libc/gen/setjmp.3

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.Dd $Mdocdate: May 31 2007 $
.Dt SETJMP 3
.Os
.Sh NAME
.Nm sigsetjmp ,
.Nm siglongjmp ,
.Nm setjmp ,
.Nm longjmp ,
.Nm _setjmp ,
.Nm _longjmp ,
.Nm longjmperror
.Nd non-local jumps
.Sh SYNOPSIS
.Fd #include <setjmp.h>
.Ft int
.Fn sigsetjmp "sigjmp_buf env" "int savemask"
.Ft void
.Fn siglongjmp "sigjmp_buf env" "int val"
.Ft int
.Fn setjmp "jmp_buf env"
.Ft void
.Fn longjmp "jmp_buf env" "int val"
.Ft int
.Fn _setjmp "jmp_buf env"
.Ft void
.Fn _longjmp "jmp_buf env" "int val"
.Ft void
.Fn longjmperror void
.Sh DESCRIPTION
The
.Fn sigsetjmp ,
.Fn setjmp ,
and
.Fn _setjmp
functions save their calling environment in
.Fa env .
Each of these functions returns 0.
.Pp
The corresponding
.Fn longjmp
functions restore the environment saved by the most recent
invocation of the respective
.Fn setjmp
function.
They then return so that program execution continues as if the corresponding
invocation of the
.Fn setjmp
call had just returned the value specified by
.Fa val ,
instead of 0.
The value specified by
.Fa val
must be non-zero; a 0 value is treated as 1 to allow the programmer
to differentiate between a direct invocation of
.Fn setjmp
and a return via
.Fn longjmp .
.Pp
Pairs of calls may be intermixed; i.e., both
.Fn sigsetjmp
and
.Fn siglongjmp
as well as
.Fn setjmp
and
.Fn longjmp
combinations may be used in the same program.
However, individual calls may not \(em e.g., the
.Fa env
argument to
.Fn setjmp
may not be passed to
.Fn siglongjmp .
.Pp
The
.Fn longjmp
routines may not be called after the routine which called the
.Fn setjmp
routines returns.
.Pp
All accessible objects have values as of the time the
.Fn longjmp
routine was called, except that the values of objects of automatic storage
invocation duration that do not have the
.Li volatile
type and have been changed between the
.Fn setjmp
invocation and
.Fn longjmp
call are indeterminate.
.Pp
The
.Fn setjmp Ns / Ns Fn longjmp
function pairs save and restore the signal mask while the
.Fn _setjmp Ns / Ns Fn _longjmp
function pairs save and restore only the register set and the stack (see
.Xr sigmask 3 ) .
.Pp
The
.Fn sigsetjmp Ns / Ns Fn siglongjmp
function pairs save and restore the signal mask if the argument
.Fa savemask
is non-zero.
Otherwise, only the register set and the stack are saved.
.Pp
In other words,
.Fn setjmp Ns / Ns Fn longjmp
are functionally equivalent to
.Fn sigsetjmp Ns / Ns Fn siglongjmp
when
.Fn sigsetjmp
is called with a non-zero
.Fa savemask
argument.
Conversely,
.Fn _setjmp Ns / Ns Fn _longjmp
are functionally equivalent to
.Fn sigsetjmp Ns / Ns Fn siglongjmp
when
.Fn sigsetjmp
is called with a zero-value
.Fa savemask .
.Pp
The
.Fn sigsetjmp Ns / Ns Fn siglongjmp
interfaces are preferred for maximum portability.
.Sh ERRORS
If the contents of the
.Fa env
are corrupted or correspond to an environment that has already returned,
the
.Fn longjmp
routine calls the routine
.Xr longjmperror 3 .
If
.Fn longjmperror
returns, the program is aborted (see
.Xr abort 3 ) .
The default version of
.Fn longjmperror
prints the message
.Dq Li longjmp botch
to standard error and returns.
User programs wishing to exit more gracefully should write their own
versions of
.Fn longjmperror .
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr sigaltstack 2 ,
.Xr signal 3
.Sh STANDARDS
The
.Fn setjmp
and
.Fn longjmp
functions conform to
.St -ansiC .
The
.Fn sigsetjmp
and
.Fn siglongjmp
functions conform to
.St -p1003.1-90 .
.Sh CAVEATS
Historically, on
.At V ,
the
.Fn setjmp Ns / Ns Fn longjmp
functions have been equivalent to the
.Bx
.Fn _setjmp Ns / Ns Fn _longjmp
functions and do not restore the signal mask.
Because of this discrepancy, the
.Fn sigsetjmp Ns / Ns Fn siglongjmp
interfaces should be used if portability is desired.
.Pp
Use of
.Fn longjmp
or
.Fn siglongjmp
from inside a signal handler is not as easy as it might seem.
Generally speaking, all possible code paths between the
.Fn setjmp
and
.Fn longjmp
must be signal race safe, as discussed in
.Xr signal 3 .
Furthermore, the code paths must not do resource management
(such as
.Xr open 2
or
.Xr close 2 )
without blocking the signal in question, or resources might
be mismanaged.
Obviously this makes
.Fn longjmp
much less useful than previously thought.