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MMAP(2) BSD System Calls Manual MMAP(2)
NAME
mmap -- map files or devices into memory
SYNOPSIS
#include <sys/mman.h>
void *
mmap(void *addr, size_t len, int prot, int flags, int fildes,
off_t offset);
DESCRIPTION
The mmap function causes the pages starting at addr and continuing for at
most len bytes to be mapped from the object described by fildes, starting
at byte offset offset. If offset or len is not a multiple of the page-size, pagesize,
size, the mapped region may extend past the specified range.
If addr is non-zero, it is used as a hint to the system. (As a conve-nience convenience
nience 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_EXEC Pages may be executed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
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 (copy-on-write) 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_ANON Map anonymous memory not associated with any specific
file. Mac OS X specific: the file descriptor used for
creating MAP_ANON regions can be used to pass some
Mach VM flags, and can be specified as -1 if no such
flags are associated with the region. Mach VM flags
are defined in <mach/vm_statistics.h> and the ones
that currently apply to mmap are:
VM_FLAGS_PURGABLE to create Mach purgable (i.e.
volatile) memory VM_MAKE_TAG(tag) to associate an
8-bit tag with the region
<mach/vm_statistics.h> defines some preset tags (with
a VM_MEMORY_ prefix). Users are encouraged to use
tags between 240 and 255. Tags are used by tools such
as vmmap(1) to help identify specific memory regions.
MAP_FILE Mapped from a regular file or character-special device
memory. (This is the default mapping type, and need
not be specified.)
MAP_FIXED Do not permit the system to select a different address
than the one specified. If the specified address can-not cannot
not be used, mmap will fail. If MAP_FIXED is speci-fied, specified,
fied, addr must be a multiple of the pagesize. Use of
this option is discouraged.
MAP_HASSEMAPHORE Notify the kernel that the region may contain sema-phores semaphores
phores and that special handling may be necessary.
MAP_PRIVATE Modifications are private (copy-on-write).
MAP_SHARED Modifications are shared.
MAP_NOCACHE Pages in this mapping are not retained in the kernel's
memory cache. If the system runs low on memory, pages
in MAP_NOCACHE mappings will be among the first to be
reclaimed. This flag is intended for mappings that
have little locality and provides a hint to the kernel
that pages in this mapping are unlikely to be needed
again in the near future.
Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
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.
RETURN VALUES
Upon successful completion, mmap returns a pointer to the mapped region.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
ERRORS
Mmap() will fail if:
[EACCES] Fildes is not open for reading.
[EACCES] The flags PROT_WRITE and MAP_SHARED are specified as
part of the flags and prot parameters and fildes is
not open for writing.
[EBADF] fildes is not a valid file descriptor for an open
file.
[EINVAL] MAP_FIXED is specified and the parameter is not page
aligned.
[EINVAL] fildes does not reference a regular or character spe-cial special
cial file.
[EINVAL] flags does not include either MAP_PRIVATE or
MAP_SHARED.
[EINVAL] len is not greater than zero.
[EINVAL] offset is not a multiple of the page size, as returned
by sysconf(3).
[EMFILE] The limit on mapped regions (per process or system) is
exceeded.
[ENODEV] The file type for fildes is not supported for mapping.
[ENOMEM] MAP_FIXED is specified and the address range specified
exceeds the address space limit for the process.
[ENOMEM] MAP_FIXED is specified and the address specified by
the addr parameter isn't available.
[ENOMEM] MAP_ANON is specified and insufficient memory is
available.
[ENXIO] Addresses in the specified range are invalid for
fildes.
[EOVERFLOW] Addresses in the specified range exceed the maximum
offset set for fildes.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
The include file <sys/types.h> is necessary.
COMPATIBILITY
mmap() now returns with errno set to EINVAL in places that historically
succeeded. The rules have changed as follows:
oo The flags parameter must specify either MAP_PRIVATE or MAP_SHARED.
oo The size parameter must not be 0.
oo The off parameter must be a multiple of pagesize, as returned by
sysconf().
SEE ALSO
getpagesize(2), madvise(2), mincore(2), mlock(2), mprotect(2), msync(2),
munmap(2), sysconf(3), compat(5)
4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution
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