---
name: "makecontext"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/makecontext"
---

# makecontext(3)

modify and exchange user thread contexts

## NAME

**makecontext**, **swapcontext** – modify and exchange user thread contexts

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<ucontext.h\>**

*void*\

**makecontext**(*ucontext\_t \*ucp*, *void \*funcvoid*, *int argc*, *...*);

*int*\

**swapcontext**(*ucontext\_t \*oucp*, *const ucontext\_t \*ucp*);

## DESCRIPTION

The **makecontext**() function modifies the user thread context pointed to by *ucp*, which must have previously been initialized by a call to [getcontext(3)](/getcontext/3.md) and had a stack allocated for it. The context is modified so that it will continue execution by invoking **func**() with the arguments provided. The *argc* argument must be equal to the number of additional arguments of type *int* provided to **makecontext**() and also equal to the number of arguments of type *int* to **func**(); otherwise , the behavior is undefined.

The *ucp-\>uc\_link* argument must be initialized before calling **makecontext**() and determines the action to take when **func**() returns: if equal to NULL, the process exits; otherwise, **setcontext**(*ucp-\>uc\_link*) is implicitly invoked.

The **swapcontext**() function saves the current thread context in *\*oucp* and makes *\*ucp* the currently active context.

## RETURN VALUES

If successful, **swapcontext**() returns zero; otherwise -1 is returned and the global variable *errno* is set appropriately.

## ERRORS

The **swapcontext**() function will fail if:

- \[ENOMEM\]

  There is not enough stack space in *ucp* to complete the operation.

## SEE ALSO

[setcontext(3)](/setcontext/3.md), [ucontext(3)](/ucontext/3.md)

## STANDARDS

The **makecontext**() and **swapcontext**() functions conform to X/Open System Interfaces and Headers Issue 5 (“XSH5”) and IEEE Std 1003.1-2001 (“POSIX.1”).

The IEEE Std 1003.1-2004 (“POSIX.1”) revision marked the functions **makecontext**() and **swapcontext**() as obsolete, citing portability issues and recommending the use of POSIX threads instead. The IEEE Std 1003.1-2008 (“POSIX.1”) revision removed the functions from the specification.

The standard does not clearly define the type of integer arguments passed to *func* via **makecontext**(); portable applications should not rely on the implementation detail that it may be possible to pass pointer arguments to functions.

## HISTORY

The **makecontext**() and **swapcontext**() functions first appeared in AT&T System V Release 4 UNIX.