Common compile-time gcc errors and warnings and their possible causes

•   
  	'myvar' undeclared (first use this function)
    
    

  This tells you that the variable myvar hasn't been declared. This means that you forgot to provide a declaration (such as int myvar) to this scope. The variable needs to be declared either within the same set of braces {}, or in an enclosing scope:

  
  	{ int myvar;
  		{
  			/* myvar is still known here */
  		}
  	}
  
  

  or at the top level (outside ANY set of braces) so that it is globally known.

  ---

•   
  	In function `main':
  	warning: implicit declaration of function `f'
    
    

  This tells you that the declaration for the function f doesn't occur until after it is called in function main. main will be compiled with the assumption that f returns an int (C is defined that way). If f returns something else, you will see another message later (see the next common error). This warning is resolved by either moving the function f so that it occurs before main in the source file, or better yet, by copying f's prototype to a place before main:

     
  	float f (void);		/* Notice the semicolon: this is a prototype */
  				/*  i.e. This is a declaration, not a        */
  				/*  for the function f().                    */
  
  	void main (void)
  	{
  		float a;
  		...
  		a = f();
  		...
  	}
  
  	float f (void)		/* NO semicolon: this is the function def'n */
  	{
  		...
  	}
     
     

  ---

•   
  	ttt.c: At top level:
  	ttt.c:9: warning: type mismatch with previous external decl
  	ttt.c:5: warning: previous external decl of `f'
  	ttt.c:9: warning: type mismatch with previous implicit declaration
  	ttt.c:5: warning: previous implicit declaration of `f'
  	ttt.c:9: warning: `f' was previously implicitly declared to return `int'
    
    

  This tells you that when f() is defined starting about line 9, the type it is declared with differs from that implied by calling it on line 5. Because there was no prototype or definition for f() before it was called on line 5 in main(), it was assumed to return an int as per the C language definition. This is called an implicit declaration. When the definition for f() declared it to return a float, it mismatched with the assumed declaration. The solution is to prototype f() before it is called in main() or to move the definition of f() before main() in the source. See the example in the previous common error.

  ---

• parse error before 'x', where x is some symbol or identifier.

  This can be many things, including (but not limited to):

  • a missing semicolon at the end of the previous statement.
  • a missing comment delimiter (*/) at the end of the last comment.
  • an undefined macro or type definition (such as using FILE without #including stdio.h).

  ---

• unterminated comment

  This means that a comment was opened with (/*) but not closed by the end of the file. You are missing the corresponding (*/).