NetBSD/share/man/man2/ucontext.2

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.Dd February 25, 2018
.Dt UCONTEXT 2
.Os
.Sh NAME
.Nm ucontext
.Nd user context
.Sh SYNOPSIS
.In ucontext.h
.Fn _UC_MACHINE_SP "&uc"
.Fn _UC_MACHINE_FP "&uc"
.Fn _UC_MACHINE_PC "&uc"
.Fn _UC_MACHINE_INTRV "&uc"
.Fn _UC_MACHINE_SET_PC "&uc"
.Sh DESCRIPTION
The
.Vt 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.
.Pp
The
.Vt ucontext_t
structure includes the following members:
.Bd -literal -offset indent
ucontext_t *uc_link
sigset_t    uc_sigmask
stack_t     uc_stack
mcontext_t  uc_mcontext
.Ed
.Pp
The
.Fa uc_link
member points to the context that will be resumed after the function
specified when modifying a context using
.Xr makecontext 3
has returned.
If
.Fa uc_link
is a
.Dv NULL
pointer, then the context is the main context,
and the process will exit with an exit status of 0 upon return.
.Pp
The
.Fa uc_sigmask
member is the set of signals that are blocked when the context is
activated.
Further information can be found in
.Xr sigprocmask 2 .
.Pp
The
.Fa uc_stack
member defines the stack used by the context.
Further information can be found in
.Xr sigaltstack 2 .
.Pp
The
.Fa 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.
.Pp
The following auxiliary macros are available to access some of that
machine-specific information:
.Bl -tag -width ".Fn _UC_MACHINE_SET_PC" -offset indent
.It Fn _UC_MACHINE_SP
\(em read stack pointer,
.It Fn _UC_MACHINE_FP
\(em read frame pointer (base pointer),
.It Fn _UC_MACHINE_PC
\(em read program counter,
.It Fn _UC_MACHINE_INTRV
\(em read interrupt vector,
.It Fn _UC_MACHINE_SET_PC
\(em write program counter.
.El
.Pp
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
.Fl ( fomit-frame-pointer )
or emit function prologues only before parts that need them
.Fl ( fshrink-wrap ) .
.Sh SEE ALSO
.Xr _exit 2 ,
.Xr getcontext 2 ,
.Xr setcontext 2 ,
.Xr sigaltstack 2 ,
.Xr sigprocmask 2 ,
.Xr makecontext 3 ,
.Xr swapcontext 3
.Sh STANDARDS
The
.Vt ucontext_t
type conforms to
.St -xsh5
and
.St -p1003.1-2001 .
The
.St -p1003.1-2008
revision removed the
.Vt ucontext_t
from the specification.