mmap gives segmentation fault - pls help

thanks for the link but it is confusing. some of the expert advised that the whole file will be loaded and some advised that it will not be loaded.

A seg fault is not caused by the system not having enough memory.
A seg fault would be from something like trying to write to a location in memory that is not allowed (read only) or not initialized.
perhaps you are unmapping too early?

Can you show the code where you are setting the length?

i used gdb and i am getting seg fault at memcpy. the code is long, so i am putting parts of it
while(1)
{
      memcpy(&x,&a[x],sizeof(unsigned int));
}

size = s.st_size;
        a = (unsigned int*) mmap(NULL, size, PROT_WRITE|PROT_READ, MAP_SHARED, fp, 0);

how is the while loop terminating..

i assume.. you map the file first and used it in the loop.

If the address where file is mapped is 'a' and the size mapped is size, then only memory from a to (a+size - 1)  is valid. If you cross that , you will get seg fault. Ofcourse, the end of the mapped segment might be rounded up to a page boundary, but from the programs logic, it shouldnt cross a+size -1 while reading from the mmapped region...

so put a terminating condition on the loop.

Ah I see you are trying to write to 0, this will cause a seq fault on certain operating systems that implement mmap_min_addr protection using LSM http://en.wikipedia.org/wiki/Linux_Security_Modules.

while(1)
{
      memcpy(&x,&a[x],sizeof(unsigned int));
}


also &a[x] dont look correct to me here.... if x is very large.. you will very easily get seg fault

trying it  now..thakns a millon

Also put a check for the return value for mmap... make sure its not failing before proceeding further..