INT04-J. Do not attempt to store signed values in the char integral type

The char type is the only unsigned primitive type in Java. As a result, a signed value cannot be stored and retrieved successfully from a variable of type char. In particular, comparing a value of type char with -1 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 is a common error [[Pugh 2008]].

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 compared with -1 to try to detect EOF. This conversion leaves the value of c as 0xffff (Character.MAX_VALUE) instead of -1. As a result, this test 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 in data. If the value of type int returned by read() is not -1, then it can be safely cast to a value of 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 possibly memory leaks. Furthermore, comparing a value of type char with -1 never evaluates to true. This error can result in a denial-of-service.

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.

Other Languages

This guideline appears in the C Secure Coding Standard as FIO34-C. Use int to capture the return value of character IO functions.

This guideline appears in the C++ Secure Coding Standard as FIO34-CPP. Use int to capture the return value of character IO functions.

Bibliography

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

INT03-J. Do not cast numeric types to wider floating-point types without range checking      06. Integers (INT)      INT05-J. Use shift operators correctly