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NAME    [Toc]    [Back]

     mmap - map files or devices into memory

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <sys/mman.h>

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

DESCRIPTION    [Toc]    [Back]

     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   Pages may not be accessed.

     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 enforcable.

     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, MAP_PRIVATE or MAP_COPY must be specified.  Sharing,
 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 regions is
                 used only for naming, and may be specified as -1 if no name
                 is associated with the region.

     MAP_FILE    Mapped from a regular file or character-special device memory.

     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 execve(2) system calls.

                 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

                 Modifications made by this process are private, however modifications
 made by other processes using MAP_SHARED will be

     MAP_SHARED  Modifications are shared.

     MAP_COPY    Modifications are private, including other processes.

     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.

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     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

     [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.  len was less than zero.

     [ENODEV]           fd did not reference a regular or character special

     [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    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     The MAP_COPY flag is not implemented.  The current MAP_COPY semantics are
     the same as those of the MAP_PRIVATE flag.

BSD                              May 11, 1995                              BSD
[ Back ]
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