The flags argument 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. 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 must be -1. The offset argument 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. If a MAP_FIXED request is successful, the mapping established by mmap replaces any previous mappings for the process’ pages in the range from addr to addr + len. 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
This flag never operated as advertised and is no longer supported. Please refer to minherit(2) for further information.
MAP_NOCORE
Region is not included in a core file.
MAP_NOSYNC
Causes data dirtied via this VM map to be flushed to physical media only when necessary (usually by the pager) rather than gratuitously. Typically this prevents the update daemons from flushing pages dirtied through such maps and thus allows efficient sharing of memory across unassociated processes using a file-backed shared memory map. Without this option any VM pages you dirty may be flushed to disk every so often (every 30-60 seconds usually) which can create performance problems if you do not need that to occur (such as when you are using shared file-backed mmap regions for IPC purposes). Note that VM/file system coherency is maintained whether you use MAP_NOSYNC or not. This option is not portable across Unix platforms (yet), though some may implement the same behavior by default.
WARNING ! Extending a file with ftruncate(2), thus creating a big hole, and then filling the hole by modifying a shared mmap can lead to severe file fragmentation. In order to avoid such fragmentation you should always pre-allocate the file’s backing store by write ing zero’s into the newly extended area prior to modifying the area via your mmap. The fragmentation problem is especially sensitive to MAP_NOSYNC pages, because pages may be flushed to disk in a totally random order.
The same applies when using MAP_NOSYNC to implement a file-based shared memory store. It is recommended that you create the backing store by write ing zero’s to the backing file rather than ftruncate ing it. You can test file fragmentation by observing the KB/t (kilobytes per transfer) results from an "iostat 1" while reading a large file sequentially, e.g. using "dd if=filename of=/dev/null bs=32k".
The fsync(2) system call will flush all dirty data and metadata associated with a file, including dirty NOSYNC VM data, to physical media. The sync(8) command and sync(2) system call generally do not flush dirty NOSYNC VM data. The msync(2) system call is obsolete since BSD implements a coherent file system buffer cache. However, it may be used to associate dirty VM pages with file system buffers and thus cause them to be flushed to physical media sooner rather than later.
MAP_PRIVATE
Modifications are private.
MAP_SHARED
Modifications are shared.
MAP_STACK
MAP_STACK implies MAP_ANON, and offset of 0. The fd argument must be -1 and prot must include at least PROT_READ and PROT_WRITE. This option creates a memory region that grows to at most len bytes in size, starting from the stack top and growing down. The stack top is the starting address returned by the call, plus len bytes. The bottom of the stack at maximum growth is the starting address returned by the call.
The close(2) system call does not unmap pages, see munmap(2) for further information.
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.
The flag PROT_READ was specified as part of the prot argument and fd was not open for reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the flags and prot argument and fd was not open for writing.
[EBADF]
The fd argument is not a valid open file descriptor.
[EINVAL]
MAP_FIXED was specified and the addr argument was not page aligned, or part of the desired address space resides out of the valid address space for a user process.
[EINVAL]
The len argument was negative.
[EINVAL]
MAP_ANON was specified and the fd argument was not -1.
[EINVAL]
MAP_ANON has not been specified and fd did not reference a regular or character special file.
[EINVAL]
The offset argument was not page-aligned. (See BUGS below.)
[ENOMEM]
MAP_FIXED was specified and the addr argument was not available. MAP_ANON was specified and insufficient memory was available. The system has reached the per-process mmap limit specified in the vm.max_proc_mmap sysctl.
