V4L2 mmap()
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Name

v4l2-mmap — Map device memory into application address space

Synopsis

#include <unistd.h>
#include <sys/mman.h>
void *mmap(void *start,
 size_t length,
 int prot,
 int flags,
 int fd,
 off_t offset);
 

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 a NULL 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_plane length 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 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.

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. 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.

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 union offset field for the single-planar API, and the same value as returned by the driver in the struct v4l2_plane m union mem_offset field for the multi-planar API.

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 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.

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 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.

ENOMEM

Not enough physical or virtual memory was available to complete the request.

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