IEEE 754 floating-point implementations allow the programmer to
decide whether traps will occur for each of the exceptions, by setting
bits in the control word. In C, traps result in the program
receiving the SIGFPE signal; see Signal Handling.
Note: IEEE 754 says that trap handlers are given details of
the exceptional situation, and can set the result value. C signals do
not provide any mechanism to pass this information back and forth.
Trapping exceptions in C is therefore not very useful.
It is sometimes necessary to save the state of the floating-point unit
while you perform some calculation. The library provides functions
which save and restore the exception flags, the set of exceptions that
generate traps, and the rounding mode. This information is known as the
floating-point environment.
The functions to save and restore the floating-point environment all use
a variable of type fenv_t to store information. This type is
defined in fenv.h. Its size and contents are
implementation-defined. You should not attempt to manipulate a variable
of this type directly.
To save the state of the FPU, use one of these functions:
— Function: int fegetenv (fenv_t *envp)
Store the floating-point environment in the variable pointed to by
envp.
The function returns zero in case the operation was successful, a
non-zero value otherwise.
— Function: int feholdexcept (fenv_t *envp)
Store the current floating-point environment in the object pointed to by
envp. Then clear all exception flags, and set the FPU to trap no
exceptions. Not all FPUs support trapping no exceptions; if
feholdexcept cannot set this mode, it returns nonzero value. If it
succeeds, it returns zero.
The functions which restore the floating-point environment can take these
kinds of arguments:
Pointers to fenv_t objects, which were initialized previously by a
call to fegetenv or feholdexcept.
The special macro FE_DFL_ENV which represents the floating-point
environment as it was available at program start.
Implementation defined macros with names starting with FE_ and
having type fenv_t *.
If possible, the GNU C Library defines a macro FE_NOMASK_ENV
which represents an environment where every exception raised causes a
trap to occur. You can test for this macro using #ifdef. It is
only defined if _GNU_SOURCE is defined.
Some platforms might define other predefined environments.
To set the floating-point environment, you can use either of these
functions:
— Function: int fesetenv (const fenv_t *envp)
Set the floating-point environment to that described by envp.
The function returns zero in case the operation was successful, a
non-zero value otherwise.
— Function: int feupdateenv (const fenv_t *envp)
Like fesetenv, this function sets the floating-point environment
to that described by envp. However, if any exceptions were
flagged in the status word before feupdateenv was called, they
remain flagged after the call. In other words, after feupdateenv
is called, the status word is the bitwise OR of the previous status word
and the one saved in envp.
The function returns zero in case the operation was successful, a
non-zero value otherwise.
To control for individual exceptions if raising them causes a trap to
occur, you can use the following two functions.
Portability Note: These functions are all GNU extensions.
— Function: int feenableexcept (int excepts)
This functions enables traps for each of the exceptions as indicated by
the parameter except. The individual excepetions are described in
Status bit operations. Only the specified exceptions are
enabled, the status of the other exceptions is not changed.
The function returns the previous enabled exceptions in case the
operation was successful, -1 otherwise.
— Function: int fedisableexcept (int excepts)
This functions disables traps for each of the exceptions as indicated by
the parameter except. The individual excepetions are described in
Status bit operations. Only the specified exceptions are
disabled, the status of the other exceptions is not changed.
The function returns the previous enabled exceptions in case the
operation was successful, -1 otherwise.
— Function: int fegetexcept (int excepts)
The function returns a bitmask of all currently enabled exceptions. It
returns -1 in case of failure.
Published under the terms of the GNU General Public License
