PHYSIO(9) | Kernel Developer's Manual | PHYSIO(9)

# NAME

`physio` — initiate I/O on raw devices

# SYNOPSIS

int
`physio`(void (*strategy)(buf_t *), buf_t *bp, dev_t dev, int flags, void (*minphys)(buf_t *), struct uio *uio);

# DESCRIPTION

The `physio`() is a helper function typically called from character device read and write routines to start I/O on a user process buffer. It calls back on the provided strategy routine one or more times to complete the transfer described by uio. The maximum amount of data to transfer with each call to strategy is determined by the minphys routine.

Since uio normally describes user space addresses, `physio`() needs to lock the appropriate data area into memory before each transaction with strategy (see uvm_vslock(9) and uvm_vsunlock(9)). The `physio`() function always awaits the completion of the entire requested transfer before returning, unless an error condition is detected earlier. In all cases, the buffer passed in bp is locked (marked as “busy”) for the duration of the entire transfer.

A break-down of the arguments follows:

strategy
  The device strategy routine to call for each chunk of data to initiate device I/O.
bp
  The buffer to use with the strategy routine. The buffer flags will have `B_BUSY`, `B_PHYS`, and `B_RAW` set when passed to the strategy routine. If `NULL`, a buffer is allocated from a system pool.
dev
  The device number identifying the device to interact with.
flags
  Direction of transfer; the only valid settings are `B_READ` or `B_WRITE`.
minphys
  A device specific routine called to determine the maximum transfer size that the device's strategy routine can handle.
uio
  The description of the entire transfer as requested by the user process. Currently, the results of passing a uio structure with the ‘uio_segflg’ set to anything other than `UIO_USERSPACE`, are undefined.

# RETURN VALUES

If successful `physio`() returns 0. `EFAULT` is returned if the address range described by uio is not accessible by the requesting process. `physio`() will return any error resulting from calls to the device strategy routine, by examining the `B_ERROR` buffer flag and the ‘b_error’ field. Note that the actual transfer size may be less than requested by uio if the device signals an “end of file” condition.

# SEE ALSO

read(2), write(2)

September 12, 2019 | NetBSD 11.0