What does the compiler error “missing binary operator before token” mean?

Question

I recently got the following error when trying to compile with gcc:

error: missing binary operator before token \"(\"

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Answer 1

This is not a compiler error, it is a preprocessor error. It occurs when the preprocessor encounters invalid syntax while trying to evaluate an expression in a #if or #elif directive.

One common cause is the sizeof operator in an #if directive:

For example:

  #define NBITS (sizeof(TYPE)*8)
  //later
  #if (NBITS>16)    //ERROR

This is an error because sizeof is evaluated by the compiler, not the preprocesor.

Type casts are also not valid preprocessor syntax:

  #define ALLBITS ((unsigned int) -1)
  //later
  #if (ALLBITS>0xFFFF)    //ERROR

The rules for what can be in a valid expression are here.

Note also that #if will evaluate an undefined macro as 0, unless it looks like it takes arguments, in which case you also get this error:

So if THIS is undefined:

#if THIS == 0  //valid, true

#if THIS > 0 //valid, false

#if THIS() == 0  //invalid. ERROR

Typos in your #if statement can also cause this message.