Chapter 13. Using mlock() system calls on RHEL for Real Time

Prerequisites

• You have root privileges or the CAP_IPC_LOCK capability to use mlockall() or mlock() on large buffers

Procedure

• The following code locks pages with mlock() system call:

  #include <stdlib.h>
  #include <unistd.h>
  #include <sys/mman.h>
  
  void *alloc_workbuf(size_t size)
  {
    void *ptr;
    int retval;
  
    // alloc memory aligned to a page, to prevent two mlock() in the same page.
    retval = posix_memalign(&ptr, (size_t) sysconf(_SC_PAGESIZE), size);
  
    // return NULL on failure
    if (retval)
      return NULL;
  
    // lock this buffer into RAM
    if (mlock(ptr, size)) {
      free(ptr);
      return NULL;
    }
  
    return ptr;
  }
  
  void free_workbuf(void *ptr, size_t size) {
    // unlock the address range
    munlock(ptr, size);
  
    // free the memory
    free(ptr);
  }

Prerequisites

• You have root permissions on the system.

Procedure

• To use mlockall() and munlockall() real-time system calls :

  • Lock all mapped pages by using mlockall() system call:

    #include <sys/mman.h>
    int mlockall (int flags)

  • Unlock all mapped pages by using munlockall() system call:

    #include <sys/mman.h>
    int munlockall (void)

    Tip

    For more information, see the capabilities(7), move_pages(2), mlock(2), mlock(3), posix_memalign(3), and posix_memalign(3p) man pages on your system.

Prerequisites

• You have root permissions on the system.

Procedure

• To map a specific process-address space:

  #include <sys/mman.h>
  #include <stdlib.h>
  
  void *alloc_workbuf(size_t size)
  {
   void *ptr;
  
   ptr = mmap(NULL, size, PROT_READ | PROT_WRITE,
              MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, -1, 0);
  
   if (ptr == MAP_FAILED)
    return NULL;
  
   return ptr;
  }
  
  void
  free_workbuf(void *ptr, size_t size)
  {
   munmap(ptr, size);
  }

Verification

• When the mmap() function completes successfully, it returns a pointer to the mapped area. On error, it returns the MAP_FAILED value and sets a errno to indicate the error.
• When the munmap() function completes successfully, it returns 0. On error, it returns -1 and sets an errno to indicate the error.