UCONTEXT(2)

UCONTEXT(2) | System Calls Manual | UCONTEXT(2)

# NAME

`ucontext` —
user context

# SYNOPSIS

`#include <ucontext.h>`

`_UC_MACHINE_SP`(&uc);

`_UC_MACHINE_FP`(&uc);

`_UC_MACHINE_PC`(&uc);

`_UC_MACHINE_INTRV`(&uc);

`_UC_MACHINE_SET_PC`(&uc);

# DESCRIPTION

The ucontext_t is a structure type which is used to describe the context of a thread of control within the execution of a process. A thread's context includes its stack, saved registers, and list of blocked signals.

The ucontext_t structure includes the following members:

    ucontext_t *uc_link
    sigset_t    uc_sigmask
    stack_t     uc_stack
    mcontext_t  uc_mcontext

The uc_link member points to the context that will be resumed after the function specified when modifying a context using makecontext(3) has returned. If uc_link is a `NULL` pointer, then the context is the main context, and the process will exit with an exit status of 0 upon return.

The uc_sigmask member is the set of signals that are blocked when the context is activated. Further information can be found in sigprocmask(2).

The uc_stack member defines the stack used by the context. Further information can be found in sigaltstack(2).

The uc_mcontext member defines the machine state associated with the context; it may consist of general registers, floating point registers and other machine-specific information. Its description is beyond the scope of this manual page; portable applications should not access this structure member.

The following auxiliary macros are available to access some of that machine-specific information:

`_UC_MACHINE_SP`()
  — read stack pointer,
`_UC_MACHINE_FP`()
  — read frame pointer (base pointer),
`_UC_MACHINE_PC`()
  — read program counter,
`_UC_MACHINE_INTRV`()
  — read integer return value,
`_UC_MACHINE_SET_PC`()
  — write program counter.

The frame pointer macro does not guarantee to retrieve a reliable value and should not be used in a code unless no other debugging format is easily accessible. A compiler might optimize the frame pointer register in a function, reusing it as a general purpose register storage (`-fomit-frame-pointer`) or emit function prologues only before parts that need them (`-fshrink-wrap`).

# SEE ALSO

_exit(2), getcontext(2), setcontext(2), sigaltstack(2), sigprocmask(2), makecontext(3), swapcontext(3)

# STANDARDS

The ucontext_t type conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”) and IEEE Std 1003.1-2001 (“POSIX.1”). The IEEE Std 1003.1-2008 (“POSIX.1”) revision removed the ucontext_t from the specification.

February 25, 2018 | NetBSD 9.5