INFO: Common Programming Errors in the C Language (22321)

SUMMARY

The text below lists some of the most common errors that occur programming in the C language. Any one of these items can cause unpredictable results, such as invalid data. Some are caught by the compiler and reported as errors or warnings.

• Using an automatic variable that has not been initialized
• Omitting a closing comment delimiter
• Using an array index greater than the length of the array (In C, array indexes run from zero to <length>-1.)
• Omitting a semicolon or a closing brace
• Using an uninitialized pointer
• Using a forward slash when a backslash is required (for example, substituting "/n" for "\n.")
• Using "=" in a comparison where "==" is desired
• Overwriting the null terminator for a string
• Prematurely terminating a function declaration with a semicolon (The compiler often flags the "orphan" block of code as a syntax error.)
• Specifying the values of variables in a scanf() statement instead of their addresses
• Failing to declare the return type for a function
• Assuming an expression evaluation order when using an expression with side effects (For example, a[i] = i++; is ambiguous and dangerous.)
• Failing to account that a static variable in a function is initialized only once
• Omitting a "break" from a case in a switch statement (Execution "falls through" to subsequent cases.)
• Using "break" to exit a block of code associated with an if statement (The break statement exits a block of code associated with a for, switch, or while statement.)
• Comparing a "char" variable against EOF (-1). The following idiom results in an infinite loop when char is unsigned. Note that char is signed by default, so the following will only fail when using the "/J" compiler option:

  char c;
  while ((c = getchar()) != EOF)
     {
     }