8.63. V4L2 mmap()¶
8.63.1. Name¶
v4l2-mmap - Map device memory into application address space
8.63.2. Synopsis¶
#include <unistd.h>
#include <sys/mman.h>
-
void *
mmap
(void *start, size_t length, int prot, int flags, int fd, off_t offset)¶
8.63.3. Arguments¶
start
- Map 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 aNULL
pointer here. length
- Length of the memory area to map. This must be the same value as
returned by the driver in the struct
v4l2_buffer
length
field for the single-planar API, and the same value as returned by the driver in the struct v4l2_planelength
field for the multi-planar API. prot
The
prot
argument describes the desired memory protection. Regardless of the device type and the direction of data exchange it should be set toPROT_READ
|PROT_WRITE
, permitting read and write access to image buffers. Drivers should support at least this combination of flags.Note
- The Linux
videobuf
kernel module, which is used by some drivers supports onlyPROT_READ
|PROT_WRITE
. When the driver does not support the desired protection, the mmap() function fails. - 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 such case.
- The Linux
flags
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.MAP_FIXED
requests that the driver selects no other address than the one specified. If the specified address cannot be used, mmap() will fail. IfMAP_FIXED
is specified,start
must be a multiple of the pagesize. Use of this option is discouraged.One of the
MAP_SHARED
orMAP_PRIVATE
flags must be set.MAP_SHARED
allows applications to share the mapped memory with other (e. g. child-) processes.Note
The Linux
videobuf
module which is used by some drivers supports onlyMAP_SHARED
.MAP_PRIVATE
requests copy-on-write semantics. V4L2 applications should not set theMAP_PRIVATE
,MAP_DENYWRITE
,MAP_EXECUTABLE
orMAP_ANON
flags.fd
- File descriptor returned by open().
offset
- Offset of the buffer in device memory. This must be the same value
as returned by the driver in the struct
v4l2_buffer
m
unionoffset
field for the single-planar API, and the same value as returned by the driver in the struct v4l2_planem
unionmem_offset
field for the multi-planar API.
8.63.4. Description¶
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 ioctl VIDIOC_QUERYBUF ioctl. Buffers must be allocated with the ioctl VIDIOC_REQBUFS ioctl before they can be queried.
To unmap buffers the munmap() function is used.
8.63.5. Return Value¶
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:
- EBADF
fd
is not a valid file descriptor.- EACCES
fd
is not open for reading and writing.- EINVAL
The
start
orlength
oroffset
are not suitable. (E. g. they are too large, or not aligned on aPAGESIZE
boundary.)The
flags
orprot
value is not supported.No buffers have been allocated with the ioctl VIDIOC_REQBUFS ioctl.
- ENOMEM
- Not enough physical or virtual memory was available to complete the request.