发现很多文章都没有把pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1) 详细计算过程写出来,自己专门计算了一遍,附在文末。本例为man mmap中的原样实例程序,加上了一些额外的无关代码及打印输出信息 让总字节数超过一页大小sysconf(_SC_PAGE_SIZE)以方便查看调试信息
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int main(int argc, char *argv[])
{
char *addr;
int fd;
struct stat sb;
off_t offset, pa_offset;
size_t length;
ssize_t s;
if (argc < 3 || argc > 4) {
fprintf(stderr, "%s file offset [length]\n", argv[0]);
exit(EXIT_FAILURE);
}
fd = open(argv[1], O_RDONLY);
if (fd == -1)
handle_error("open");
if (fstat(fd, &sb) == -1)
handle_error("fstat");
offset = atoi(argv[2]);
pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
if (offset >= sb.st_size) {
fprintf(stderr, "offset is past end of file\n");
exit(EXIT_FAILURE);
}
if (argc == 4) {
length = atoi(argv[3]);
if (offset + length > sb.st_size)
length = sb.st_size - offset;
} else {
length = sb.st_size - offset;
}
addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
MAP_PRIVATE, fd, pa_offset);
if (addr == MAP_FAILED)
handle_error("mmap");
printf("length = %ld\n", length);
printf("offset = %ld\n", offset);
printf("pa_offset = %ld\n", pa_offset);
printf("length + offset - pa_offset = %ld\n", length + offset - pa_offset);
s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
if (s != length) {
if (s == -1)
handle_error("write");
fprintf(stderr, "partial write");
exit(EXIT_FAILURE);
}
munmap(addr, length + offset - pa_offset);
close(fd);
exit(EXIT_SUCCESS);
}
编译命令:gcc mmap.c
程序执行格式: ./a.out file offset [length]
格式解析:./a.out +file 文件名 + offset偏移量 +length可选长度
./a.out mmap.c 10000
length = 2266
offset = 10000
pa_offset = 8192
假设文件mmap.c文件大小为12266 ,
length =文件总大小(12266) - 偏移量offset(1000) = 2266
4096(sysconf(_SC_PAGE_SIZE)一页大小)
0001 0000 0000 0000 = 4096(sysconf(_SC_PAGE_SIZE)一页大小)
0000 1111 1111 1111 = (sysconf(_SC_PAGE_SIZE) - 1) =4095
~(0000 1111 1111 1111)
= 1111 0000 0000 0000 = ~ (sysconf(_SC_PAGE_SIZE) - 1) =61440
pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1)
=10000 & 61440
=(0010 0111 0001 0000) & (1111 0000 0000 0000)
=(0010 0000 0000 0000)
=8192 //pa_offset即内存中的页偏移量
本文内容由网友自发贡献,版权归原作者所有,本站不承担相应法律责任。如您发现有涉嫌抄袭侵权的内容,请联系:hwhale#tublm.com(使用前将#替换为@)