MLOCK(2) BSD Programmer's Manual MLOCK(2) NNAAMMEE mmlloocckk, mmuunnlloocckk - lock (unlock) physical pages in memory SSYYNNOOPPSSIISS ##iinncclluuddee <<ssyyss//ttyyppeess..hh>> ##iinncclluuddee <<ssyyss//mmmmaann..hh>> _i_n_t mmlloocckk(_c_a_d_d_r___t _a_d_d_r, _s_i_z_e___t _l_e_n); _i_n_t mmuunnlloocckk(_c_a_d_d_r___t _a_d_d_r, _s_i_z_e___t _l_e_n); DDEESSCCRRIIPPTTIIOONN The mmlloocckk system call locks into memory the physical pages associated with the virtual address range starting at _a_d_d_r for _l_e_n bytes. The mmuunnlloocckk call unlocks pages previously locked by one or more mmlloocckk calls. For both, the _a_d_d_r parameter should be aligned to a multiple of the page size. If the _l_e_n parameter is not a multiple of the page size, it will be rounded up to be so. The entire range must be allocated. After an mmlloocckk call, the indicated pages will cause neither a non- resident page or address-translation fault until they are unlocked. They may still cause protection-violation faults or TLB-miss faults on archi- tectures with software-managed TLBs. The physical pages remain in memory until all locked mappings for the pages are removed. Multiple processes may have the same physical pages locked via their own virtual address mappings. A single process may likewise have pages multiply-locked via different virtual mappings of the same pages or via nested mmlloocckk calls on the same address range. Unlocking is performed explicitly by mmuunnlloocckk or implicitly by a call to mmuunnmmaapp which deallocates the unmapped address range. Locked mappings are not inherited by the child process after a fork(2). Since physical memory is a potentially scarce resource, processes are limited in how much they can lock down. A single process can mmlloocckk the minimum of a system-wide ``wired pages'' limit and the per-process RLIMIT_MEMLOCK resource limit. RREETTUURRNN VVAALLUUEESS A return value of 0 indicates that the call succeeded and all pages in the range have either been locked or unlocked. A return value of -1 in- dicates an error occurred and the locked status of all pages in the range remains unchanged. In this case, the global location _e_r_r_n_o is set to in- dicate the error. EERRRROORRSS MMlloocckk() will fail if: [EINVAL] The address given is not page aligned or the length is neg- ative. [EAGAIN] Locking the indicated range would exceed either the system or per-process limit for locked memory. [ENOMEM] Some portion of the indicated address range is not allocat- ed. There was an error faulting/mapping a page. MMuunnlloocckk() will fail if: [EINVAL] The address given is not page aligned or the length is neg- ative. [ENOMEM] Some portion of the indicated address range is not allocat- ed. Some portion of the indicated address range is not locked. SSEEEE AALLSSOO fork(2), mmap(2), munmap(2), setrlimit(2), getpagesize(3) BBUUGGSS Unlike The Sun implementation, multiple mmlloocckk calls on the same address range require the corresponding number of mmuunnlloocckk calls to actually un- lock the pages, i.e. mmlloocckk nests. This should be considered a conse- quence of the implementation and not a feature. The per-process resource limit is a limit on the amount of virtual memory locked, while the system-wide limit is for the number of locked physical pages. Hence a process with two distinct locked mappings of the same physical page counts as 2 pages against the per-process limit and as only a single page in the system limit. HHIISSTTOORRYY The mmlloocckk() and mmuunnlloocckk() functions first appeared in 4.4BSD. 4.4BSD June 2, 1993 2