Last change: June 19, 2005
The Single UNIX 3, C99 specification for setjmp() and longjmp() is silent about what part of the floating point environment shall be saved by a setjmp() and restored by a corresponding longjmp() return, except for a mention in the longjmp() spec of the example that the floating-point status flags are allowed to change between the two, and should not be restored.
In particular, it is silent about the floating-point control modes, such as the IEEE 754 rounding modes FE_TONEAREST, FE_TOWARDZERO, FE_DOWNWARD, and FE_UPWARD.
We noticed that the setjmp/longjmp behavior changed between the Panther and Tiger versions of Mac OS X (10.3.x and 10.4.x), from not restoring to restoring the rounding directions; and when we asked about that on the darwin-dev Apple mailing list (May 28, 2005), Matt Watson suggested it would be good to check what other systems do. Several list participants graciously ran the C program we supplied to tell whether longjmp() restores the rounding modes, and the results are tabulated here:
| system | restores mode | contributer |
| darwin 7.9.0 (panther) gcc 3.3 | no | David Williams |
| darwin 8.1.0 (tiger) gcc 3.3 gcc 4.0.0 |
yes yes |
David Williams |
| FreeBSD | yes | Matt Watson |
| FreeBSD 5.3-RELEASE-p1 i386 gcc version 3.4.2 |
yes | Andrew Gallatin |
| HP-UX B.11.11 U 9000/785 HP ANSI C++ B3910B A.03.37 |
no | Benoit Foucher |
| HP-UX B.11.11 9000/800 HP ANSI C++ B3910B A.03.52 |
no | Kevin Harris |
| HP-UX B.11.23 ia64 HP aC++/ANSI C B3910B A.05.55 [Dec 04 2003] |
yes | Eric Gouriou |
| IBM AIX 5.2 / VisualAge 6.0 | no | Benoit Foucher |
| IBM AIX 5.3 Power5, xlc vers 6 | no | Andrew Gallatin |
| Linux (GNU) 2.4.29 i686 gcc version 3.3.1 |
no | David Williams |
| Linux (SuSE) 2.6.5-7.97-pseries64 ppc64 (SLES 9) gcc version 3.3.3 |
no | Andrew Gallatin |
| SGI IRIX 6.5.27 / MIPSpro 7.4.3 | yes | Steve Allen |
| Solaris 9 Forte Developer 7 C 5.4 2002/03/09 | no | Benoit Foucher |
| SunOS stonehenge 5.8 Generic_108528-18 sun4u sparc Sun WorkShop 6 update 2 C 5.3 Patch 111679-09 2002/11/12 C++ 5.3 Patch 111685-12 2002/12/16 |
no no |
Kyle Dean |
| SunOS 5.10 i86pc gcc 3.3.2 | no | Andrew Gallatin |
We personally prefer that setjmp() not save the
rounding mode for restoration by longjmp(). The
situation should be the same as for changing without restoring
the original mode in functions, which Single UNIX 3 allows (see
the Application Usage section of the spec for
fenv.h), with an admonition to document such changes.
Indeed, setjmp() and longjmp() are typically
embedded in different functions, where a change of mode is
otherwise allowed.
To have setjmp/longjmp automatically restore the
floating-point rounding mode seems to us a gratuitous,
unnecessary restriction on the programmer.
Accessed
times since June 19, 2005.