mmap man page on 4.4BSD

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MMAP(2)			    BSD System Calls Manual		       MMAP(2)

     mmap — map files or devices into memory

     #include <sys/types.h>
     #include <sys/mman.h>

     mmap(caddr_t 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 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.

     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.	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 be -1.
		 The offset parameter is ignored.

     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(2) system calls.

		 Modifications are private.

     MAP_SHARED	 Modifications are shared.

     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.

     Upon successful completion, mmap returns a pointer to the mapped region.
     Otherwise, a value of -1 is returned and errno is set to indicate the

     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]		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.  did not reference a regular or character special
			file.  Len was less than zero.

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

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

4th Berkeley Distribution     September 27, 2021     4th Berkeley Distribution

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