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@(#)mmap.2 8.4 (Berkeley) 5/11/95
.Dd May 11, 1995 .Dt MMAP 2 .Os .Sh NAME .Nm mmap .Nd map files or devices into memory .Sh LIBRARY .Lb libc .Sh SYNOPSIS .Fd #include \*[Lt]sys/mman.h\*[Gt] .Ft void * .Fn mmap "void *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. t Dv PROT_NONE Pages may not be accessed. .El
p f -symbolic Note that, due to hardware limitations, on some platforms .Dv PROT_WRITE may imply .Dv PROT_READ , and .Dv PROT_READ may imply .Dv PROT_EXEC . Portable programs should not rely on these flags being separately enforceable. .Ef
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. Note that either .Dv MAP_SHARED , .Dv MAP_PRIVATE or .Dv MAP_COPY must be specified. 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 regions is used only for naming, and may be specified as -1 if no name is associated with the region. t 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 execve 2 system calls. t Dv MAP_PRIVATE Modifications made by this process are private, however modifications made by other processes using .Dv MAP_SHARED will be seen. t Dv MAP_SHARED Modifications are shared. t Dv MAP_COPY Modifications are private, including other processes. .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
.Dv MAP_FAILED
is returned and
.Va errno
is set to indicate the error.
The symbol
.Dv MAP_FAILED
is defined in the header
.Ao Pa sys/mman.h Ac .
No successful return from
.Fn mmap
will return the value
.Dv MAP_FAILED .
.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 len
was less than zero.
t Bq Er ENODEV .Fa fd
did not reference a regular or character special file.
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.
t Bq Er EOVERFLOW .Fa fd
references a regular file and the value of
.Fa offset
plus
.Fa len
would exceed the offset maximum established in its open file description.
.El
.Sh SEE ALSO
.Xr madvise 2 ,
.Xr mincore 2 ,
.Xr mlock 2 ,
.Xr mprotect 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr getpagesize 3
.Sh BUGS
The
.Dv MAP_COPY
flag is not implemented.
The current
.Dv MAP_COPY
semantics are the same as those of the
.Dv MAP_PRIVATE
flag.
Indexes created Wed Oct 15 16:09:53 GMT 2025