# <fenv.h>

**[<fenv.h>](https://en.cppreference.com/w/c/header/fenv)** provides access to the floating-point environment: exception flags that record invalid operations, overflows, and underflows, plus rounding mode control. Essential for checking whether computations silently produced NaN or infinity.

Use it to detect floating-point anomalies and test numerical edge cases portably.

## Example

This example checks which floating-point exception flags are set after various computations.

```c
// compile: gcc -o fenvexample fenvexample.c -lm
// run: ./fenvexample
// description: detect floating-point exceptions (invalid, overflow, etc.)

#include <fenv.h>
#include <math.h>
#include <stdio.h>

int main() {
    volatile double x;
    
    feclearexcept(FE_ALL_EXCEPT);
    x = sqrt(-1.0);
    if (fetestexcept(FE_INVALID))
        printf("sqrt(-1.0) set FE_INVALID\n");
    
    feclearexcept(FE_ALL_EXCEPT);
    x = 1.0 / 0.0;
    if (fetestexcept(FE_DIVBYZERO))
        printf("1.0/0.0 set FE_DIVBYZERO\n");
    
    feclearexcept(FE_ALL_EXCEPT);
    x = 1e308 * 1e308;
    if (fetestexcept(FE_OVERFLOW))
        printf("1e308*1e308 set FE_OVERFLOW\n");
    
    return 0;
}
```

## Common functions and macros

**Exception flags** (bit flags for fetestexcept, feclearexcept):
- `FE_INEXACT`: result not representable
- `FE_UNDERFLOW`: underflow to zero or subnormal
- `FE_OVERFLOW`: result too large to represent
- `FE_DIVBYZERO`: division by zero
- `FE_INVALID`: invalid operation (sqrt(-1), NaN comparison)
- `FE_ALL_EXCEPT`: all exception flags

**Exception handling**:
- `feclearexcept(excepts)`: clear specified flags
- `fetestexcept(excepts)`: test which flags are set
- `fegetexceptflag(&state, excepts)`: save flags to variable
- `fesetexceptflag(&state, excepts)`: restore flags from variable

**Rounding modes**:
- `FE_DOWNWARD`, `FE_TONEAREST`, `FE_TOWARDZERO`, `FE_UPWARD`
- `fegetround()`: query current rounding mode
- `fesetround(mode)`: set rounding mode

**Environment save/restore**:
- `fegetenv(&env)`: save full FP environment
- `fesetenv(&env)`: restore FP environment
- `feupdateenv(&env)`: restore and re-raise current exceptions