10.13 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

$_gdb_setting_str (setting)

Return the value of the GDB setting as a string. setting is any setting that can be used in a set or show command (see Controlling GDB).

(gdb) show print frame-arguments
Printing of non-scalar frame arguments is "scalars".
(gdb) p $_gdb_setting_str("print frame-arguments")
$1 = "scalars"
(gdb) p $_gdb_setting_str("height")
$2 = "30"
(gdb)

$_gdb_setting (setting)

Return the value of the GDB setting. The type of the returned value depends on the setting.

The value type for boolean and auto boolean settings is int. The boolean values off and on are converted to the integer values 0 and 1. The value auto is converted to the value -1.

The value type for integer settings is either unsigned int or int, depending on the setting.

Some integer settings accept an unlimited value. Depending on the setting, the set command also accepts the value 0 or the value -1 as a synonym for unlimited. For example, set height unlimited is equivalent to set height 0.

Some other settings that accept the unlimited value use the value 0 to literally mean zero. For example, set history size 0 indicates to not record any GDB commands in the command history. For such settings, -1 is the synonym for unlimited.

See the documentation of the corresponding set command for the numerical value equivalent to unlimited.

The $_gdb_setting function converts the unlimited value to a 0 or a -1 value according to what the set command uses.

(gdb) p $_gdb_setting_str("height")
$1 = "30"
(gdb) p $_gdb_setting("height")
$2 = 30
(gdb) set height unlimited
(gdb) p $_gdb_setting_str("height")
$3 = "unlimited"
(gdb) p $_gdb_setting("height")
$4 = 0

(gdb) p $_gdb_setting_str("history size")
$5 = "unlimited"
(gdb) p $_gdb_setting("history size")
$6 = -1
(gdb) p $_gdb_setting_str("disassemble-next-line")
$7 = "auto"
(gdb) p $_gdb_setting("disassemble-next-line")
$8 = -1
(gdb)

Other setting types (enum, filename, optional filename, string, string noescape) are returned as string values.

$_gdb_maint_setting_str (setting)

Like the $_gdb_setting_str function, but works with maintenance set variables.

$_gdb_maint_setting (setting)

Like the $_gdb_setting function, but works with maintenance set variables.

$_shell (command-string)

Invoke a shell to execute command-string. command-string must be a string. The shell runs on the host machine, the machine GDB is running on. Returns the command’s exit status. On Unix systems, a command which exits with a zero exit status has succeeded, and non-zero exit status indicates failure. When a command terminates on a fatal signal whose number is N, GDB uses the value 128+N as the exit status, as is standard in Unix shells. Note that N is a host signal number, not a target signal number. If you’re native debugging, they will be the same, but if cross debugging, the host vs target signal numbers may be completely unrelated. Please consult your host operating system’s documentation for the mapping between host signal numbers and signal names. The shell to run is determined in the same way as for the shell command. See Shell Commands.

(gdb) print $_shell("true")
$1 = 0
(gdb) print $_shell("false")
$2 = 1
(gdb) p $_shell("echo hello")
hello
$3 = 0
(gdb) p $_shell("foobar")
bash: line 1: foobar: command not found
$4 = 127

This may also be useful in breakpoint conditions. For example:

(gdb) break function if $_shell("some command") == 0

In this scenario, you’ll want to make sure that the shell command you run in the breakpoint condition takes the least amount of time possible. For example, avoid running a command that may block indefinitely, or that sleeps for a while before exiting. Prefer a command or script which analyzes some state and exits immediately. This is important because the debugged program stops for the breakpoint every time, and then GDB evaluates the breakpoint condition. If the condition is false, the program is re-resumed transparently, without informing you of the stop. A quick shell command thus avoids significantly slowing down the debugged program unnecessarily.

Note: unlike the shell command, the $_shell convenience function does not affect the $_shell_exitcode and $_shell_exitsignal convenience variables.

The following 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.

$_caller_is(name[, number_of_frames])

Returns one if the calling function’s name is equal to name. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

Example:

(gdb) backtrace
#0  bottom_func ()
    at testsuite/gdb.python/py-caller-is.c:21
#1  0x00000000004005a0 in middle_func ()
    at testsuite/gdb.python/py-caller-is.c:27
#2  0x00000000004005ab in top_func ()
    at testsuite/gdb.python/py-caller-is.c:33
#3  0x00000000004005b6 in main ()
    at testsuite/gdb.python/py-caller-is.c:39
(gdb) print $_caller_is ("middle_func")
$1 = 1
(gdb) print $_caller_is ("top_func", 2)
$1 = 1

$_caller_matches(regexp[, number_of_frames])

Returns one if the calling function’s name matches the regular expression regexp. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

$_any_caller_is(name[, number_of_frames])

Returns one if any calling function’s name is equal to name. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

This function differs from $_caller_is in that this function checks all stack frames from the immediate caller to the frame specified by number_of_frames, whereas $_caller_is only checks the frame specified by number_of_frames.

$_any_caller_matches(regexp[, number_of_frames])

Returns one if any calling function’s name matches the regular expression regexp. Otherwise it returns zero.

If the optional argument number_of_frames is provided, it is the number of frames up in the stack to look. The default is 1.

This function differs from $_caller_matches in that this function checks all stack frames from the immediate caller to the frame specified by number_of_frames, whereas $_caller_matches only checks the frame specified by number_of_frames.

$_as_string(value)

This convenience function is considered deprecated, and could be removed from future versions of GDB. Use the ‘%V’ format specifier instead (see %V Format Specifier).

Return the string representation of value.

This function is useful to obtain the textual label (enumerator) of an enumeration value. For example, assuming the variable node is of an enumerated type:

(gdb) printf "Visiting node of type %s\n", $_as_string(node)
Visiting node of type NODE_INTEGER

$_cimag(value)

$_creal(value)

Return the imaginary ($_cimag) or real ($_creal) part of the complex number value.

The type of the imaginary or real part depends on the type of the complex number, e.g., using $_cimag on a float complex will return an imaginary part of type float.

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

help function

Print a list of all convenience functions.