On Sun, Aug 27, 2006 at 12:37:52PM +1000, Kandan Venkataraman wrote:
> Hi
> I want to track any pages that have been written to a mmap file using a
> segv handler.
>
> I have given an example program below to illustrate my idea.
>
>
> What I want to know is whether the modifications to the mmap file will
> be guaranteed to be unaffected by the interruption of the signal
> handler, i.e. would the resultant changes in the mmap file be as if the
> mmap file had write access set.
>
>
> It can also be assumped that only the current process would be writing
> to the mmap file.
>
>
> Thanks
> Kandan
>
Can't you accomplish the same goal, in a much more portable way using inotify?
Neil
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <unistd.h>
> #include <sys/wait.h>
> #include <sys/stat.h>
> #include <sys/mman.h>
> #include <fcntl.h>
> #include <string.h>
> #include <assert.h>
> #include <signal.h>
>
>
> typedef struct TwoInts_tag {
> int i;
> int j;
>
>
>
> } TwoInts;
>
>
> const int maxPages = 10;
>
> char* start = 0;
> int fd;
> TwoInts *array = 0;
> int pageSize;
> int max;
> int elemsPerPage;
>
>
> void memory_fault(int signum, siginfo_t *info, void *ptr)
> {
> char *faultAddr = (char *)info->si_addr;
>
>
> /* we only handle the faults within our known range , anything else
> is really an illegal access */
> if (faultAddr >= start && faultAddr < start + (pageSize * maxPages))
> {
>
>
> int faultPageIdx = (faultAddr - start)/pageSize;
>
>
> fprintf(stderr, "fault at page index %d\n", faultPageIdx);
>
>
> if (mprotect(start + (faultPageIdx * pageSize), pageSize,
> PROT_READ | PROT_WRITE)) {
> perror("mprotect failed");
> abort();
> }
> }
> else
> abort();
>
>
>
> }
>
>
> int main()
> {
> struct sigaction act;
>
> act.sa_sigaction = memory_fault;
> act.sa_flags = SA_SIGINFO;
> sigemptyset (&act.sa_mask);
>
>
> int status = sigaction (SIGSEGV, &act, NULL);
> if (status) {
> perror ("sigaction");
> exit (1);
> }
>
>
> pageSize = getpagesize();
>
>
> elemsPerPage = pageSize/sizeof(TwoInts);
>
>
> max = elemsPerPage * maxPages;
>
>
> fprintf(stderr, "max elements is %d\n", max);
>
>
> assert(pageSize % sizeof(TwoInts) == 0);
>
>
> /* open/create the file */
> if ((fd = open ("kandan", O_RDWR | O_CREAT | O_TRUNC, S_IRWXU)) < 0)
> {
> fprintf(stderr, "can't create file for writing");
> exit(1);
> }
>
>
> /* set the lenght of file */
> if (ftruncate(fd, maxPages * pageSize) == -1) {
> fprintf(stderr, "error on ftruncate\n");
> exit(1);
> }
>
>
> /* mmap the file with read access*/
>
>
> if ((start = mmap(0, maxPages * pageSize, PROT_READ, MAP_SHARED, fd,
> 0)) == MAP_FAILED) {
>
>
> perror("mmap error");
> exit(1);
> }
>
>
> array = (TwoInts *)start;
>
>
> array[0].i = 5;
>
>
> fprintf(stderr, "value = %d\n", array[0].i);
>
>
> array[0].i = 7;
>
>
> fprintf(stderr, "value = %d\n", array[0].i);
>
>
> array[1].i = 9;
>
>
> fprintf(stderr, "value = %d\n", array[1].i);
>
>
> array[elemsPerPage].i = 14;
>
>
> fprintf(stderr, "value = %d\n", array[elemsPerPage].i);
>
>
> close(fd);
>
>
> return 0;
>
>
> }
> -
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--
/***************************************************
*Neil Horman
*Software Engineer
*gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
***************************************************/
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