HP-UX Reference (11i v2 07/12) - 2 System Calls (vol 5)

m
mmap(2) mmap(2)
implementation selects a value for pa, it never places a mapping at address 0, replaces any extant map-
ping, or maps into dynamic memory allocation areas.
The off argument is constrained to be aligned and sized according to the value returned by
sysconf()
when passed _SC_PAGESIZE or _SC_PAGE_SIZE
. When MAP_FIXED is specified, the argument
addr must also meet these constraints. The implementation performs mapping operations over whole
pages. Thus, while the argument len need not meet a size or alignment constraint, the implementation will
include, in any unmapping operation, any partial page specified by the range [pa, pa+len].
The
MAP_FILE and MAP_ANONYMOUS
flags control whether the region to be mapped is a mapped file
region or an anonymous shared memory region. Exactly one of these flags must be selected.
If
MAP_FILE is set in flags, a new mapped file region is created, mapping the file associated with fildes.
off specifies the file byte offset at which the mapping starts. This offset must be a multiple of the page size
returned by
sysconf(_SC_PAGE_SIZE)
. If the end of the mapped file region is beyond the end of the
file, any reference to an address in the mapped file region corresponding to an offset beyond the end of the
file results in the delivery of a
SIGBUS signal to the process, unless the address lies in the last partial
page corresponding to the range beyond the end of the file. The last partial page mapping the range
beyond the end of the file is always initialized to zeros, and any modified portions of the last page of a file
which are beyond the file’s end are not written back to the file.
If
MAP_ANONYMOUS is set in flags, a new memory region is created and initialized to all zeros. This
memory region can be shared only with descendants of the current process. If the fildes argument is not
-1, an [EINVAL] error is generated. This is not enforced in the current release, but will be enforced in the
next release. The value of off is meaningless because there is no underlying file object for the memory
region.
If MAP_NORESERVE is set in flags, no swap space is initially reserved for the private mapping. Without
this flag, the creation of a
MAP_PRIVATE region reserves swap space equal to the size of the mapping.
When a page in the mapping is first modified (written to), a private page is created and the swap space
reserved is used to hold the private copy of the data in the event of a page-out. An initial write into a page
of a MAP_NORESERVE mapping produces results which depend on the current availability of system swap
space, since the swap space reservation occurs at the time of the first write and only for the affected page.
If the swap space reservation can be made for the page, the write succeeds as described above. If not, the
write fails and a
SIGBUS signal is posted to the writing process for the effective virtual address.
madvise(...,MADV_DONTNEED)
on a MAP_NORESERVE object will release swap space reservations
for relevant pages.
Specifying
MAP_GLOBAL in a call to mmap() forces the allocation to happen from the global quadrant as
compared to the private quadrants. See setmemwindow(1M) for further details.
If MAP_IO is set in flags:
• The region will represent ranges of I/O memory instead of core memory. This allows direct access
to memory on I/O devices. I/O space is always mapped with both read and write access rights,
regardless of the actual permissions specified by the prot argument. Multiple processes can con-
currently mmap() a single I/O memory range. It is the responsibility of the processes to syn-
chronize their accesses. Successive calls to
mmap() to map the same I/O range must be identical
to the first mapping. Identical mappings have the same address and size.
• The process can additionally share I/O space (mapped via iomap) with a kernel driver. However,
this is only possible if the driver maps in the I/O space with user read/write access rights using the
appropriate driver I/O mapping services. Any I/O space mapped by drivers with kernel read/write
access rights can not be concurrently mapped by processes using mmap().
• Calls using MAP_IO implicitly also use MAP_SHARED and MAP_VARIABLE as mentioned above.
Explicit use of these flags is encouraged for forward compatibility, but not required. The required
arguments to mmap() with MAP_IO set are: addr is set to NULL, fildes is set to -1, and off is set
to the starting physical address of the I/O range to be mapped. This address will be either 40, 44
or 64-bits and use of MAP_IO for 32-bit processes is therefore restricted to users of mmap64().
• The MAP_IO functionality is currently restricted to Real Time processes or users with the
DEVOPS privilege.
Note that the flags MAP_MEM_INTERLEAVED, MAP_MEM_LOCAL, and MAP_MEM_FIRST_TOUCH
involve the placement of physical memory. This is important only on Cache Coherent Non-Uniform
Memory Architecture (ccNUMA) systems. They will have no effect otherwise. For more information, see
mpctl(2). These flags are hints to the system. If memory of the desired type is not available, the next most
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