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MMAP(2)                   OpenBSD Programmer's Manual                  MMAP(2)

     mmap - map files or devices into memory

     #include <sys/types.h>
     #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 offset or len is not a multiple of the pagesize,
     the mapped region may extend past the specified range.

     If addr is non-zero, it is used as a hint to the system.  (As a conve-
     nience 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     No permissions.

     The flags parameter specifies the type of the mapped object, mapping op-
     tions, and whether modifications made to the mapped copy of the page are
     private to the process or are to be shared with other references.  Shar-
     ing, mapping type, and options are specified in the flags argument by
     OR'ing the following values:

     MAP_ANON    Map anonymous memory not associated with any specific file.
                 The file descriptor used for creating MAP_ANON must currently
                 be -1 indicating no name is associated with the region.

     MAP_FILE    Mapped from a regular file or character-special device memo-
                 ry.  (This is the default mapping type, and need not be spec-

     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.

                 Notify the kernel that the region may contain semaphores and
                 that special handling may be necessary.

                 Permit regions to be inherited across exec(3) system calls.

                 Modifications are private.

     MAP_SHARED  Modifications are shared.

     MAP_COPY    Modifications are private and, unlike MAP_PRIVATE, modifica-
                 tions made by others are not visible.  This option is depre-
                 cated, shouldn't be used and behaves just like MAP_PRIVATE in
                 the current implementation.

     The close(2) function does not unmap pages; see munmap(2) for further in-

     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 de-
     vice to which swapping should be done.

     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 param-
                   eter 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.

     [EBADF]       fd is not a valid open file descriptor.

     [EINVAL]      MAP_FIXED was specified and the addr parameter was not page
                   aligned.  fd did not reference a regular or character spe-
                   cial file.

     [ENOMEM]      MAP_FIXED was specified and the addr parameter wasn't
                   available.  MAP_ANON was specified and insufficient memory
                   was available.

     madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munmap(2),

     Due to a limitation of the current vm system (see uvm(9)), mapping de-
     scriptors PROT_WRITE without also specifying PROT_READ is useless (re-
     sults in a segmentation fault when first accessing the mapping).  This
     means that such descriptors must be opened with O_RDWR, which requires
     both read and write permissions on the underlying object.

OpenBSD 3.8                      June 4, 1993                                2