7.63. V4L2 mmap()¶
v4l2-mmap - Map device memory into application address space
#include <unistd.h> #include <sys/mman.h>
mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset)¶
- Map the buffer to this address in the application’s address space.
MAP_FIXEDflag is specified,
startmust 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
- Length of the memory area to map. This must be the same value as
returned by the driver in the struct
lengthfield for the single-planar API, and the same value as returned by the driver in the struct
lengthfield for the multi-planar API.
protargument describes the desired memory protection. Regardless of the device type and the direction of data exchange it should be set to
PROT_WRITE, permitting read and write access to image buffers. Drivers should support at least this combination of flags.
- The Linux
videobufkernel module, which is used by some drivers supports only
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
flagsparameter 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_FIXEDrequests that the driver selects no other address than the one specified. If the specified address cannot be used, mmap() will fail. If
startmust be a multiple of the pagesize. Use of this option is discouraged.
One of the
MAP_PRIVATEflags must be set.
MAP_SHAREDallows applications to share the mapped memory with other (e. g. child-) processes.
videobufmodule which is used by some drivers supports only
MAP_PRIVATErequests copy-on-write semantics. V4L2 applications should not set the
- File descriptor returned by open().
- Offset of the buffer in device memory. This must be the same value
as returned by the driver in the struct
offsetfield for the single-planar API, and the same value as returned by the driver in the struct
mem_offsetfield for the multi-planar API.
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.
7.63.5. Return Value¶
On success mmap() returns a pointer to the mapped buffer. On
MAP_FAILED (-1) is returned, and the
errno variable is set
appropriately. Possible error codes are:
fdis not a valid file descriptor.
fdis not open for reading and writing.
offsetare not suitable. (E. g. they are too large, or not aligned on a
protvalue is not supported.
No buffers have been allocated with the ioctl VIDIOC_REQBUFS ioctl.
- Not enough physical or virtual memory was available to complete the request.