mmap(2)mmap(2)Namemmap - maps memory of a character device
caddr_t mmap(addr, len, prot, flags, fd, off)
addr Specifies the address space of the calling process where
begins mapping the memory associated with the specified char‐
len Specifies the number of bytes to map.
prot Specifies the protection flag for the mapping. The protec‐
tion flag is the bitwise inclusive OR of these valid protec‐
tion flag bits defined in or
flags Specifies the mapping flag. The mapping flag is the bitwise
inclusive OR of these valid flag mapping bits defined in or
fd Specifies the file descriptor for the character device being
off Specifies the offset in bytes into the character device's
The system call maps the memory of a character device (fd) starting at
a specified offset (off) for a specified number of bytes (len) into the
address space of the calling process. The routine does not map into
private data, stack, or text segments. This routine does not replace
any existing mappings. All the mapped regions are inherited by a child
process on a fork.
When calling you need to:
· Align the offset
You must align the offset (off) on a page boundary as returned by
the system call.
· Indicate the number of bytes to map
The number of bytes to map (len) need not be a multiple of the
page size as returned by However, will round the value of len to
the nearest multiple of the page size as returned by and perform
the mapping over whole pages.
· Specify the protection for the mapping
You specify the protection for the mapping by ORing the prot argu‐
ment with one or more of the constants or If you specify only both
VAX and RISC architectures will grant read access. If you specify
only then both VAX and RISC architectures will grant read and
· Indicate the address at which the mapping occurs
The addr and flags arguments are used by to determine the actual
address at which the mapping is done. You specify the mapping
flag by ORing the flags argument with one or more of the constants
Based on the values you specify for addr and flags, performs the
mapping as follows:
- If addr is equal to zero, selects the address at which
the mapping is done.
- If addr is not equal to zero and you specified the map‐
ping is done at the address specified in addr. You can
obtain the boundary on which addr must be aligned by
calling the system call.
- If addr is not equal to zero and you did not specify the
mapping is done at the address rounded down to the
boundary returned by The value returned by is machine-
You must observe the following restrictions when calling
· Maximum number of mapped regions for each process
The shared memory functionality upon which is implemented requires
that you not exceed the maximum number of shared memory segments
allowed for each process. The default value is six shared memory
segments for each process. You can configure (in the system con‐
figuration file) the number of shared memory segments for each
process by using the constant
· Maximum number of mapped regions on the system
The shared memory functionality imposes a limit on the maximum
number of mapped regions on the system. The default limit is 100,
sufficient to support a maximum configuration of all devices that
can be mapped. The maximum configuration is 2*21 devices (2 VME
adapters, 21 devices for each bus).
· A process mapping the same region of device memory
A process cannot map the same region of device memory more than
once into its address space. This limitation is a result of the
lack of multiple attach capability for shared memory. However, a
single process can map different regions of device memory simulta‐
neously into its address space. Two regions of memory are identi‐
cal only if they start at the same offset in device memory and are
of the same length.
· Use of the system call
The system call does not work on the addresses mapped by a call to
The reason is that does not work for shared memory and therefore
cannot be used to change protection for the address space mapped
If successful, returns the address where the mapping was done. Other‐
wise, it returns -1 and sets the errno argument to one of the errors
listed in the Diagnostics section.
The call to fails if one or more of the following is true:
[EBADF] The device, represented by the file descriptor fd, is not
[EACCES] The device, represented by the file descriptor fd, is not
open for read operations and you specified the protection
flag in prot. Or, the device is not open for write opera‐
tions and you specified the protection flag in prot.
[ENXIO] Addresses in the range [ off, off + len ] are invalid for
this device, represented by the file descriptor fd.
[EINVAL] You did not specify either or in the flags argument.
[EINVAL] You did not specify either or in the prot argument.
[EINVAL] The file descriptor, fd, is not a valid file descriptor for a
[ENODEV] The file descriptor, fd, refers to a device that does not
support the system call.
[ENOMEM] There is not enough room in the address space to perform the
See Alsogetpagesize(2), getsysinfo(2), mmapalignment(1), munmap(2)mmap(2)