Goto and Labels in C

Introduction

Goto in C:

The goto statement is known as the jump statement in C. The goto is used to transfer the control of a program to a predefined label. The goto statement has one more application, which is, for a particular condition it performs repeat operation on some part of the code.

Labels in C:

The Labels in C are used with goto and switch statements. It can be defined as an identifier which is required for the goto statement. It is required as an identifier to a location where the branch is to be made.

Syntax:

goto label;

Theory:

The awareness about “goto” and “labels'' is widely known to every programmer who has gone through the surface of C programming. It is used in C programming to perform the jump operation. By jump we mean, jump within a C function.

Note:  In C, the GCC (GNU Compiler Collection) provides an extension which is called “local labels”.

Goto Statement in C

The goto statement is known as the jump statement in C. The goto is used to transfer the control of a program to a predefined label. C offers the infinitely-abusable goto statement, and labels to branch to. This means that the label is required as an identifier to a location where the branch is to be made.

The goto statement is not necessary, and in general it is quite easy to write code without the use of this statement. But there are a few scenarios where goto may play a vital role and resolve the problems the user may face in those conditions.

The most common scenario is to abandon processing in some deeply nested structure, such as breaking out of two or more loops at once.

Note: Here, the break statement may resolve the issue but one must remember that it cannot be used directly since it only exits from the innermost loop.

      for ( ... )
           for ( ... ) {
               ...
               if (condition_causing_disaster)
                   goto error;
           }
       ...
   error: error_statement

In the above lines of code, the former suggested organization is compatible if the error-handling code is non-trivial, and if the occurrence of errors is bound to be in several places.

Label Statement in C

A Label in C is used with goto and switch statements. A label is followed by a colon and what makes it special is that it has the same form as a variable name. Also,it can be adjoined with any statement in the same function as the goto. The label differs from the other statements of its kind is the scope. The scope of a label is the entire function.

Application

Example 1:   

   for (i = 0; i < n; i++)
           for (j = 0; j < m; j++)
               if (x[i] == y[j])


                   goto found;  //no common element found
       …


   found:
       /* found one common element : x [i] == y [j] */
       ...

Code explanation:

The above code is an attempt to solve the problem of determining whether two arrays x and y have an element in common. But this statement can be written in a more elegant way or we can say the easy approach. The above code uses the goto statement to solve the given condition.

But the code consisting of a goto statement can always be written without using this statement. It may cause the programmer trouble with repeated tests (testing conditions) or an extra variable.

Example 2:

  found = 0;
   for (i = 0; i < n && !found; i++)
       for (j = 0; j < m && !found; j++)


           if (x [i] == y [j])
               found = 1;
   if (found)
       // found one common element : x [i] == y [j]
       ...
   else
       // didn't find any common element 
       ...

It is commonly observed that the code that relies on goto statements is generally harder to understand. Not only is it harder to decode but also to maintain as compared to code without gotos.

Conclusion

The goto and label statements do come with its own advantages and the programmer tasks by reducing the number of lines of code (eg: reducing the number of testing conditions), but it is also true that they make the program quite hard to understand and decode.