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@(#)mmap.2 8.4 (Berkeley) 5/11/95
.Dd "May 11, 1995" .Dt MMAP 2 .Os BSD 4 .Sh NAME .Nm mmap .Nd map files or devices into memory .Sh SYNOPSIS .Fd #include <sys/types.h> .Fd #include <sys/mman.h> .Ft caddr_t .Fn mmap "caddr_t addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset" .Sh DESCRIPTION The .Nm mmap function causes the pages starting at .Fa addr and continuing for at most .Fa len bytes to be mapped from the object described by .Fa fd , starting at byte offset .Fa offset . If .Fa 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.
p If .Fa 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 .Fa addr is zero, an address will be selected by the system. The actual starting address of the region is returned. A successful .Fa mmap deletes any previous mapping in the allocated address range.
p The protections (region accessibility) are specified in the .Fa prot argument by .Em or Ns 'ing the following values:
p l -tag -width MAP_FIXEDX t Dv PROT_EXEC Pages may be executed. t Dv PROT_READ Pages may be read. t Dv PROT_WRITE Pages may be written. .El
p The .Fa 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 .Fa flags argument by .Em or Ns 'ing the following values:
p
l -tag -width MAP_FIXEDX t Dv MAP_ANON Map anonymous memory not associated with any specific file.
The file descriptor used for creating
.Dv MAP_ANON
must be -1.
The
.Fa offset
parameter is ignored.
.It Dv MAP_FILE
Mapped from a regular file or character-special device memory.
t Dv MAP_FIXED Do not permit the system to select a different address than the one
specified.
If the specified address cannot be used,
.Nm mmap
will fail.
If MAP_FIXED is specified,
.Fa addr
must be a multiple of the pagesize.
Use of this option is discouraged.
t Dv MAP_HASSEMAPHORE Notify the kernel that the region may contain semaphores and that special
handling may be necessary.
t Dv MAP_INHERIT Permit regions to be inherited across
.Xr exec 2
system calls.
t Dv MAP_PRIVATE Modifications are private.
t Dv MAP_SHARED Modifications are shared.
.El
p The .Xr close 2 function does not unmap pages, see .Xr munmap 2 for further information.
p
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,
.Dv MAP_SWAP ,
in which
the file descriptor argument specifies a file or device to which swapping
should be done.
.Sh RETURN VALUES
Upon successful completion,
.Nm mmap
returns a pointer to the mapped region.
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Mmap
will fail if:
l -tag -width Er t Bq Er EACCES The flag
.Dv PROT_READ
was specified as part of the
.Fa prot
parameter and
.Fa fd
was not open for reading.
The flags
.Dv MAP_SHARED
and
.Dv PROT_WRITE
were specified as part of the
.Fa flags
and
.Fa prot
parameters and
.Fa fd
was not open for writing.
t Bq Er EBADF .Fa Fd
is not a valid open file descriptor.
t Bq Er EINVAL One of
.Dv MAP_ANON
or
.Dv MAP_FILE
was not specified as part of the
.Fa flags
parameter.
.Dv MAP_FIXED
was specified and the
.Fa addr
parameter was not page aligned or was outside of the
valid address range for a process.
.Dv MAP_ANON was specified and
.Fa fd
was not -1.
.Fa Fd
did not reference a regular or character special file.
.Fa Len
was less than zero.
t Bq Er ENOMEM .Dv MAP_FIXED
was specified and the
.Fa addr
parameter wasn't available.
.Dv MAP_ANON
was specified and insufficient memory was available.
.Sh "SEE ALSO"
.Xr getpagesize 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr mprotect 2 ,
.Xr madvise 2 ,
.Xr mincore 2
Indexes created Sun Oct 19 02:09:48 GMT 2025