The C Programming Language, Ansi C
goto
statement, and labels to branch
to. Formally, the
goto
statement is never necessary, and in practice it
is almost always easy to write code without it. We have not used
goto
in this book.
Nevertheless, there are a few situations where gotos may find a place. The most common is to abandon processing in some deeply nested structure, such as breaking out of two or more loops at once. The break statement cannot be used directly since it only exits from the innermost loop. Thus:
for ( ... )
for ( ... ) {
...
if (disaster)
goto error;
}
...
error:
/* clean up the mess */
This organization is handy if the error-handling code is non-trivial, and if
errors can occur in several places.
A label has the same form as a variable name, and is followed by a colon. It can be attached to any statement in the same function as the goto. The scope of a label is the entire function.
As another example, consider the problem of determining whether two arrays a and b have an element in common. One possibility is
for (i = 0; i < n; i++)
for (j = 0; j < m; j++)
if (a[i] == b[j])
goto found;
/* didn't find any common element */
...
found:
/* got one: a[i] == b[j] */
...
Code involving a
goto
can always be written without one, though perhaps
at the price of some repeated tests or an extra variable. For example, the
array search becomes
found = 0;
for (i = 0; i < n && !found; i++)
for (j = 0; j < m && !found; j++)
if (a[i] == b[j])
found = 1;
if (found)
/* got one: a[i-1] == b[j-1] */
...
else
/* didn't find any common element */
...
With a few exceptions like those cited here, code that relies on
goto
statements is generally harder to understand and to maintain than code without
goto
s. Although we are not dogmatic about the matter, it does seem that
goto