Updated: May 06, 2022

Change memory protection


#include <sys/mman.h>

int mprotect( void * addr, 
              size_t len,
              int prot );


The beginning of the range of addresses whose protection you want to change.
The length of the range of addresses, in bytes.
The new access capabilities for the mapped memory region(s). You can combine the following bits, which are defined in <sys/mman.h>:
  • PROT_EXEC — the region can be executed.
    Note: To successfully use this flag:
    • Your process must have the PROCMGR_AID_PROT_EXEC ability enabled. For more information, see procmgr_ability().

    For more information about abilities, see procmgr_ability().

  • PROT_NOCACHE — disable caching of the region (for example, to access dual ported memory).
    Note: On 32- and 64-bit ARM targets, PROT_NOCACHE causes RAM to be mapped as normal noncached, but non-RAM to be mapped as strongly ordered device memory. For finer control, see shm_ctl_special().
  • PROT_NONE — the region can't be accessed.
  • PROT_READ — the region can be read.
  • PROT_WRITE — the region can be written.
    Note: In order to simultaneously set PROT_EXEC and PROT_WRITE, your process must have the PROCMGR_AID_PROT_WRITE_AND_EXEC ability enabled (in addition to PROCMGR_AID_PROT_EXEC). For more information, see procmgr_ability().



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The mprotect() function changes the access protections on any mappings residing in the range starting at addr, and continuing for len bytes.


An error occurred (errno is set).
Note: If mprotect() fails, the protections on some of the pages in the address range starting at addr and continuing for len bytes may have been changed.


One of the following occurred:
  • The memory object wasn't opened for read, regardless of the protection specified.
  • The memory object wasn't opened for write, and you specified PROT_WRITE for a MAP_SHARED type mapping.
  • You specified PROT_EXEC for a memory-mapped file mapping, the file doesn't have execute permission for the client process, and procnto was started with the -mX option.
The prot argument specifies PROT_WRITE on a MAP_PRIVATE mapping, and there's insufficient memory resources to reserve for locking the private pages (if required).
The addresses in the range starting at addr and continuing for len bytes are outside the range allowed for the address space of a process, or specify one or more pages that are not mapped.

The prot argument specifies PROT_WRITE on a MAP_PRIVATE mapping, and locking the private pages (if required) would need more space than the system can supply to reserve for doing so.

The function mprotect() isn't supported by this implementation.
The calling process doesn't have the required permission (see procmgr_ability()), or it attempted to set PROT_EXEC for a region of memory covered by an untrusted memory-mapped file.


POSIX 1003.1

Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes