSIZE=%d off=%d len=%d\n",
(int)buf.st_size, (int)READSIZE, (int)off, (int)len);
*/
prot = PROT_READ;
flags = MAP_PRIVATE;
addr = mmap(NULL, len, prot, flags, fd, off);
fclose(fp);
{
int i, n;
char *head, *tail;
size_t size;
char line[1024];
tail = addr + len - 1;
n = 20;
for (i = 0; i < n; ++i) {
head = memchrr(addr, tail - 1, '\n');
if (head == NULL) {
size = tail - addr;
memcpy(line, addr, size);
line[size] = '\0';
} else {
size = tail - head - 1;
memcpy(line, head + 1, size);
line[size] = '\0';
tail = head;
}
printf("%s\n", line);
if (head == NULL) {
break;
}
}
}
munmap(addr, len);
return 0;
}
运行结果为：
./last20 | tac | cat -n
line[size] = '\0';
} else {
size = tail - head - 1;
memcpy(line, head + 1, size);
line[size] = '\0';
tail = head;
}
printf("%s\n", line);
if (head == NULL) {
break;
}
}
}
munmap(addr, len);
return 0;
}