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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 -1any 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 is a common error \[[Pugh 2008|AA. Bibliography#Pugh 08]\].

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 to try in an attempt to detect EOF. This conversion leaves the value of c as 0xffff (Character.MAX_VALUE) instead of -1. As a result, this 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 in data. If When 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, 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.

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

\[[API 2006|AA. Bibliography#API 06]\] Class {{InputStream}}
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 4.2|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.2] "Primitive Types and Values"
\[[Pugh 2008|AA. Bibliography#Pugh 08]\] "Waiting for the end"

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