> Ah, OK, this is becuase mmap doesn't actually set up the page tables
> by default. Try adding MAP_POPULATE to the flags.
Here's an updated version of the program, with an added a '-r' option,
that performs a read access on the mapping before doing the write
(basically equivalent to MAP_POPULATE, but portable).
Please try this on a tmpfs file.
Thanks,
Miklos
---
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/wait.h>
static const char *filename;
static int msync_flag = MS_ASYNC;
static int msync_fork = 0;
static int msync_read = 0;
static void print_times(const char *msg)
{
struct stat stbuf;
stat(filename, &stbuf);
printf("%s\t%li\t%li\t%li\n", msg, stbuf.st_ctime, stbuf.st_mtime,
stbuf.st_atime);
}
static void do_msync(void *addr, int len)
{
int res;
if (!msync_fork) {
res = msync(addr, len, msync_flag);
if (res == -1) {
perror("msync");
exit(1);
}
} else {
int pid = fork();
if (pid == -1) {
perror("fork");
exit(1);
}
if (!pid) {
int fd = open(filename, O_RDWR);
if (fd == -1) {
perror("open");
exit(1);
}
addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
perror("mmap");
exit(1);
}
res = msync(addr, len, msync_flag);
if (res == -1) {
perror("msync");
exit(1);
}
exit(0);
}
wait(NULL);
}
}
static void usage(const char *progname)
{
fprintf(stderr, "usage: %s filename [-sfr]\n", progname);
fprintf(stderr, " -s: use MS_SYNC instead of MS_ASYNC\n");
fprintf(stderr, " -f: fork and perform msync in a different process\n");
fprintf(stderr, " -r: do a read access before each write access\n");
exit(1);
}
int main(int argc, char *argv[])
{
int res;
char *addr;
char tmp[32];
int fd;
if (argc < 2)
usage(argv[0]);
filename = argv[1];
if (argc > 2) {
char *s;
if (argc > 3)
usage(argv[0]);
s = argv[2];
if (s[0] != '-' || !s[1])
usage(argv[0]);
for (s++; *s; s++) {
switch (*s) {
case 's':
msync_flag = MS_SYNC;
break;
case 'f':
msync_fork = 1;
break;
case 'r':
msync_read = 1;
break;
default:
usage(argv[0]);
}
}
}
fd = open(filename, O_RDWR | O_TRUNC | O_CREAT, 0666);
if (fd == -1) {
perror(filename);
return 1;
}
print_times("begin");
sleep(1);
write(fd, "wxyz\n", 4);
print_times("write");
sleep(1);
addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
perror("mmap");
return 1;
}
print_times("mmap");
sleep(1);
if (msync_read) {
sprintf(tmp, "fetch %c", addr[1]);
print_times(tmp);
sleep(1);
}
addr[1] = 'b';
print_times("store b");
sleep(1);
do_msync(addr, 4);
print_times("msync");
sleep(1);
if (msync_read) {
sprintf(tmp, "fetch %c", addr[2]);
print_times(tmp);
sleep(1);
}
addr[2] = 'c';
print_times("store c");
sleep(1);
do_msync(addr, 4);
print_times("msync");
sleep(1);
if (msync_read) {
sprintf(tmp, "fetch %c", addr[3]);
print_times(tmp);
sleep(1);
}
addr[3] = 'd';
print_times("store d");
sleep(1);
res = munmap(addr, 4);
if (res == -1) {
perror("munmap");
return 1;
}
print_times("munmap");
sleep(1);
res = close(fd);
if (res == -1) {
perror("close");
return 1;
}
print_times("close");
sleep(1);
sync();
print_times("sync");
return 0;
}
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