The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008
Copyright © 2001-2008 The IEEE and The Open Group


mprotect - set protection of memory mapping


#include <sys/mman.h>

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


The mprotect() function shall change the access protections to be that specified by prot for those whole pages containing any part of the address space of the process starting at address addr and continuing for len bytes. The parameter prot determines whether read, write, execute, or some combination of accesses are permitted to the data being mapped. The prot argument should be either PROT_NONE or the bitwise-inclusive OR of one or more of PROT_READ, PROT_WRITE, and PROT_EXEC.

If an implementation cannot support the combination of access types specified by prot, the call to mprotect() shall fail.

An implementation may permit accesses other than those specified by prot; however, no implementation shall permit a write to succeed where PROT_WRITE has not been set or shall permit any access where PROT_NONE alone has been set. Implementations shall support at least the following values of prot: PROT_NONE, PROT_READ, PROT_WRITE, and the bitwise-inclusive OR of PROT_READ and PROT_WRITE. If PROT_WRITE is specified, the application shall ensure that it has opened the mapped objects in the specified address range with write permission, unless MAP_PRIVATE was specified in the original mapping, regardless of whether the file descriptors used to map the objects have since been closed.

The implementation may require that addr be a multiple of the page size as returned by sysconf().

The behavior of this function is unspecified if the mapping was not established by a call to mmap().

When mprotect() fails for reasons other than [EINVAL], the protections on some of the pages in the range [addr,addr+len) may have been changed.


Upon successful completion, mprotect() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.


The mprotect() function shall fail if:

The prot argument specifies a protection that violates the access permission the process has to the underlying memory object.
The prot argument specifies PROT_WRITE over a MAP_PRIVATE mapping and there are insufficient memory resources to reserve for locking the private page.
Addresses in the range [addr,addr+len) are invalid for the address space of a process, or specify one or more pages which are not mapped.
The prot argument specifies PROT_WRITE on a MAP_PRIVATE mapping, and it would require more space than the system is able to supply for locking the private pages, if required.
The implementation does not support the combination of accesses requested in the prot argument.

The mprotect() function may fail if:

The addr argument is not a multiple of the page size as returned by sysconf().

The following sections are informative.




Most implementations require that addr is a multiple of the page size as returned by sysconf().






mmap , sysconf

XBD <sys/mman.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Aligned with mprotect() in the POSIX Realtime Extension as follows:

Issue 6

The mprotect() function is marked as part of the Memory Protection option.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The normative text is updated to avoid use of the term "must" for application requirements.

Issue 7

SD5-XSH-ERN-22 is applied, deleting erroneous APPLICATION USAGE.

Austin Group Interpretation 1003.1-2001 #078 is applied, clarifying page alignment requirements.

The mprotect() function is moved from the Memory Protection option to the Base.

End of informative text.


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