FIO14-J. Use an int to capture the return value of functions that read a character or byte

The abstract InputStream.read() method reads a single byte from an input source, and returns its value as an int, in the range 0 to 255. It will return -1 only when the end of the input stream has been reached. The similar Reader.read() method reads a single character, and returns its value as an int, in the range 0-65,535. It also returns -1 only when the end of the stream has been reached. Both methods are meant to be overridden by subclasses.

These methods are often used to read a byte or character from a stream. Unfortunately many programmers prematurely convert the resulting int back to a byte or char before checking whether they have reached the end of the stream (signaled by a return value of -1). Programs must check for end of stream (e.g., -1) before narrowing the return value to a byte or char.

This guideline applies to any InputStream or Reader subclass that provide an implementation of the read() method. This guideline is a specific instance of NUM15-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data.

Noncompliant Code Example (byte)

This noncompliant code example casts the value returned by the read() method directly to a value of type byte, and then compares this value with -1 in an attempt to detect the end of the stream.

FileInputStream in;
// initialize stream 
byte data;
while ((data = (byte) in.read()) != -1) { 
  // ... 
}

When the read() method in this noncompliant code example returns the byte value 0xFF, it will be indistinguishable from the -1 value used to indicate the end of stream, because the byte value is promoted and sign extended to an int before being compared with -1.

Compliant Solution (byte)

Use a variable of type int to capture the return value of the byte input method. When the value returned by read() is not -1, it can be safely cast to type byte. When read() returns 0XFF, the comparison will test 0x000000FF against 0xFFFFFFFF and fail.

FileInputStream in;
// initialize stream 
int inbuff;
byte data;
while ((inbuff = in.read()) != -1) { 
  data = (byte) inbuff;
  // ...  
}

Noncompliant Code Example (char)

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 the end of stream. This conversion leaves the value of c as 0xffff (e.g., Character.MAX_VALUE) instead of -1. Consequently, the test for the end of stream never evaluates to true.

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

Compliant Solution (char)

Use a variable of type int to capture the return value of the character input method. When the value of returned by read() is not -1, it can be safely cast to type char.

FileReader in;
// initialize stream 
int inbuff;
char data;
while ((inbuff = in.read()) != -1) { 
  data = (char) inbuff;
  // ...  
}

Risk Assessment

Historically, using a narrow type to capture the return value of a byte input function has resulted in significant vulnerabilities, including command injection attacks; see CA-1996-22 advisory. Consequently, the severity of this error is high.

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"

FIO13-J. Do not rely on the write() method to output integers outside the range 0 to 255      12. Input Output (FIO)      FIO15-J. Do not store excess or sensitive information within cookies when using Java Servlets