What is a null statement in C?

Question

I want to know exactly, what is a null statement in C programming language? And explain a typical use of it.

I found the following segment of code.

for (j=6; j>0; j++)
;

And

for (j=6; j>0; j++)

Solution 1

From the msdn page:

The "null statement" is an expression statement with the expression missing. It is useful when the syntax of the language calls for a statement but no expression evaluation. It consists of a semicolon.

Null statements are commonly used as placeholders in iteration statements or as statements on which to place labels at the end of compound statements or functions.

know more: https://msdn.microsoft.com/en-us/library/1zea45ac.aspx

---

And explain a typical use of it.

When you want to find the index of first occurrence of a certain character in a string

int a[50] = "lord of the rings";
int i;

for(i = 0; a[i] != 't'; i++)
    ;//null statement
//as no operation is required

Solution 2

A null statement is a statement that doesn't do anything, but exists for syntactical reasons.

while ((*s++ = *t++))
    ; /* null statement */

In this case the null statement provides the body of the while loop.

or (disclaimer: bad code)

if (condition1)
    if (condition2)
        dosomething();
    else
        ; /* null statement */
else
    dosomethingelse();

In this case the inner else and null statement keeps the outer else from binding to the inner if.

Solution 3

From C11, §6.8.4.1, 6.8.3 Expression and null statements:

A null statement (consisting of just a semicolon) performs no operations.

The standard also provides a couple of common uses of it:

EXAMPLE 2 In the program fragment

      char *s;
      /* ... */
      while (*s++ != '\0')
              ;

a null statement is used to supply an empty loop body to the iteration statement.

6 EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound statement.

      while (loop1) {
            /* ... */
            while (loop2) {
                    /* ... */
                    if (want_out)
                            goto end_loop1;
                    /* ... */
            }
            /* ... */
      end_loop1: ;
      }