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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

           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 9.99                    September 8, 2019                   NetBSD 9.99