libc/sys/mmap.2

.\"	$NetBSD: mmap.2,v 1.55 2019/09/08 17:24:49 sevan Exp $
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.\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
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.Dd September 8, 2019
.Dt MMAP 2
.Os
.Sh NAME
.Nm mmap
.Nd map files or devices into memory
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/mman.h
.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.
.Pp
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
.Nm
deletes any previous mapping in the allocated address range.
.Pp
The protections (region accessibility) are specified in the
.Fa prot
argument by
.Em OR Ns 'ing
the following values:
.Bl -tag -width PROT_WRITEXX -offset indent
.It Dv PROT_EXEC
Pages may be executed.
.It Dv PROT_READ
Pages may be read.
.It Dv PROT_WRITE
Pages may be written.
.It Dv PROT_NONE
Placeholder when requesting no access permission.
.El
.Pp
As a
.Nx
extension, the
.Dv PROT_MPROTECT
macro can be used to request additional permissions for later use with
.Fn mprotect 2 .
For example
.Dv PROT_MPROTECT(PROT_READ)
requests that future
.Dv PROT_READ
mappings are allowed and can be enabled using
.Xr mprotect 2 ,
but does not currently grant read mappings to the returned memory segment.
This is necessary for switching pages between writable and executable
when PaX MPROTECT restrictions are in place.
See
.Xr mremap 2
for a sample use case.
.Pp
.Bf -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
.Pp
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
or
.Dv MAP_PRIVATE
must be specified.
Sharing, mapping type and options are specified in the
.Fa flags
argument by
.Em OR Ns 'ing
the following values:
.Bl -tag -width MAP_HASSEMAPHOREXX -offset indent
.It Dv MAP_ALIGNED(n)
Request that the allocation be aligned to the given boundary.
The parameter
.Ar n
should be the base 2 logarithm of the desired alignment (e.g., to
request alignment to 16K, use 14 as the value for n).
The alignment must be equal to or greater than the platform's page
size as returned by
.Xr sysconf 3
with the
.Dv _SC_PAGESIZE
request.
The following constants are defined for convenience:
.Bl -bullet -compact -offset indent
.It
.Dv MAP_ALIGNMENT_64KB
.It
.Dv MAP_ALIGNMENT_16MB
.It
.Dv MAP_ALIGNMENT_4GB
.It
.Dv MAP_ALIGNMENT_1TB
.It
.Dv MAP_ALIGNMENT_256TB
.It
.Dv MAP_ALIGNMENT_64PB
.El
.It Dv MAP_ANON
Map anonymous memory not associated with any specific file.
The file descriptor is not used for creating
.Dv MAP_ANON
regions, and must be specified as \-1.
The mapped memory will be zero filled.
.It Dv MAP_ANONYMOUS
Synonymous with
.Dv MAP_ANON .
.It Dv MAP_FILE
Mapped from a regular file or character-special device memory.
Read accesses beyond the end of of the file or device but less
than the current page size will be zero-filled.
Write accesses beyond the end of the file or device but less
than the current page size will not affect the file or device.
References beyond the end of file that are beyond the current
page size will result in the delivery of
.Dv SIGBUS
signal.
.It 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
.Dv MAP_FIXED
is specified,
.Fa addr
must be a multiple of the pagesize.
Use of this option is discouraged.
.It Dv MAP_HASSEMAPHORE
Notify the kernel that the region may contain semaphores and that special
handling may be necessary.
.It Dv MAP_INHERIT
Permit regions to be inherited across
.Xr execve 2
system calls.
.It Dv MAP_NORESERVE
Only reserve address space, but do not reserve swap space or any other
resources for this mapping.
Access to the address space is not guaranteed and may result in a segmentation
violation.
Unimplemented.
.It Dv MAP_PRIVATE
Modifications made by this process are private, however modifications made by
other processes using
.Dv MAP_SHARED
will be seen.
.It Dv MAP_REMAPDUP
Only valid for
.Xr mremap 2 .
.It Dv MAP_RENAME
Assign the referenced private pages to the file descriptor provided.
Unimplemented.
.It Dv MAP_SHARED
Modifications are shared.
.It Dv MAP_STACK
Allocate a memory segment that can be used either for a process or thread stack.
This currently has no effect, but its use is reserved for architectures
that might require special treatment of that address space.
Unimplemented.
.It Dv MAP_TRYFIXED
Attempt to use the address
.Fa addr
even if it falls within the normally protected process data or
text segment memory regions.
If the requested region of memory
is actually present in the memory map, a different address will
be selected as if
.Dv MAP_TRYFIXED
had not been specified.
If
.Fa addr
is
.Dv NULL ,
this flag is ignored and the system will select a mapping address.
.It Dv MAP_WIRED
Lock the mapped region into memory as with
.Xr mlock 2 .
.El
.Pp
The
.Xr close 2
function does not unmap pages, see
.Xr munmap 2
for further information.
.Pp
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.
.Pp
If
.Dv MAP_FIXED
is not specified, the system will attempt to place the mapping in an
unused portion of the address space chosen to minimize possible
collision between mapped regions and the heap.
.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
.In sys/mman.h .
No successful return from
.Fn mmap
will return the value
.Dv MAP_FAILED .
.Sh ERRORS
.Fn mmap
will fail if:
.Bl -tag -width Er
.It 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.
.Pp
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.
.Pp
PaX mprotect restrictions prohibit the requested protection.
.It Bq Er EBADF
.Fa fd
is not a valid open file descriptor.
.It 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.
.Pp
.Dv MAP_ANON
was specified and
.Fa fd
was not \-1.
.It Bq Er ENODEV
.Fa fd
did not reference a regular or character special file.
.It Bq Er ENOMEM
.Dv MAP_FIXED
was specified and the
.Fa addr
parameter wasn't available.
.Pp
.Dv MAP_ANON
was specified and insufficient memory was available.
.It 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 mremap 2 ,
.Xr msync 2 ,
.Xr munmap 2 ,
.Xr getpagesize 3 ,
.Xr sysconf 3
.Sh STANDARDS
The
.Fn mmap
function conforms to
.St -p1003.1b-93 .
.Sh HISTORY
The
.Fn mmap
interface was first designed in
.Bx 4.2 .