Is it possible to use “munmap” without knowing the length of data which was originally mapped?
I’m porting code and have had to change "malloc"s and "free"s to “mmap” and “munmap” to ensure that the cache isn’t used. (Otherwise I get problems with a DMA transfer). My problem is that the pointer to the memory is easily available but the length isn’t so readily available because it wasn’t required for the “free” command.
Is there a default value which i can give for the length in the “munmap” command which ensures that all the mapped memory associated with this pointer is unmapped?
There is a simple solution. Replace malloc/free with something like my_malloc and my_free.
void * my_malloc( size_t size )
{
int *pt;
pt = mmap ( …, size + sizeof ( size ) );
*pt = size;
return &pt[1];
}
my_free(void *pointer)
{
size_t size;
int *pt = pointer;
--pt;
size = *pt;
munmap ( pt, size );
}
The idea is to store the size in the allocated memory area but return a pointer that is just after the memory area that holds the size. I prefer to put it at the beginning instead of at the end because it’s less likely to be overwritten
If Dma would be a problem. either make the header big enough to create a dummy space that keeps the aligment inline with the harware requirement or write the size at the end. But as I said the end is bad…
