Two signals are associated with mapped regions:

SIGBUS

This signal is generated when a process attempts to access a region of a mapping that is no longer valid—for example, because the file was truncated after it was mapped.

SIGSEGV

This signal is generated when a process attempts to write to a region that is mapped read-only.

munmap()

Linux provides the munmap() system call for removing a mapping created with mmap():

  #include <sys/mman.h>

  int munmap (void *addr, size_t len);

A call to munmap() removes any mappings that contain pages located anywhere in the process address space starting at addr, which must be page-aligned, and continuing for len bytes. Once the mapping has been removed, the previously associated mem ory region is no longer valid, and further access attempts result in a SIGSEGV signal.

Normally, munmap() is passed the return value and the len parameter from a previous invocation of mmap() .

On success, munmap() returns 0 ; on failure, it returns
-1 , and errno is set appropriately. The only standard errno value is EINVAL , which specifies that one or more parameters were invalid.

As an example, the following snippet unmaps any memory regions with pages contained in the interval [addr,addr+len] :

  if (munmap (addr, len) == -1)
          perror ("munmap");

Please check back next week for the continuation of this article.