---
name: "intro"
section: "3rtapi"
platform: "linux"
package: "linuxcnc-uspace-dev"
version: "1:2.9.4-2+deb13u1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/intro/3rtapi"
---

# intro(3rtapi)

Introduction to the RTAPI API

## NAME

rtapi - Introduction to the RTAPI API

## DESCRIPTION

RTAPI is a library providing a uniform API for several real time operating systems.  As of LinuxCNC 2.7, POSIX threads and RTAI are supported.

## HEADER FILES

### rtapi.h

The file **rtapi.h** defines the RTAPI for both realtime and non-realtime code.  This is a change from Rev 2, where the non-realtime API was defined in ulapi.h and used different function names.  The symbols RTAPI and ULAPI are used to determine which mode is being compiled, RTAPI for realtime and ULAPI for non-realtime.

### rtapi\_math.h

The file rtapi\_math.h defines floating-point functions and constants. It should be used instead of \<math.h\> in rtapi real-time components.

### rtapi\_string.h

The file rtapi\_string.h defines string-related functions. It should be used instead of \<string.h\> in rtapi real-time components.

### rtapi\_byteorder.h

This file defines the preprocessor macros RTAPI\_BIG\_ENDIAN, RTAPI\_LITTLE\_ENDIAN, and RTAPI\_FLOAT\_BIG\_ENDIAN as true or false depending on the characteristics of the target system.  It should be used instead of **\<endian.h\>** (userspace) or **\<linux/byteorder.h\>** (kernel space).

### rtapi\_limits.h

This file defines the minimum and maximum value of some fundamental integral types, such as INT\_MIN and INT\_MAX.  This should be used instead of **\<limits.h\>** because that header file is not available to kernel modules.

## REALTIME CONSIDERATIONS

### Non-realtime code

Certain functions are not available in non-realtime code.  This includes functions that perform direct device access such as **rtapi\_inb(3)**.

### Init/cleanup code

Certain functions may only be called from realtime init/cleanup code. This includes functions that perform memory allocation, such as **rtapi\_shmem\_new(3)**.

### Realtime code

Only a few functions may be called from realtime code.  This includes functions that perform direct device access such as **rtapi\_inb(3)**. It excludes most Linux kernel APIs such as do\_gettimeofday(3) and many rtapi APIs such as rtapi\_shmem\_new(3).

### Simulator

For an RTAPI module to be buildable in the "sim" environment (fake realtime system without special privileges), it must not use **any** linux kernel APIs, and must not use the RTAPI APIs for direct device access such as **rtapi\_inb(3)**.  This automatically includes any hardware device drivers, and also devices which use Linux kernel APIs to do things like create special devices or entries in the **/proc** filesystem.

## RTAPI STATUS CODES

Except as noted in specific manual pages, RTAPI returns negative errno values for errors, and nonnegative values for success.