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MMAP(2) System Calls Manual MMAP(2)
NAME
mmap - map files or devices into memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
void *
mmap(void *addr, size_t len, int prot, int flags, int fd, 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 fd, starting at
byte offset offset. If 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.
If 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 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.
PROT_NONE Placeholder when requesting no access permission.
As a NetBSD extension, the PROT_MPROTECT macro can be used to request
additional permissions for later use with mprotect(2). For example
PROT_MPROTECT(PROT_READ) requests that future PROT_READ mappings are
allowed and can be enabled using 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 mremap(2) for a sample use case.
Note that, due to hardware limitations, on some platforms PROT_WRITE may
imply PROT_READ, and PROT_READ may imply PROT_EXEC. Portable programs
should not rely on these flags being separately enforceable.
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. Note
that either MAP_SHARED or MAP_PRIVATE must be specified. Sharing,
mapping type and options are specified in the flags argument by OR'ing
the following values:
MAP_ALIGNED(n) Request that the allocation be aligned to the
given boundary. The parameter 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 sysconf(3) with the _SC_PAGESIZE
request. The following constants are defined
for convenience:
⊕ MAP_ALIGNMENT_64KB
⊕ MAP_ALIGNMENT_16MB
⊕ MAP_ALIGNMENT_4GB
⊕ MAP_ALIGNMENT_1TB
⊕ MAP_ALIGNMENT_256TB
⊕ MAP_ALIGNMENT_64PB
MAP_ANON Map anonymous memory not associated with any
specific file. The file descriptor is not used
for creating MAP_ANON regions, and must be
specified as -1. The mapped memory will be
zero filled.
MAP_ANONYMOUS Synonymous with MAP_ANON.
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 SIGBUS signal.
MAP_FIXED Do not permit the system to select a different
address than the one specified. If the
specified address cannot be used, mmap will
fail. If MAP_FIXED is specified, addr must be
a multiple of the pagesize. Use of this option
is discouraged.
MAP_HASSEMAPHORE Notify the kernel that the region may contain
semaphores and that special handling may be
necessary.
MAP_INHERIT Permit regions to be inherited across execve(2)
system calls.
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.
MAP_PRIVATE Modifications made by this process are private,
however modifications made by other processes
using MAP_SHARED will be seen.
MAP_REMAPDUP Only valid for mremap(2).
MAP_RENAME Assign the referenced private pages to the file
descriptor provided. Unimplemented.
MAP_SHARED Modifications are shared.
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.
MAP_TRYFIXED Attempt to use the address 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 MAP_TRYFIXED had not been
specified. If addr is NULL, this flag is
ignored and the system will select a mapping
address.
MAP_WIRED Lock the mapped region into memory as with
mlock(2).
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.
If 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.
RETURN VALUES
Upon successful completion, mmap returns a pointer to the mapped region.
Otherwise, a value of MAP_FAILED is returned and errno is set to indicate
the error. The symbol MAP_FAILED is defined in the header <sys/mman.h>.
No successful return from mmap() will 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.
PaX mprotect restrictions prohibit the requested
protection.
[EBADF] fd is not a valid open file descriptor.
[EINVAL] MAP_FIXED was specified and the addr parameter was not
page aligned or was outside of the valid address range
for a process.
MAP_ANON was specified and fd was not -1.
[ENODEV] fd did not reference a regular or character special
file.
[ENOMEM] MAP_FIXED was specified and the addr parameter wasn't
available.
MAP_ANON was specified and insufficient memory was
available.
[EOVERFLOW] fd references a regular file and the value of offset
plus len would exceed the offset maximum established
in its open file description.
SEE ALSO
madvise(2), mincore(2), mlock(2), mprotect(2), mremap(2), msync(2),
munmap(2), getpagesize(3), sysconf(3)
STANDARDS
The mmap() function conforms to IEEE Std 1003.1b-1993 ("POSIX.1b").
HISTORY
The mmap() interface was first designed in 4.2BSD.
NetBSD 9.99 September 8, 2019 NetBSD 9.99