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MMAP(2)                       System Calls Manual                      MMAP(2)

     mmap - map files or devices into memory

     Standard C Library (libc, -lc)

     #include <sys/mman.h>

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

     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

     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_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

           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.

           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

     The close(2) function does not unmap pages, see munmap(2) for further

     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.

     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.

     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

     [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

     [ENOMEM]           MAP_FIXED was specified and the addr parameter wasn't

                        MAP_ANON was specified and insufficient memory was

     [EOVERFLOW]        fd references a regular file and the value of offset
                        plus len would exceed the offset maximum established
                        in its open file description.

     madvise(2), mincore(2), mlock(2), mprotect(2), mremap(2), msync(2),
     munmap(2), getpagesize(3), sysconf(3)

     The mmap() function conforms to IEEE Std 1003.1b-1993 ("POSIX.1b").

     The mmap() interface was first designed in 4.2BSD.

NetBSD 10.99                   September 8, 2019                  NetBSD 10.99