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MMAP(2) | System Calls Manual | MMAP(2) |
NAME¶
mmap — allocate memory, or map files or devices into memoryLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <sys/mman.h> void *mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
DESCRIPTION¶
The mmap() system call 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:PROT_NONE
- Pages may not be accessed.
PROT_READ
- Pages may be read.
PROT_WRITE
- Pages may be written.
PROT_EXEC
- Pages may be executed.
MAP_ANON
- Map anonymous memory not associated with any specific file.
The file descriptor used for creating
MAP_ANON
must be -1. The offset argument must be 0. MAP_FIXED
- Do not permit the system to select a different address than
the one specified. If the specified address cannot be used,
mmap() will fail. If
MAP_FIXED
is specified, addr must be a multiple of the pagesize. If aMAP_FIXED
request is successful, the mapping established by mmap() replaces any previous mappings for the process' pages in the range from addr to addr + len. Use of this option is discouraged. MAP_HASSEMAPHORE
- Notify the kernel that the region may contain semaphores and that special handling may be necessary.
MAP_INHERIT
- This flag never operated as advertised and is no longer supported. Please refer to minherit(2) for further information.
MAP_NOCORE
- Region is not included in a core file.
MAP_NOSYNC
- 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/file
system 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. WARNING! Extending a file with ftruncate(2), thus creating a big hole, and then filling the hole by modifying a shared mmap() 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 toMAP_NOSYNC
pages, because pages may be flushed to disk in a totally random order. The same applies when usingMAP_NOSYNC
to implement a file-based shared memory store. It is recommended that you create the backing store by write()ing zero's to the backing file rather than ftruncate()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) system call 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 file system buffer cache. However, it may be used to associate dirty VM pages with file system buffers and thus cause them to be flushed to physical media sooner rather than later. MAP_PRIVATE
- Modifications are private.
MAP_SHARED
- Modifications are shared.
MAP_STACK
MAP_STACK
impliesMAP_ANON
, and offset of 0. The fd argument must be -1 and prot must include at leastPROT_READ
andPROT_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.
MAP_SWAP
, in which the file descriptor argument
specifies a file or device to which swapping should be done.
NOTES¶
Although this implementation does not impose any alignment restrictions on the offset argument, a portable program must only use page-aligned values.RETURN VALUES¶
Upon successful completion, mmap() returns a pointer to the mapped region. Otherwise, a value ofMAP_FAILED
is
returned and errno is set to indicate the error.
ERRORS¶
The mmap() system call will fail if:- [
EACCES
] - The flag
PROT_READ
was specified as part of the prot argument and fd was not open for reading. The flagsMAP_SHARED
andPROT_WRITE
were specified as part of the flags and prot argument and fd was not open for writing. - [
EBADF
] - The fd argument is not a valid open file descriptor.
- [
EINVAL
] MAP_FIXED
was specified and the addr argument was not page aligned, or part of the desired address space resides out of the valid address space for a user process.- [
EINVAL
] - The len argument was equal to zero.
- [
EINVAL
] MAP_ANON
was specified and the fd argument was not -1.- [
EINVAL
] MAP_ANON
was specified and the offset argument was not 0.- [
ENODEV
] MAP_ANON
has not been specified and fd did not reference a regular or character special file.- [
ENOMEM
] MAP_FIXED
was specified and the addr argument was not 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.
SEE ALSO¶
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), getpagesize(3), make.conf(5)BUGS¶
The len argument is limited to the maximum file size or available userland address space. Files may not be able to be made more than 1TB large on 32 bit systems due to file systems restrictions and bugs, but address space is far more restrictive. Larger files may be possible on 64 bit systems. The previous documented limit of 2GB was a documentation bug. That limit has not existed since FreeBSD 2.2.July 26, 2009 | Debian |