mmap(2) - NetBSD Manual Pages

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MMAP(2)                   NetBSD Programmer's 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 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
     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   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 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.  Note
     that either MAP_SHARED, MAP_PRIVATE or MAP_COPY must be specified.  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 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 memo-
                 ry.

     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_PRIVATE
                 Modifications made by this process are private, however modi-
                 fications made by other processes using MAP_SHARED will be
                 seen.

     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 in-
     formation.

     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.


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 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 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 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), msync(2), munmap(2),
     getpagesize(3)


BUGS

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

NetBSD 1.6.1                     May 11, 1995                                2