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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



makecontext, swapcontext - manipulate user contexts


#include <ucontext.h>

void makecontext(ucontext_t *ucp, void (*func)(), int argc...);
int swapcontext(ucontext_t *restrict oucp,
     const ucontext_t *restrict ucp);


The makecontext() function modifies the context specified by ucp, which has been initialized using getcontext(2). When this context is resumed using swapcontext() or setcontext(2), execution continues by calling the function func, passing it the arguments that follow argc in the makecontext() call. The value of argc must match the number of pointer-sized integer arguments passed to func, otherwise the behavior is undefined.

Before a call is made to makecontext(), the context being modified should have a stack allocated for it. The stack is assigned to the context by initializing the uc_stack member.

The uc_link member is used to determine the context that will be resumed when the context being modified by makecontext() returns. The uc_link member should be initialized prior to the call to makecontext(). If the uc_link member is initialized to NULL, the thread executing func will exit when func returns. See pthread_exit(3C).

The swapcontext() function saves the current context in the context structure pointed to by oucp and sets the context to the context structure pointed to by ucp.

If the ucp or oucp argument points to an invalid address, the behavior is undefined and errno may be set to EFAULT.

Return Values

On successful completion, swapcontext() returns 0. Otherwise, −1 is returned and errno is set to indicate the error.


The swapcontext() function will fail if:


The ucp argument does not have enough stack left to complete the operation.

The swapcontext() function may fail if:


The ucp or oucp argument points to an invalid address.


Example 1 Alternate execution context on a stack whose memory was allocated using mmap().
#include <stdio.h>
#include <ucontext.h>
#include <sys/mman.h>

assign(long a, int *b)
        *b = (int)a;

main(int argc, char **argv)
        ucontext_t uc, back;
        size_t sz = 0x10000;
        int value = 0;


        uc.uc_stack.ss_sp = mmap(0, sz,
            MAP_PRIVATE | MAP_ANON, -1, 0);
        uc.uc_stack.ss_size = sz;
        uc.uc_stack.ss_flags = 0;

        uc.uc_link = &back;

        makecontext(&uc, assign, 2, 100L, &value);
        swapcontext(&back, &uc);

        printf("done %d\n", value);

        return (0);


These functions are useful for implementing user-level context switching between multiple threads of control within a process (co-processing). More effective multiple threads of control can be obtained by using native support for multithreading. See threads(7).


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

getcontext(2), mmap(2), sigaction(2), sigprocmask(2), pthread_exit(3C), ucontext.h(3HEAD), attributes(7), standards(7), threads(7)


The semantics of the uc_stack member of the ucontext_t structure have changed as they apply to inputs to makecontext(). Prior to Solaris 10, the ss_sp member of the uc_stack structure represented the high memory address of the area reserved for the stack. The ss_sp member now represents the base (low memory address), in keeping with other uses of ss_sp.

This change in the meaning of ss_sp is now the default behavior. The –D__MAKECONTEXT_V2_SOURCE compilation flag used in Solaris 9 update releases to access this behavior is obsolete.

Binary compatibility has been preserved with releases prior to Solaris 10. Before recompiling, applications that use makecontext() must be updated to reflect this behavior change. The example below demonstrates a typical change that must be applied:

--- example1_s9.c       Thu Oct  3 11:58:17 2002
+++ example1.c  Thu Jun 27 13:28:16 2002
@@ -27,12 +27,9 @@
        uc.uc_stack.ss_sp = mmap(0, sz,
            MAP_PRIVATE | MAP_ANON, -1, 0);
-       uc.uc_stack.ss_sp = (char *)uc.uc_stack.ss_sp + sz - 8;
        uc.uc_stack.ss_size = sz;
        uc.uc_stack.ss_flags = 0;

        uc.uc_link = &back

        makecontext(&uc, assign, 2, 100L, &value);