NUM04-J. Avoid using the char integral type to hold signed values

The char type is the only unsigned primitive type in Java; its representable range of values contains only non-negative values. Consequently, attempting to store and retrieve a value from a variable of any of the other integral primitive types will produce unexpected results when that value is negative. Similarly, comparing a value of type char with any negative number will never yield true. However, because the method read() returns -1 to indicate the End of File (EOF) condition, it is tempting to compare the character returned by read() with -1. This common error [[Pugh 2008]] is illustrated below.

Noncompliant Code Example

This noncompliant code example casts the value of type int returned by the read() method directly to a value of type char, which is then compared with -1 in an attempt to detect EOF. This conversion leaves the value of c as 0xffff (Character.MAX_VALUE) instead of -1. As a result, the test for EOF never evaluates to true.

char c;
while ((c = (char) in.read()) != -1) { 
  // ... 
}

Compliant Solution

Always use a signed type of sufficient size to store signed data. To be compliant, use a value of type int to check for EOF while reading data. When the value of type int returned by read() is not -1, it can be safely cast to type char.

int c;
while ((c = in.read()) != -1) { 
  ch = (char) c; 
}

Risk Assessment

Storing signed data in a variable of the unsigned type char can lead to misinterpreted data and possible memory leaks. Furthermore, comparing a value of type char with -1 never evaluates to true. This error can result in a denial-of-service, for example, when code fails to detect EOF.

Automated Detection

FindBugs version 1.3.9 can detect violations of this guideline with the INT: Bad comparison of nonnegative value with negative constant detector.

Related Vulnerabilities

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

Related Guidelines

C Secure Coding Standard: FIO34-C. Use int to capture the return value of character IO functions

C++ Secure Coding Standard: FIO34-CPP. Use int to capture the return value of character IO functions

Bibliography

[[API 2006]] Class InputStream
[[JLS 2005]] Section 4.2 "Primitive Types and Values"
[[Pugh 2008]] "Waiting for the end"

NUM10-J. Beware of precision loss when converting primitive integers to floating-point      03. Integers (INT)      NUM05-J. Use shift operators correctly