VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O


int ioctl(int fd, int request, struct v4l2_requestbuffers *argp);



File descriptor returned by open().





This ioctl is used to initiate memory mapped or user pointer I/O. Memory mapped buffers are located in device memory and must be allocated with this ioctl before they can be mapped into the application's address space. User buffers are allocated by applications themselves, and this ioctl is merely used to switch the driver into user pointer I/O mode.

To allocate device buffers applications initialize three fields of a v4l2_requestbuffers structure. They set the type field to the respective stream or buffer type, the count field to the desired number of buffers, and memory must be set to V4L2_MEMORY_MMAP. When the ioctl is called with a pointer to this structure the driver attempts to allocate the requested number of buffers and stores the actual number allocated in the count field. It can be smaller than the number requested, even zero, when the driver runs out of free memory. A larger number is possible when the driver requires more buffers to function correctly.[1] When memory mapping I/O is not supported the ioctl returns an EINVAL error code.

Applications can call VIDIOC_REQBUFS again to change the number of buffers, however this cannot succeed when any buffers are still mapped. A count value of zero frees all buffers, after aborting or finishing any DMA in progress, an implicit VIDIOC_STREAMOFF.

To negotiate user pointer I/O, applications initialize only the type field and set memory to V4L2_MEMORY_USERPTR. When the ioctl is called with a pointer to this structure the driver prepares for user pointer I/O, when this I/O method is not supported the ioctl returns an EINVAL error code.

Table 1. struct v4l2_requestbuffers

__u32countThe number of buffers requested or granted. This field is only used when memory is set to V4L2_MEMORY_MMAP.
enum v4l2_buf_typetypeType of the stream or buffers, this is the same as the struct v4l2_format type field. See Table 3-2 for valid values.
enum v4l2_memorymemoryApplications set this field to V4L2_MEMORY_MMAP or V4L2_MEMORY_USERPTR.
__u32reserved[2]A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:


The driver supports multiple opens and I/O is already in progress, or reallocation of buffers was attempted although one or more are still mapped.


The buffer type (type field) or the requested I/O method (memory) is not supported.



For example video output requires at least two buffers, one displayed and one filled by the application.