C99 Section 7.12.1 defines two types of errors that relate specifically to math functions in math.h [[ISO/IEC 9899:1999]]:

a domain error occurs if an input argument is outside the domain over which the mathematical function is defined.

a range error occurs if the mathematical result of the function cannot be represented in an object of the specified type, due to extreme magnitude.

An example of a domain error is the square root of a negative number, such as sqrt(-1.0), which has no meaning in real arithmetic. Similarly, ten raised to the one-millionth power, pow(10., 1e6), likely cannot be represented in an implementation's floating point representation and consequently constitutes a range error.

In both cases, the function will return some value, but the value returned is not the correct result of the computation.

Many math errors can be prevented by carefully bounds checking the arguments before calling functions, and taking alternative action if the bounds are violated. In particular, the following functions should be bounds checked as follows:

However, for some functions it is not practical to use bounds checking to prevent all errors.  In the above pow example, the bounds check does not prevent the pow(10., 1e6) range error. In these cases detection must be used, either in addition to bounds checking or instead of bounds checking.

acos(x), asin(x)

Noncompliant Code Example

The following noncompliant code computes the arc cosine of the variable x

double x;
double result;

/* Set the value for x */

result = acos(x);

However, this code may produce a domain error if x is not in the range [-1, +1].

Compliant Solution

The following compliant solution uses bounds checking to ensure that there is not a domain error.

double x;
double result;

/* Set the value for x */

if ( isnan(x) || isless(x,-1) || isgreater(x, 1) ){
  /* handle domain error */
}

result = acos(x);

atan2(y, x)

Noncompliant Code Example

The following noncompliant code computes the arc tangent of the two variables x and y.

double x;
double y;
double result;

/* Set the value for x and y */

result = atan2(y, x);

However, this code may produce a domain error if both x and y are zero.

Compliant Solution

The following compliant solution tests the arguments to ensure that there is not a domain error.

double x;
double y;
double result;

/* Set the value for x and y */

if ( (x == 0.f) && (y == 0.f) ) {
  /* handle domain error */
}

result = atan2(y, x);

log(x), log10(x)

Noncompliant Code Example

The following noncompliant code determines the natural logarithm of x.

double x;
double result;

/* Set the value for x */

result = log(x);

However, this code may produce a domain error if x is negative and a range error if x is zero.

Compliant Solution

The following compliant solution tests the suspect arguments to ensure that no domain errors or range errors are raised.

double x;
double result;

/* Set the value for x */

if (isnan(x) || islessequal(x, 0)) {
  /* handle domain and range errors */
}

result = log(x);

pow(x, y)

Noncompliant Code Example

The following noncompliant code raises x to the power of y.

double x;
double y;
double result;

result = pow(x, y);

However, this code may produce a domain error if x is negative and y is not an integer, or if x is zero and y is zero. A domain error or range error may occur if x is zero and y is negative, and a range error may occur if the result cannot be represented as a double.

Noncompliant Code Example

This code only performs bounds checking on x and y. It prevents domain errors and some range errors, but does not prevent range errors where the result cannot be represented as a double (see the Error Checking and Detection section below regarding ways to mitigate the effects of a range error).

double x;
double y;
double result;

if (((x == 0.f) && islessequal(y, 0)) ||
    (isless(x, 0))) {
  /* handle domain and range errors */
}

result = pow(x, y);

sqrt(x)

Noncompliant Code Example

The following noncompliant code determines the square root of x

double x;
double result;

result = sqrt(x);

However, this code may produce a domain error if x is negative.

Compliant Solution

The following compliant solution tests the suspect argument to ensure that no domain error is raised.

double x;
double result;

if (isless(x, 0)){
  /* handle domain error */
}

result = sqrt(x);

lgamma(x), tgamma(x)

The "too large" and "too small" conditions for a range error in the gamma functions are vague enough to make it very difficult to catch all possible range errors simply by looking at the input.

