10.12 Convenience Functions

gdb also supplies some convenience functions. These have a syntax similar to convenience variables. A convenience function can be used in an expression just like an ordinary function; however, a convenience function is implemented internally to gdb.

These functions do not require gdb to be configured with Python support, which means that they are always available.

$_isvoid (expr)

Return one if the expression expr is void. Otherwise it returns zero.

A void expression is an expression where the type of the result is void. For example, you can examine a convenience variable (see Convenience Variables) to check whether it is void:

          (gdb) print $_exitcode
          $1 = void
          (gdb) print $_isvoid ($_exitcode)
          $2 = 1
          (gdb) run
          Starting program: ./a.out
          [Inferior 1 (process 29572) exited normally]
          (gdb) print $_exitcode
          $3 = 0
          (gdb) print $_isvoid ($_exitcode)
          $4 = 0

In the example above, we used $_isvoid to check whether $_exitcode is void before and after the execution of the program being debugged. Before the execution there is no exit code to be examined, therefore $_exitcode is void. After the execution the program being debugged returned zero, therefore $_exitcode is zero, which means that it is not void anymore.

The void expression can also be a call of a function from the program being debugged. For example, given the following function:

          void
          foo (void)
          {
          }

The result of calling it inside gdb is void:

          (gdb) print foo ()
          $1 = void
          (gdb) print $_isvoid (foo ())
          $2 = 1
          (gdb) set $v = foo ()
          (gdb) print $v
          $3 = void
          (gdb) print $_isvoid ($v)
          $4 = 1

These functions require gdb to be configured with Python support.

$_memeq(buf1, buf2, length)

Returns one if the length bytes at the addresses given by buf1 and buf2 are equal. Otherwise it returns zero.

$_regex(str, regex)

Returns one if the string str matches the regular expression regex. Otherwise it returns zero. The syntax of the regular expression is that specified by Python's regular expression support.

$_streq(str1, str2)

Returns one if the strings str1 and str2 are equal. Otherwise it returns zero.

$_strlen(str)

Returns the length of string str.

gdb provides the ability to list and get help on convenience functions.

help function

Print a list of all convenience functions.