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MMAP(2) System Calls Manual MMAP(2)

mmap — allocate memory, or map files or devices into memory

library “libc”

#include <sys/types.h>
#include <sys/mman.h>

void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);

The () function causes the pages starting at addr and continuing for at most len bytes to be mapped from the object described by fd, starting at byte offset offset. If len is not a multiple of the pagesize, the mapped region may extend past the specified range. Any such extension beyond the end of the mapped object will be zero-filled.

If addr is non-zero, it is used as a hint to the system. (As a convenience to the system, the actual address of the region may differ from the address supplied.) If addr is zero, an address will be selected by the system. The actual starting address of the region is returned. A successful mmap deletes any previous mapping in the allocated address range.

The protections (region accessibility) are specified in the prot argument by or'ing the following values:

Pages may not be accessed.
Pages may be read.
Pages may be written.
Pages may be executed.

The flags parameter specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. Sharing, mapping type and options are specified in the flags argument by or'ing the following values:

Map anonymous memory not associated with any specific file. The file descriptor used for creating MAP_ANON must be -1. The offset parameter is ignored.
This flag is an alias for MAP_ANON and is provided for compatibility.
Do not permit the system to select a different address than the one specified. If the specified address contains other mappings those mappings will be replaced. If the specified address cannot otherwise be used, () will fail. If MAP_FIXED is specified, addr must be a multiple of the pagesize.
Try to do a fixed mapping but fail if another mapping already exists in the space instead of overwriting the mapping.

When used with MAP_STACK This flag creates a grow-down stack area with the specified maximum stack size. This flag is no longer special-cased and will be converted to a normal anonymous mmap(), meaning that other mmap() calls cannot sub-map ungrown areas returned by prior MAP_STACK maps using TRYFIXED. The entire area is now applicable to the mapping.

Note that the kernel itself can still create auto-grow areas but will do so for the user stack in order to maintain backwards compatibility with older code that might otherwise assume it can map below the user stack (in particular, older pthread libraries). This compatibility is deprecated and will be removed in a future release.

Notify the kernel that the region may contain semaphores and that special handling may be necessary.
Region is not included in a core file.
Causes data dirtied via this VM map to be flushed to physical media only when necessary (usually by the pager) rather than gratuitously. Typically this prevents the update daemons from flushing pages dirtied through such maps and thus allows efficient sharing of memory across unassociated processes using a file-backed shared memory map. Without this option any VM pages you dirty may be flushed to disk every so often (every 30-60 seconds usually) which can create performance problems if you do not need that to occur (such as when you are using shared file-backed mmap regions for IPC purposes). Note that VM/filesystem coherency is maintained whether you use MAP_NOSYNC or not. This option is not portable across UNIX platforms (yet), though some may implement the same behavior by default.

! Extending a file with ftruncate(2), thus creating a big hole, and then filling the hole by modifying a shared () can lead to severe file fragmentation. In order to avoid such fragmentation you should always pre-allocate the file's backing store by write()ing zero's into the newly extended area prior to modifying the area via your mmap(). The fragmentation problem is especially sensitive to MAP_NOSYNC pages, because pages may be flushed to disk in a totally random order.

The same applies when using MAP_NOSYNC to implement a file-based shared memory store. It is recommended that you create the backing store by ()ing zero's to the backing file rather than ()ing it. You can test file fragmentation by observing the KB/t (kilobytes per transfer) results from an “iostat 1” while reading a large file sequentially, e.g., using “dd if=filename of=/dev/null bs=32k”.

The fsync(2) function will flush all dirty data and metadata associated with a file, including dirty NOSYNC VM data, to physical media. The sync(8) command and sync(2) system call generally do not flush dirty NOSYNC VM data. The msync(2) system call is obsolete since BSD implements a coherent filesystem buffer cache. However, it may be used to associate dirty VM pages with filesystem buffers and thus cause them to be flushed to physical media sooner rather than later.

Modifications are private.
Modifications are shared.
Map the area as a stack. MAP_ANON is implied. Offset should be 0, fd must be -1, and prot should include at least PROT_READ and PROT_WRITE. This option creates a memory region that grows to at most len bytes in size, starting from the stack top and growing down. The stack top is the starting address returned by the call, plus len bytes. The bottom of the stack at maximum growth is the starting address returned by the call.

The entire area is reserved from the point of view of other () calls, even if not faulted in yet.

Note that unless MAP_FIXED or MAP_TRYFIXED is used, you cannot count on the returned address matching the hint you have provided.

Memory accessed via this map is not linearly mapped and will be governed by a virtual page table. The base address of the virtual page table may be set using mcontrol(2) with MADV_SETMAP. Virtual page tables work with anonymous memory but there is no way to populate the page table so for all intents and purposes MAP_VPAGETABLE can only be used when mapping file descriptors. Since the kernel will update the VPTE_M bit in the virtual page table, the mapping must R+W even though actual access to the memory will be properly governed by the virtual page table.

Addressable backing store is limited by the range supported in the virtual page table entries. The kernel may implement a page table abstraction capable of addressing a larger range within the backing store then could otherwise be mapped into memory.

The close(2) function does not unmap pages, see munmap(2) for further information.

The current design does not allow a process to specify the location of swap space. In the future we may define an additional mapping type, MAP_SWAP, in which the file descriptor argument specifies a file or device to which swapping should be done.

Upon successful completion, mmap() returns a pointer to the mapped region. Otherwise, a value of MAP_FAILED is returned and errno is set to indicate the error.

Mmap() will fail if:

[]
The flag PROT_READ was specified as part of the prot parameter and fd was not open for reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the flags and prot parameters and fd was not open for writing.
[]
fd is not a valid open file descriptor.
[]
was specified and the addr parameter was not page aligned, or part of the desired address space resides out of the valid address space for a user process.
[]
Len was negative.
[]
was specified and the fd parameter was not -1.
[]
has not been specified and fd did not reference a regular or character special file.
[]
Offset was not page-aligned.
[]
was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available. The system has reached the per-process mmap limit specified in the vm.max_proc_mmap sysctl.

madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), getpagesize(3)

January 18, 2015 DragonFly-5.6.1