Noncompliant Code Example

This noncompliant code example attempts to prevent domain and range errors, but a range error occurs nonetheless.

float x = -90.5;

if ((x == 0) || (x < 0 && x == nearbyint(x))) {
  /* handle error */
}

float f = tgamma(x);

Compliant Solution

This compliant solution detects the underflow by using the methods described below in the Error Checking and Detection section.

float x = -90.5;

if ((x == 0) || (x < 0 && x == nearbyint(x))) {
  /* handle error */
}

feclearexcept(FE_ALL_EXCEPT);

float f = tgamma(x);

if (fetestexcept(FE_UNDERFLOW) != 0) {
  printf("Underflow detected\n");
}

Error Checking and Detection

The exact treatment of error conditions from math functions is quite complicated. C99 Section 7.12.1 defines the following behavior for floating point overflow [[ISO/IEC 9899:1999]]

A floating result overflows if the magnitude of the mathematical result is finite but so large that the mathematical result cannot be represented without extraordinary roundoff error in an object of the specified type. If a floating result overflows and default rounding is in effect, or if the mathematical result is an exact infinity from finite arguments (for example log(0.0)), then the function returns the value of the macro HUGE_VAL, HUGE_VALF, or HUGE_VALL according to the return type, with the same sign as the correct value of the function; if the integer expression math_errhandling & MATH_ERRNO is nonzero, the integer expression errno acquires the value ERANGE; if the integer expression math_errhandling & MATH_ERREXCEPT is nonzero, the ''divide-by-zero'' floating-point exception is raised if the mathematical result is an exact infinity and the ''overflow'' floating-point exception is raised otherwise.

It is best not to check for errors by comparing the returned value against HUGE_VAL or 0 for several reasons:

• These are in general valid (albeit unlikely) data values.
• Making such tests requires detailed knowledge of the various error returns for each math function.
• There are three different possibilities, -HUGE_VAL, 0, and HUGE_VAL, and you must know which are possible in each case.
• Different versions of the library have differed in their error-return behavior.

It is also difficult to check for math errors using errno because an implementation might not set it. For real functions, the programmer can tell whether the implementation sets errno by checking whether math_errhandling & MATH_ERRNO is nonzero. For complex functions, the C99 Section 7.3.2 simply states "an implementation may set errno but is not required to" [[ISO/IEC 9899:1999]].

Compliant Solution (Error Checking)

The most reliable way to test for errors is by checking arguments beforehand, as in the following compliant solution:

if (/* arguments will cause a domain or range error */) {
  /* handle the error */
}
else {
  /* perform computation */
}

For functions where argument validation is difficult, including pow(), erfc(), lgamma(), and tgamma(), one can employ the following approach. This approach uses C99 standard functions for floating point errors.

#include <math.h>
#if defined(math_errhandling) \
  && (math_errhandling & MATH_ERREXCEPT)
#include <fenv.h>
#endif

/* ... */

#if defined(math_errhandling) \
  && (math_errhandling & MATH_ERREXCEPT)
  feclearexcept(FE_ALL_EXCEPT);
#endif
errno = 0;

/* call the function */

#if !defined(math_errhandling) \
  || (math_errhandling & MATH_ERRNO)
if (errno != 0) {
  /* handle error */
}
#endif
#if defined(math_errhandling) \
  && (math_errhandling & MATH_ERREXCEPT)
if (fetestexcept(FE_INVALID
               | FE_DIVBYZERO
               | FE_OVERFLOW) != 0)
{
  /* handle error */
}
#endif

See FLP03-C. Detect and handle floating point errors for more details on how to detect floating point errors.

Implementation Details

System V Interface Definition, Third Edition

The System V Interface Definition, Third Edition (SVID3) provides more control over the treatment of errors in the math library. The user can provide a function named matherr that is invoked if errors occur in a math function. This function can print diagnostics, terminate the execution, or specify the desired return-value. The matherr() function has not been adopted by C99, so its use is not generally portable.

Risk Assessment

Failure to properly verify arguments supplied to math functions may result in unexpected results.

Automated Detection

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this rule.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899:1999]] Section 7.3, "Complex arithmetic <complex.h>", and Section 7.12, "Mathematics <math.h>"
[[Plum 85]] Rule 2-2
[[Plum 89]] Topic 2.10, "conv - conversions and overflow"

FLP31-C. Do not call functions expecting real values with complex values      05. Floating Point (FLP)       FLP33-C. Convert integers to floating point for floating point operations