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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95 29.\" $FreeBSD: src/lib/libc/sys/mmap.2,v 1.22.2.12 2002/02/27 03:40:13 dd Exp $ 30.\" 31.Dd January 18, 2015 32.Dt MMAP 2 33.Os 34.Sh NAME 35.Nm mmap 36.Nd allocate memory, or map files or devices into memory 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/types.h 41.In sys/mman.h 42.Ft void * 43.Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset" 44.Sh DESCRIPTION 45The 46.Fn mmap 47function causes the pages starting at 48.Fa addr 49and continuing for at most 50.Fa len 51bytes to be mapped from the object described by 52.Fa fd , 53starting at byte offset 54.Fa offset . 55If 56.Fa len 57is not a multiple of the pagesize, the mapped region may extend past the 58specified range. 59Any such extension beyond the end of the mapped object will be zero-filled. 60.Pp 61If 62.Fa addr 63is non-zero, it is used as a hint to the system. 64(As a convenience to the system, the actual address of the region may differ 65from the address supplied.) 66If 67.Fa addr 68is zero, an address will be selected by the system. 69The actual starting address of the region is returned. 70A successful 71.Fa mmap 72deletes any previous mapping in the allocated address range. 73.Pp 74The protections (region accessibility) are specified in the 75.Fa prot 76argument by 77.Em or Ns 'ing 78the following values: 79.Pp 80.Bl -tag -width PROT_WRITE -compact 81.It Dv PROT_NONE 82Pages may not be accessed. 83.It Dv PROT_READ 84Pages may be read. 85.It Dv PROT_WRITE 86Pages may be written. 87.It Dv PROT_EXEC 88Pages may be executed. 89.El 90.Pp 91The 92.Fa flags 93parameter specifies the type of the mapped object, mapping options and 94whether modifications made to the mapped copy of the page are private 95to the process or are to be shared with other references. 96Sharing, mapping type and options are specified in the 97.Fa flags 98argument by 99.Em or Ns 'ing 100the following values: 101.Bl -tag -width MAP_HASSEMAPHORE 102.It Dv MAP_ANON 103Map anonymous memory not associated with any specific file. 104The file descriptor used for creating 105.Dv MAP_ANON 106must be \-1. 107The 108.Fa offset 109parameter is ignored. 110.It Dv MAP_ANONYMOUS 111This flag is an alias for 112.Dv MAP_ANON 113and is provided for compatibility. 114.\".It Dv MAP_FILE 115.\"Mapped from a regular file or character-special device memory. 116.It Dv MAP_FIXED 117Do not permit the system to select a different address than the one 118specified. 119If the specified address contains other mappings those mappings will 120be replaced. 121If the specified address cannot otherwise be used, 122.Fn mmap 123will fail. 124If 125.Dv MAP_FIXED 126is specified, 127.Fa addr 128must be a multiple of the pagesize. 129.It Dv MAP_TRYFIXED 130Try to do a fixed mapping but fail if another mapping already exists in 131the space instead of overwriting the mapping. 132.Pp 133When used with MAP_STACK this flag allows one MAP_STACK mapping to be 134made within another (typically the master user stack), as long as 135no pages have been faulted in the area requested. 136.It Dv MAP_HASSEMAPHORE 137Notify the kernel that the region may contain semaphores and that special 138handling may be necessary. 139.It Dv MAP_NOCORE 140Region is not included in a core file. 141.It Dv MAP_NOSYNC 142Causes data dirtied via this VM map to be flushed to physical media 143only when necessary (usually by the pager) rather than gratuitously. 144Typically this prevents the update daemons from flushing pages dirtied 145through such maps and thus allows efficient sharing of memory across 146unassociated processes using a file-backed shared memory map. Without 147this option any VM pages you dirty may be flushed to disk every so often 148(every 30-60 seconds usually) which can create performance problems if you 149do not need that to occur (such as when you are using shared file-backed 150mmap regions for IPC purposes). Note that VM/filesystem coherency is 151maintained whether you use 152.Dv MAP_NOSYNC 153or not. This option is not portable 154across 155.Ux 156platforms (yet), though some may implement the same behavior 157by default. 158.Pp 159.Em WARNING ! 160Extending a file with 161.Xr ftruncate 2 , 162thus creating a big hole, and then filling the hole by modifying a shared 163.Fn mmap 164can lead to severe file fragmentation. 165In order to avoid such fragmentation you should always pre-allocate the 166file's backing store by 167.Fn write Ns ing 168zero's into the newly extended area prior to modifying the area via your 169.Fn mmap . 170The fragmentation problem is especially sensitive to 171.Dv MAP_NOSYNC 172pages, because pages may be flushed to disk in a totally random order. 173.Pp 174The same applies when using 175.Dv MAP_NOSYNC 176to implement a file-based shared memory store. 