startMap the buffer to this address in the
application's address space. When the MAP_FIXED
flag is specified, start must be a multiple of the
pagesize and mmap will fail when the specified address
cannot be used. Use of this option is discouraged; applications should
just specify a NULL pointer here.
lengthLength of the memory area to map. This must be the
same value as returned by the driver in the struct v4l2_buffer
length field.
protThe prot argument describes the
desired memory protection. Regardless of the device type and the
direction of data exchange it should be set to
PROT_READ | PROT_WRITE,
permitting read and write access to image buffers. Drivers should
support at least this combination of flags. Note the Linux
video-buf kernel module, which is used by the
bttv, saa7134, saa7146, cx88 and vivi driver supports only
PROT_READ | PROT_WRITE. When
the driver does not support the desired protection the
mmap() function fails.
Note device memory accesses (e. g. the memory on a graphics card with video capturing hardware) may incur a performance penalty compared to main memory accesses, or reads may be significantly slower than writes or vice versa. Other I/O methods may be more efficient in this case.
flagsThe 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.
MAP_FIXED requests that the
driver selects no other address than the one specified. If the
specified address cannot be used, mmap() will fail. If
MAP_FIXED is specified,
start must be a multiple of the pagesize. Use
of this option is discouraged.
One of the MAP_SHARED or
MAP_PRIVATE flags must be set.
MAP_SHARED allows applications to share the
mapped memory with other (e. g. child-) processes. Note the Linux
video-buf module which is used by the bttv,
saa7134, saa7146, cx88 and vivi driver supports only
MAP_SHARED. MAP_PRIVATE
requests copy-on-write semantics. V4L2 applications should not set the
MAP_PRIVATE, MAP_DENYWRITE,
MAP_EXECUTABLE or MAP_ANON
flag.
fdFile descriptor returned by open().
offsetOffset of the buffer in device memory. This must be the
same value as returned by the driver in the struct v4l2_buffer
m union offset field.
The mmap() function asks to map
length bytes starting at
offset in the memory of the device specified by
fd into the application address space,
preferably at address start. This latter
address is a hint only, and is usually specified as 0.
Suitable length and offset parameters are queried with the
VIDIOC_QUERYBUF ioctl. Buffers must be allocated with the
VIDIOC_REQBUFS ioctl before they can be queried.
To unmap buffers the munmap() function is used.
On success mmap() returns a pointer to
the mapped buffer. On error MAP_FAILED (-1) is
returned, and the errno variable is set
appropriately. Possible error codes are:
fd is not a valid file
descriptor.
fd is
not open for reading and writing.
The start or
length or offset are not
suitable. (E. g. they are too large, or not aligned on a
PAGESIZE boundary.)
The flags or
prot value is not supported.
No buffers have been allocated with the
VIDIOC_REQBUFS ioctl.
Not enough physical or virtual memory was available to complete the request.