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HP-UX 11i Version 3: February 2007

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mlock() — lock a segment of the process virtual address space in memory


#include <sys/mman.h>

int mlock( const void * addr, size_t len) ;


The mlock() system call allows the calling process to lock a segment of the process virtual address space into memory. Any addressable segment of the process' address space may be locked. Locked segments are immune to all routine swapping.

addr must be a valid address in the process virtual address space. addr + len must also be a valid address in the process virtual address space.

Locks are applied at page boundaries that encompass the range from addr to addr + len. If any address within the range is not valid, an error is returned and no locks are applied.

munlock() or munlockall() can be used to unlock memory segments (or all memory segments) locked with mlock().

Regardless of how many times a process locks a page, a single munlock() or munlockall() will unlock it. An munlock() of a page within a range specified in an mlock() call results in only the range specified in the munlock() being unlocked.

When memory is shared by multiple processes and mlocks are applied to the same physical page by multiple processes, a page remains locked until the last lock is removed from that page.

Locks applied with mlock() are not inherited by a child process.

The user must have the MLOCK privilege.

Although plock() and the mlock() family of functions may be used together in an application, each may affect the other in unexpected ways. This practice is not recommended.

Security Restrictions

Some or all of the actions associated with this system call require the MLOCK privilege. Processes owned by the superuser have this privilege. Processes owned by other users may have this privilege, depending on system configuration. See privileges(5) for more information about privileged access on systems that support fine-grained privileges.


mlock() returns the following values:


Successful completion.


Failure. The requested operation is not performed. errno is set to indicate the error.


If mlock() fails, errno is set to one of the following values:


One or more addresses in the specified range is not valid within the process address space.


There is not enough lockable memory in the system to satisfy the locking request.


The len parameter was zero.


The user does not have the MLOCK privilege.


The following call to mlock() locks the first 10 pages of the calling process in memory:

mlock(sbrk(0), 40960);


mlock(): POSIX Realtime Extensions, IEEE Std 1003.1b

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