Manual browser: mmap(2)

MMAP(2) System Calls Manual MMAP(2)


mmapmap files or devices into memory


Standard C Library (libc, -lc)


#include <sys/mman.h>

void *
mmap(void *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 convenience 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:

Pages may be executed.
Pages may be read.
Pages may be written.
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 enforceable.

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. Note that either MAP_SHARED or MAP_PRIVATE must be specified. Sharing, mapping type and options are specified in the flags argument by OR'ing the following values:

Request that the allocation be aligned to the given boundary. The parameter n should be the base 2 logarithm of the desired alignment (e.g., to request alignment to 16K, use 14 as the value for n). The alignment must be equal to or greater than the platform's page size as returned by sysconf(3) with the _SC_PAGESIZE request.
Map anonymous memory not associated with any specific file. The file descriptor is not used for creating MAP_ANON regions, and must be specified as -1. The mapped memory will be zero filled.
Mapped from a regular file or character-special device memory.
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 execve(2) system calls.
Attempt to use the address addr even if it falls within the normally protected process data or text segment memory regions. If the requested region of memory is actually present in the memory map, a different address will be selected as if MAP_TRYFIXED had not been specified. If addr is NULL, this flag is ignored and the system will select a mapping address.
Lock the mapped region into memory as with mlock(2).
Modifications made by this process are private, however modifications made by other processes using MAP_SHARED will be seen.
Modifications are shared.

The close(2) function 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.

If MAP_FIXED is not specified, the system will attempt to place the mapping in an unused portion of the address space chosen to minimize possible collision between mapped regions and the heap.


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.


mmap() will fail if:
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 writing.
fd is not a valid open file descriptor.
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.
fd did not reference a regular or character special file.
MAP_FIXED was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available.
fd references a regular file and the value of offset plus len would exceed the offset maximum established in its open file description.


The mmap() function conforms to IEEE Std 1003.1b-1993 (“POSIX.1b”).


The mmap() interface was first designed in 4.2BSD.
December 20, 2011 NetBSD 7.0