177It is recommended that you create the backing store by 178.Fn write Ns ing 179zero's to the backing file rather than 180.Fn ftruncate Ns ing 181it. 182You can test file fragmentation by observing the KB/t (kilobytes per 183transfer) results from an 184.Dq Li iostat 1 185while reading a large file sequentially, e.g. using 186.Dq Li dd if=filename of=/dev/null bs=32k . 187.Pp 188The 189.Xr fsync 2 190function will flush all dirty data and metadata associated with a file, 191including dirty NOSYNC VM data, to physical media. The 192.Xr sync 8 193command and 194.Xr sync 2 195system call generally do not flush dirty NOSYNC VM data. 196The 197.Xr msync 2 198system call is obsolete since 199.Bx 200implements a coherent filesystem buffer cache. However, it may be 201used to associate dirty VM pages with filesystem buffers and thus cause 202them to be flushed to physical media sooner rather than later. 203.It Dv MAP_PRIVATE 204Modifications are private. 205.It Dv MAP_SHARED 206Modifications are shared. 207.It Dv MAP_STACK 208Map the area as a stack. 209.Dv MAP_ANON 210is implied. 211.Fa Offset 212should be 0, 213.Fa fd 214must be -1, and 215.Fa prot 216should include at least 217.Dv PROT_READ 218and 219.Dv PROT_WRITE . 220This option creates 221a memory region that grows to at most 222.Fa len 223bytes in size, starting from the stack top and growing down. The 224stack top is the starting address returned by the call, plus 225.Fa len 226bytes. 227The bottom of the stack at maximum growth is the starting 228address returned by the call. 229.Pp 230The entire area is reserved from the point of view of other 231.Fn mmap 232calls, even if not faulted in yet. 233.Pp 234WARNING. We currently allow 235.Dv MAP_STACK 236mappings to provide a hint that points within an existing 237.Dv MAP_STACK 238mapping's space, and this will succeed as long as no page have been 239faulted in the area specified, but this behavior is no longer supported 240unless you also specify the 241.Dv MAP_TRYFIXED 242flag. 243.Pp 244Note that unless 245.Dv MAP_FIXED 246or 247.Dv MAP_TRYFIXED 248is used, you cannot count on the returned address matching the hint 249you have provided. 250.It Dv MAP_VPAGETABLE 251Memory accessed via this map is not linearly mapped and will be governed 252by a virtual page table. The base address of the virtual page table may 253be set using 254.Xr mcontrol 2 255with 256.Dv MADV_SETMAP . 257Virtual page tables work with anonymous memory but there 258is no way to populate the page table so for all intents and purposes 259.Dv MAP_VPAGETABLE 260can only be used when mapping file descriptors. Since the kernel will 261update the VPTE_M bit in the virtual page table, the mapping must R+W 262even though actual access to the memory will be properly governed by 263the virtual page table. 264.Pp 265Addressable backing store is limited by the range supported in the virtual 266page table entries. The kernel may implement a page table abstraction capable 267of addressing a larger range within the backing store then could otherwise 268be mapped into memory. 269.El 270.Pp 271The 272.Xr close 2 273function does not unmap pages, see 274.Xr munmap 2 275for further information. 276.Pp 277The current design does not allow a process to specify the location of 278swap space. 279In the future we may define an additional mapping type, 280.Dv MAP_SWAP , 281in which 282the file descriptor argument specifies a file or device to which swapping 283should be done. 284.Sh RETURN VALUES 285Upon successful completion, 286.Fn mmap 287returns a pointer to the mapped region. 288Otherwise, a value of 289.Dv MAP_FAILED 290is returned and 291.Va errno 292is set to indicate the error. 293.Sh ERRORS 294.Fn Mmap 295will fail if: 296.Bl -tag -width Er 297.It Bq Er EACCES 298The flag 299.Dv PROT_READ 300was specified as part of the 301.Fa prot 302parameter and 303.Fa fd 304was not open for reading. 305The flags 306.Dv MAP_SHARED 307and 308.Dv PROT_WRITE 309were specified as part of the 310.Fa flags 311and 312.Fa prot 313parameters and 314.Fa fd 315was not open for writing. 316.It Bq Er EBADF 317.Fa fd 318is not a valid open file descriptor. 319.It Bq Er EINVAL 320.Dv MAP_FIXED 321was specified and the 322.Fa addr 323parameter was not page aligned, or part of the desired address space 324resides out of the valid address space for a user process. 325.It Bq Er EINVAL 326.Fa Len 327was negative. 328.It Bq Er EINVAL 329.Dv MAP_ANON 330was specified and the 331.Fa fd 332parameter was not -1. 333.It Bq Er EINVAL 334.Dv MAP_ANON 335has not been specified and 336.Fa fd 337did not reference a regular or character special file. 338.It Bq Er EINVAL 339.Fa Offset 340was not page-aligned. 341(See 342.Sx BUGS 343below.) 344.It Bq Er ENOMEM 345.Dv MAP_FIXED 346was specified and the 347.Fa addr 348parameter wasn't available. 349.Dv MAP_ANON 350was specified and insufficient memory was available. 351The system has reached the per-process mmap limit specified in the 352.Va vm.max_proc_mmap 353sysctl. 354.El 355.Sh SEE ALSO 356.Xr madvise 2 , 357.Xr mincore 2 , 358.Xr mlock 2 , 359.Xr mprotect 2 , 360.Xr msync 2 , 361.Xr munlock 2 , 362.Xr munmap 2 , 363.Xr getpagesize 3 364