Thread: Can't I "munmap" a part of a given memory block?

I don't know if this is the right place to ask this (I suppose some people can consider this lower level and not strictly related to C or C++). I have allocated a memory block directly with "mmap" and while I try to unmap ("munmap" system call) a part of it, it returns the -1 code! So I don't even know what's the problem. Any ideas? And any ways to bypass this?

Example program:

Code:

#include "stdio.h"
#include "sys/mman.h"


 #define null         (void*)0
#define DEF_PROTECT  PROT_READ | PROT_WRITE
#define DEF_FLAGS    MAP_PRIVATE | MAP_ANON


 void first_ten(void* ptr) {
  printf("%d\n", munmap(ptr, 10));
}


 void last_ten(void* ptr) {
  printf("%d\n", munmap(ptr + 10, 10));
}


 int main() {
  void* mem = mmap(null, 20, DEF_PROTECT, DEF_FLAGS, 0, 0);

   /* Unmapping the first 10 bytes work */
  first_ten(mem);
   /* Unmapping the last 10 bytes DOES NOT work */
  last_ten(mem);
}

Originally Posted by Salem

Why don't you also print errno to find out the reason for the error?


10 bytes into a block is not going to be page aligned.

I'm learning how the OS manages memory so I can hopefully understand. The code for errno is "22" which is invalid argument so you are right. This will mess up my library in the end, lol!
Once again, thank you for your help and have a great day!