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MMAP(2)                     BSD System Calls Manual                    MMAP(2)

NAME
     mmap -- map files or devices into memory

SYNOPSIS
     #include <&lt;sys/mman.h>&gt;

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

DESCRIPTION
     The mmap() function causes the contents of fd, starting at offset, to be
     mapped in memory at the given addr.  The mapping will extend at least len
     bytes, subject to page alignment restrictions.

     The addr argument describes the address where the system should place the
     mapping.  If the MAP_FIXED flag is specified, the allocation will happen
     at the specified address, replacing any previously established mappings
     in its range.  Otherwise, the mapping will be placed at the available
     spot at addr; failing that it will be placed "close by".  If addr is NULL
     the system can pick any address.  Except for MAP_FIXED mappings, the sys-
     tem will never replace existing mappings.

     The len argument describes the minimum amount of bytes the mapping will
     span.  Since mmap() maps pages into memory, len may be rounded up to hit
     a page boundary.  If len is 0, the mapping will fail with EINVAL.

     If an fd and offset are specified, the resulting address may end up not
     on a page boundary, in order to align the page offset in the addr to the
     page offset in offset.

     The protections (region accessibility) are specified in the prot argu-
     ment.  It should either be PROT_NONE (no permissions) or the bitwise OR
     of one or more of 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 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.  Exactly one of the first two values must be
     specified:

           MAP_PRIVATE    Modifications are private.

           MAP_SHARED     Modifications are shared.

     Any combination of the following flags may additionally be used:

           MAP_ANON       Map anonymous memory not associated with any spe-
                          cific file.  The file descriptor used for creating
                          MAP_ANON must currently be -1 indicating no name is
                          associated with the region.

           MAP_ANONYMOUS  Synonym for MAP_ANON.

           MAP_FIXED      Demand that the mapping is placed at addr, rather
                          than having the system select a location.  addr, len
                          and offset (in the case of fd mappings) must be mul-
                          tiples of the page size.  Existing mappings in the
                          address range will be replaced.  Use of this option
                          is discouraged.

     Finally, the following flags are also provided for source compatibility
     with code written for other operating systems, but are not recommended
     for use in new OpenBSD code:

           MAP_COPY       Modifications are private and, unlike MAP_PRIVATE,
                          modifications made by others are not visible.  On
                          OpenBSD this behaves just like MAP_PRIVATE.

           MAP_FILE       Mapped from a regular file, character special file,
                          or block special file specified by file descriptor
                          fd.  On OpenBSD and all systems conforming to IEEE
                          Std 1003.1-2008 (``POSIX.1'') this is the default
                          mapping type and need not be specified.

           MAP_HASSEMAPHORE
                          Notify the kernel that the region may contain sema-
                          phores and that special handling may be necessary.
                          On OpenBSD this flag is ignored.

           MAP_INHERIT    Permit regions to be inherited across exec(3) system
                          calls.  On OpenBSD this flag is ignored.

           MAP_TRYFIXED   Attempt to use the hint provided by addr.  On
                          OpenBSD this is the default behavior.

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

RETURN VALUES
     The mmap() function returns a pointer to the mapped region if successful;
     otherwise the value MAP_FAILED is returned and the global variable errno
     is set to indicate the error.  A successful return from mmap() will never
     return the value MAP_FAILED.

ERRORS
     mmap() will fail if:

     [EACCES]           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.

     [EBADF]            fd is not a valid open file descriptor.

     [EINVAL]           MAP_PRIVATE and MAP_SHARED were both specified.

     [EINVAL]           MAP_FIXED was specified and the addr parameter was not
                        page aligned.

     [EINVAL]           addr and len specified a region that would extend
                        beyond the end of the address space.

     [EINVAL]           fd did not specify a regular, character special, or
                        block special file.

     [EINVAL]           The allocation len was 0.

     [ENOMEM]           MAP_FIXED was specified and the addr parameter wasn't
                        available.  MAP_ANON was specified and insufficient
                        memory was available.

SEE ALSO
     madvise(2), mincore(2), mlock(2), mprotect(2), mquery(2), msync(2),
     munmap(2), getpagesize(3)

STANDARDS
     The mmap() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').

HISTORY
     The mmap() system call first appeared in 4.1cBSD.

CAVEATS
     IEEE Std 1003.1-2008 (``POSIX.1'') specifies that references to pages
     beyond the end of a mapped object shall generate a SIGBUS signal; how-
     ever, OpenBSD generates a SIGSEGV signal in this case instead.

     Additionally, IEEE Std 1003.1-2008 (``POSIX.1'') specifies that mmap()
     shall fail with EINVAL if neither MAP_PRIVATE nor MAP_SHARED is specified
     by flags; however, for compatibility with old programs, OpenBSD instead
     defaults to MAP_SHARED for mappings of character special files, and to
     MAP_PRIVATE for all other mappings.  New programs should not rely on this
     behavior.

BUGS
     Due to a limitation of the current vm system (see uvm(9)), mapping
     descriptors PROT_WRITE without also specifying PROT_READ is useless
     (results in a segmentation fault when first accessing the mapping).  This
     means that such descriptors must be opened with O_RDWR, which requires
     both read and write permissions on the underlying object.

BSD                             April 28, 2017                             BSD