As programmers we generally use malloc() , free() and similar functions in order to allocate memory. They are provided by glibc() library. The actual work is done by mmap() and munmap()which is a Linux systemcall.

What Does mmap() Function?

mmap() function or system call will create a mapping in the virtual meory of the current process.The address space consist of multiple pages and each page can be mapped some resource. We can create this mapping for a resources we want to use.

Library

mmap() and munmap() functions are provided by sys/mman.h library. so in order to use we need to include them like below.

#include <sys/mman.h>

Syntax

As mmap() provides flexible memory mapping it has a lot of parameters to use.

void *mmap(void *addr, size_t lengthint " prot ", int " flags ,
           int fd, off_t offset)

• void *addr is the address we want to start mapping
• size_t lengthint is the size we want to map in as integer
• PROT_READ|PROT_WRITE|PROT_EXEC options about page
• MAP_ANON|MAP_PRIVATE options about page

Memory Mapping Types

We have two option about memory mapping for sharing.

• MAP_SHARED will map given page and this will be also visible by other processes.
• MAP_PRIVATE will map given page and this will not visible to other processes.

Example

Here is an example which takes a page from start of 2^20 . The default size of the page is 4096 byte so we will map a page with 4096 byte memory.

#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

int main(void) {
  size_t pagesize = getpagesize();

  printf("System page size: %zu bytes\n", pagesize);

  char * region = mmap(
    (void*) (pagesize * (1 << 20)),   // Map from the start of the 2^20th page
    pagesize,                         // for one page length
    PROT_READ|PROT_WRITE|PROT_EXEC,
    MAP_ANON|MAP_PRIVATE,             // to a private block of hardware memory
    0,
    0
  );
  if (region == MAP_FAILED) {
    perror("Could not mmap");
    return 1;
  }

  strcpy(region, "Hello, poftut.com");

  printf("Contents of region: %s\n", region);

  int unmap_result = munmap(region, 1 << 10);
  if (unmap_result != 0) {
    perror("Could not munmap");
    return 1;
  }
  // getpagesize
  return 0;
}

When we compile with the following command the a.out executable will be generated.

$ gcc main.c