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MMAP(2) NetBSD System Calls Manual MMAP(2)
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mmap -- map files or devices into memory
Standard C Library (libc, -lc)
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 conve-
nience 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
The protections (region accessibility) are specified in the prot argument
by OR'ing the following values:
PROT_EXEC Pages may be executed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_NONE 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, MAP_PRIVATE or MAP_COPY must be specified. Shar-
ing, mapping type and options are specified in the flags argument by
OR'ing the following values:
MAP_ALIGNED(n) Request that the allocation be aligned to the given
boundary. The parameter n should be the base 2 loga-
rithm 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 plat-
form's page size as returned by sysconf(3) with the
MAP_ANON 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.
MAP_FILE Mapped from a regular file or character-special
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 page-
size. Use of this option is discouraged.
MAP_HASSEMAPHORE Notify the kernel that the region may contain sema-
phores and that special handling may be necessary.
MAP_INHERIT Permit regions to be inherited across execve(2) sys-
MAP_TRYFIXED 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 dif-
ferent 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
MAP_WIRED Lock the mapped region into memory as with mlock(2).
MAP_PRIVATE Modifications made by this process are private, how-
ever modifications made by other processes using
MAP_SHARED will be seen.
MAP_SHARED Modifications are shared.
MAP_COPY Modifications are private, including other processes.
The close(2) function does not unmap pages, see munmap(2) for further
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 map-
ping in an unused portion of the address space chosen to minimize possi-
ble 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:
[EACCES] 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
[EBADF] fd is not a valid open file descriptor.
[EINVAL] 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
[ENODEV] fd did not reference a regular or character special
[ENOMEM] MAP_FIXED was specified and the addr parameter wasn't
available. MAP_ANON was specified and insufficient
memory was available.
[EOVERFLOW] fd references a regular file and the value of offset
plus len would exceed the offset maximum established
in its open file description.
madvise(2), mincore(2), mlock(2), mprotect(2), msync(2), munmap(2),
The MAP_COPY flag is not implemented. The current MAP_COPY semantics are
the same as those of the MAP_PRIVATE flag.
NetBSD 5.0 October 6, 2003 NetBSD 5.0