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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:
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.
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
feholdexceptcannot 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:
fenv_t objects, which were initialized previously by a
call to fegetenv or feholdexcept.
FE_DFL_ENV which represents the floating-point
environment as it was available at program start.
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:
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.
Like
fesetenv, this function sets the floating-point environment to that described by envp. However, if any exceptions were flagged in the status word beforefeupdateenvwas called, they remain flagged after the call. In other words, afterfeupdateenvis 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.
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,
-1otherwise.
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,
-1otherwise.