---
name: "physio"
section: "9"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/physio"
---

# physio(9)

initiate I/O on raw devices

## NAME

**physio** – initiate I/O on raw devices

## SYNOPSIS

**#include \<sys/param.h\>**

**#include \<sys/systm.h\>**

**#include \<sys/bio.h\>**

**#include \<sys/buf.h\>**

*int*\

**physio**(*dev\_t dev*, *struct uio \*uio*, *int ioflag*);

## 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. The maximum amount of data to transfer with each call is determined by *dev-\>si\_iosize\_max*. The **physio**() call converts the I/O request into a **strategy**() request and passes the new request to the driver's **strategy**() routine for processing.

Since *uio* normally describes user space addresses, **physio**() needs to lock those pages into memory. This is done by calling **vmapbuf**() for the appropriate pages. **physio**() always awaits the completion of the entire requested transfer before returning, unless an error condition is detected earlier.

A break-down of the arguments follows:

- *dev*

  The device number identifying the device to interact with.

- *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.

- *ioflag*

  The ioflag argument from the **read**() or **write**() function calling **physio**().

## 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)](/read/2.md), [write(2)](/write/2.md)

## HISTORY

The **physio** manual page is originally from NetBSD with minor changes for applicability with FreeBSD.

The **physio** call has been completely re-written for providing higher I/O and paging performance.