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MMAP(2) System Calls Manual MMAP(2)

mmap — map files or devices into memory

#include <sys/mman.h>

void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);

The () 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 () 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. It should either be PROT_NONE (no permissions) or the bitwise OR of one or more of the following values:

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

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. Exactly one of the first two values must be specified:

Modifications are private.
Modifications are shared.

Any combination of the following flags may additionally be used:

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.
Synonym for MAP_ANON.
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.
Indicate that the mapping is used as a stack. This flag must be used in combination with MAP_ANON and MAP_PRIVATE.
Exclude the mapping from core dumps.

Finally, the following flags are also provided for source compatibility with code written for other operating systems, but are not recommended for use in new OpenBSD code:

Modifications are private and, unlike MAP_PRIVATE, modifications made by others are not visible. On OpenBSD this behaves just like MAP_PRIVATE.
Mapped from a regular file, character special file, or block special file specified by file descriptor fd. On OpenBSD and all systems conforming to IEEE Std 1003.1-2008 (“POSIX.1”) this is the default mapping type and need not be specified.
Notify the kernel that the region may contain semaphores and that special handling may be necessary. On OpenBSD this flag is ignored.
Permit regions to be inherited across execve(2) system calls. On OpenBSD this flag is ignored.
Attempt to use the hint provided by addr. On OpenBSD this is the default behavior.

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

The mmap() function returns a pointer to the mapped region if successful; otherwise the value MAP_FAILED is returned and the global variable errno is set to indicate the error. A successful return from mmap() will never return the value MAP_FAILED.

mmap() will fail if:

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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.
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fd is not a valid open file descriptor.
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and MAP_SHARED were both specified.
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was specified and the addr parameter was not page aligned.
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addr and len specified a region that would extend beyond the end of the address space.
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fd did not specify a regular, character special, or block special file.
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fd specified a character special or block special file and the underlying device does not support memory mapping.
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The allocation len was 0.
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was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available.
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The accesses requested in the prot argument are not allowed. In particular, PROT_WRITE | mappings are not permitted unless the filesystem is mounted wxallowed and the process is link-time tagged with wxneeded. (See also kern.wxabort in sysctl(2) for a method to diagnose failure).
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The addr and len parameters specify a region which contains at least one page marked immutable.

madvise(2), mimmutable(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3), core(5)

The mmap() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

A fully functional mmap() system call first appeared in SunOS 4.0 and has been available since 4.3BSD-Net/2.

IEEE Std 1003.1-2008 (“POSIX.1”) specifies that references to pages beyond the end of a mapped object shall generate a SIGBUS signal; however, on some architectures OpenBSD generates a SIGSEGV signal in this case instead.

Additionally, IEEE Std 1003.1-2008 (“POSIX.1”) specifies that mmap() shall fail with EINVAL if neither MAP_PRIVATE nor MAP_SHARED is specified by flags; however, for compatibility with old programs, OpenBSD instead defaults to MAP_SHARED for mappings of character special files, and to MAP_PRIVATE for all other mappings. New programs should not rely on this behavior.

Due to a limitation of the current vm system (see uvm_init(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.

October 7, 2022 OpenBSD-7.6