mmap函数和sysconf函数使用页偏移例子--linux函数

#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)           /* To obtain file size */
handle_error("fstat");

offset = atoi(argv[2]);
pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
/* offset for mmap() must be page aligned */

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;
/* Can't display bytes past end of file */

} else {    /* No length arg ==> display to end of file */
length = sb.st_size - offset;
}

addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
MAP_PRIVATE, fd, pa_offset);
printf("addr is 0x%08x\n\n\n",&addr);
if (addr == MAP_FAILED)
handle_error("mmap");

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);
}

exit(EXIT_SUCCESS);
} /* main */