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MMAP(2) System Calls Manual MMAP(2)

mmap — map files or devices into memory

#include <sys/types.h>
#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 contents of fd, starting at offset, to be mapped in memory at the given addr. The mapping will extend at least len bytes, subject to page alignment restrictions.

The addr argument describes the address where the system should place the mapping. If the MAP_FIXED flag is specified, the allocation will happen at the specified address, replacing any previously established mappings in its range. Otherwise, the mapping will be placed at the available spot at addr; failing that it will be placed "close by". If addr is NULL the system can pick any address. Except for MAP_FIXED mappings, the system will never replace existing mappings.

The len argument describes the minimum amount of bytes the mapping will span. Since mmap maps pages into memory, len may be rounded up to hit a page boundary. If len is 0, the mapping will fail with EINVAL.

If an fd and offset are specified, the resulting address may end up not on a page boundary, in order to align the page offset in the addr to the page offset in offset.

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.
No permissions.

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. Sharing, mapping type, and options are specified in the flags argument by OR'ing the following values:

Map anonymous memory not associated with any specific file. The file descriptor used for creating MAP_ANON must currently be -1 indicating no name is associated with the region.
Mapped from a regular file or character-special device memory. (This is the default mapping type, and need not be specified.)
Demand that the mapping is placed at addr, rather than having the system select a location. addr, len and offset (in the case of fd mappings) must be multiples of the page size. Existing mappings in the address range will be replaced. 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(3) system calls.
Modifications are private.
Modifications are shared.
Attempt to use the hint provided by addr. This is the default mapping type and need not be specified. Use of MAP_TRYFIXED is discouraged, as it is a non-portable extension.
Modifications are private and, unlike MAP_PRIVATE, modifications made by others are not visible. This option is deprecated, shouldn't be used and behaves just like MAP_PRIVATE in the current implementation.

The close(2) function does not unmap pages; see munmap(2) for further information.

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>. A successful return from mmap will never 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.
[]
was specified and the addr parameter was not page aligned.
[]
addr and len specified a region that would extend beyond the end of the address space.
[]
fd did not reference a regular or character special file.
[]
The allocation len was 0.
[]
was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available.

madvise(2), mincore(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3)

The mmap() system call first appeared in 4.1cBSD.

Due to a limitation of the current vm system (see uvm(9)), mapping descriptors PROT_WRITE without also specifying PROT_READ is useless (results in a segmentation fault when first accessing the mapping). This means that such descriptors must be opened with O_RDWR, which requires both read and write permissions on the underlying object.

January 21, 2014 OpenBSD-5.5