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| <h4 class="subsection">3.4.3 Functions To Lock And Unlock Pages</h4> |
| |
| <p>The symbols in this section are declared in <samp><span class="file">sys/mman.h</span></samp>. These |
| functions are defined by POSIX.1b, but their availability depends on |
| your kernel. If your kernel doesn't allow these functions, they exist |
| but always fail. They <em>are</em> available with a Linux kernel. |
| |
| <p><strong>Portability Note:</strong> POSIX.1b requires that when the <code>mlock</code> |
| and <code>munlock</code> functions are available, the file <samp><span class="file">unistd.h</span></samp> |
| define the macro <code>_POSIX_MEMLOCK_RANGE</code> and the file |
| <code>limits.h</code> define the macro <code>PAGESIZE</code> to be the size of a |
| memory page in bytes. It requires that when the <code>mlockall</code> and |
| <code>munlockall</code> functions are available, the <samp><span class="file">unistd.h</span></samp> file |
| define the macro <code>_POSIX_MEMLOCK</code>. The GNU C library conforms to |
| this requirement. |
| |
| <!-- sys/mman.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Function: int <b>mlock</b> (<var>const void *addr, size_t len</var>)<var><a name="index-mlock-359"></a></var><br> |
| <blockquote> |
| <p><code>mlock</code> locks a range of the calling process' virtual pages. |
| |
| <p>The range of memory starts at address <var>addr</var> and is <var>len</var> bytes |
| long. Actually, since you must lock whole pages, it is the range of |
| pages that include any part of the specified range. |
| |
| <p>When the function returns successfully, each of those pages is backed by |
| (connected to) a real frame (is resident) and is marked to stay that |
| way. This means the function may cause page-ins and have to wait for |
| them. |
| |
| <p>When the function fails, it does not affect the lock status of any |
| pages. |
| |
| <p>The return value is zero if the function succeeds. Otherwise, it is |
| <code>-1</code> and <code>errno</code> is set accordingly. <code>errno</code> values |
| specific to this function are: |
| |
| <dl> |
| <dt><code>ENOMEM</code><dd> |
| <ul> |
| <li>At least some of the specified address range does not exist in the |
| calling process' virtual address space. |
| <li>The locking would cause the process to exceed its locked page limit. |
| </ul> |
| |
| <br><dt><code>EPERM</code><dd>The calling process is not superuser. |
| |
| <br><dt><code>EINVAL</code><dd><var>len</var> is not positive. |
| |
| <br><dt><code>ENOSYS</code><dd>The kernel does not provide <code>mlock</code> capability. |
| |
| </dl> |
| |
| <p>You can lock <em>all</em> a process' memory with <code>mlockall</code>. You |
| unlock memory with <code>munlock</code> or <code>munlockall</code>. |
| |
| <p>To avoid all page faults in a C program, you have to use |
| <code>mlockall</code>, because some of the memory a program uses is hidden |
| from the C code, e.g. the stack and automatic variables, and you |
| wouldn't know what address to tell <code>mlock</code>. |
| |
| </blockquote></div> |
| |
| <!-- sys/mman.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Function: int <b>munlock</b> (<var>const void *addr, size_t len</var>)<var><a name="index-munlock-360"></a></var><br> |
| <blockquote> |
| <p><code>munlock</code> unlocks a range of the calling process' virtual pages. |
| |
| <p><code>munlock</code> is the inverse of <code>mlock</code> and functions completely |
| analogously to <code>mlock</code>, except that there is no <code>EPERM</code> |
| failure. |
| |
| </blockquote></div> |
| |
| <!-- sys/mman.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Function: int <b>mlockall</b> (<var>int flags</var>)<var><a name="index-mlockall-361"></a></var><br> |
| <blockquote> |
| <p><code>mlockall</code> locks all the pages in a process' virtual memory address |
| space, and/or any that are added to it in the future. This includes the |
| pages of the code, data and stack segment, as well as shared libraries, |
| user space kernel data, shared memory, and memory mapped files. |
| |
| <p><var>flags</var> is a string of single bit flags represented by the following |
| macros. They tell <code>mlockall</code> which of its functions you want. All |
| other bits must be zero. |
| |
| <dl> |
| <dt><code>MCL_CURRENT</code><dd>Lock all pages which currently exist in the calling process' virtual |
| address space. |
| |
| <br><dt><code>MCL_FUTURE</code><dd>Set a mode such that any pages added to the process' virtual address |
| space in the future will be locked from birth. This mode does not |
| affect future address spaces owned by the same process so exec, which |
| replaces a process' address space, wipes out <code>MCL_FUTURE</code>. |
| See <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>. |
| |
| </dl> |
| |
| <p>When the function returns successfully, and you specified |
| <code>MCL_CURRENT</code>, all of the process' pages are backed by (connected |
| to) real frames (they are resident) and are marked to stay that way. |
| This means the function may cause page-ins and have to wait for them. |
| |
| <p>When the process is in <code>MCL_FUTURE</code> mode because it successfully |
| executed this function and specified <code>MCL_CURRENT</code>, any system call |
| by the process that requires space be added to its virtual address space |
| fails with <code>errno</code> = <code>ENOMEM</code> if locking the additional space |
| would cause the process to exceed its locked page limit. In the case |
| that the address space addition that can't be accommodated is stack |
| expansion, the stack expansion fails and the kernel sends a |
| <code>SIGSEGV</code> signal to the process. |
| |
| <p>When the function fails, it does not affect the lock status of any pages |
| or the future locking mode. |
| |
| <p>The return value is zero if the function succeeds. Otherwise, it is |
| <code>-1</code> and <code>errno</code> is set accordingly. <code>errno</code> values |
| specific to this function are: |
| |
| <dl> |
| <dt><code>ENOMEM</code><dd> |
| <ul> |
| <li>At least some of the specified address range does not exist in the |
| calling process' virtual address space. |
| <li>The locking would cause the process to exceed its locked page limit. |
| </ul> |
| |
| <br><dt><code>EPERM</code><dd>The calling process is not superuser. |
| |
| <br><dt><code>EINVAL</code><dd>Undefined bits in <var>flags</var> are not zero. |
| |
| <br><dt><code>ENOSYS</code><dd>The kernel does not provide <code>mlockall</code> capability. |
| |
| </dl> |
| |
| <p>You can lock just specific pages with <code>mlock</code>. You unlock pages |
| with <code>munlockall</code> and <code>munlock</code>. |
| |
| </blockquote></div> |
| |
| <!-- sys/mman.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Function: int <b>munlockall</b> (<var>void</var>)<var><a name="index-munlockall-362"></a></var><br> |
| <blockquote> |
| <p><code>munlockall</code> unlocks every page in the calling process' virtual |
| address space and turn off <code>MCL_FUTURE</code> future locking mode. |
| |
| <p>The return value is zero if the function succeeds. Otherwise, it is |
| <code>-1</code> and <code>errno</code> is set accordingly. The only way this |
| function can fail is for generic reasons that all functions and system |
| calls can fail, so there are no specific <code>errno</code> values. |
| |
| </blockquote></div> |
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