This document describes the obsolete level two annotation interface implemented in older gdb versions.
To produce obsolete level two annotations, start gdb with the
Annotations start with a newline character, two `control-z' characters, and the name of the annotation. If there is no additional information associated with this annotation, the name of the annotation is followed immediately by a newline. If there is additional information, the name of the annotation is followed by a space, the additional information, and a newline. The additional information cannot contain newline characters.
Any output not beginning with a newline and two `control-z' characters denotes literal output from gdb. Currently there is no need for gdb to output a newline followed by two `control-z' characters, but if there was such a need, the annotations could be extended with an `escape' annotation which means those three characters as output.
A simple example of starting up gdb with annotations is:
$ gdb --annotate=2 GNU GDB 5.0 Copyright 2000 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "sparc-sun-sunos4.1.3" ^Z^Zpre-prompt (gdb) ^Z^Zprompt quit ^Z^Zpost-prompt $
Here `quit' is input to gdb; the rest is output from gdb. The three lines beginning `^Z^Z' (where `^Z' denotes a `control-z' character) are annotations; the rest is output from gdb.
The level two annotations mechanism is known to have a number of technical and architectural limitations. As a consequence, in 2001, with the release of gdb 5.1 and the addition of gdb/mi, the annotation interface was marked as deprecated.
This chapter discusses the known problems.
The annotation interface works by interspersing markups with gdb normal command-line interpreter output. Unfortunately, this makes the annotation client dependant on not just the annotations, but also the cli output. This is because the client is forced to assume that specific gdb commands provide specific information. Any change to gdb's cli output modifies or removes that information and, consequently, likely breaks the client.
Since the gdb/mi output is independant of the cli, it does not have this problem.
The annotation interface relies on value annotations (see Value Annotations) and the display mechanism as a way of obtaining up-to-date value information. These mechanisms are not scalable.
In a graphical environment, where many values can be displayed simultaneously, a serious performance problem occurs when the client tries to first extract from gdb, and then re-display, all those values. The client should instead only request and update the values that changed.
The gdb/mi Variable Objects provide just that mechanism.
The annotation interface assumes that a variable's value can only be changed when the target is running. This assumption is not correct. A single assignment to a single variable can result in the entire target, and all displayed values, needing an update.
The gdb/mi Variable Objects include a mechanism for efficiently reporting such changes.
The gdb/mi interface includes a dedicated test directory (gdb/gdb.mi), and any addition or fix to gdb/mi must include testsuite changes.
The annotation mechanism was implemented by interspersing cli print statements with various annotations. As a consequence, any cli output change can alter the annotation output.
Since the gdb/mi output is independant of the cli, and the gdb/mi is increasingly implemented independant of the cli code, its long term maintenance is much easier.
By using the `interp mi' command, it is possible for annotation clients to invoke gdb/mi commands, and hence access the gdb/mi. By doing this, existing annotation clients have a migration path from this obsolete interface to gdb/mi.
To issue a command to gdb without affecting certain aspects of the state which is seen by users, prefix it with `server '. This means that this command will not affect the command history, nor will it affect gdb's notion of which command to repeat if <RET> is pressed on a line by itself.
The server prefix does not affect the recording of values into the value
history; to print a value without recording it into the value history,
output command instead of the
Value Annotations have been removed. gdb/mi instead provides Variable Objects.
When a value is printed in various contexts, gdb uses annotations to delimit the value from the surrounding text.
If a value is printed using
^Z^Zvalue-history-begin history-number value-flags history-string ^Z^Zvalue-history-value the-value ^Z^Zvalue-history-end
where history-number is the number it is getting in the value history, history-string is a string, such as `$5 = ', which introduces the value to the user, the-value is the output corresponding to the value itself, and value-flags is `*' for a value which can be dereferenced and `-' for a value which cannot.
If the value is not added to the value history (it is an invalid float
or it is printed with the
output command), the annotation is similar:
^Z^Zvalue-begin value-flags the-value ^Z^Zvalue-end
When gdb prints an argument to a function (for example, in the output
backtrace command), it annotates it as follows:
^Z^Zarg-begin argument-name ^Z^Zarg-name-end separator-string ^Z^Zarg-value value-flags the-value ^Z^Zarg-end
where argument-name is the name of the argument,
separator-string is text which separates the name from the value
for the user's benefit (such as `='), and value-flags and
the-value have the same meanings as in a
When printing a structure, gdb annotates it as follows:
^Z^Zfield-begin value-flags field-name ^Z^Zfield-name-end separator-string ^Z^Zfield-value the-value ^Z^Zfield-end
where field-name is the name of the field, separator-string
is text which separates the name from the value for the user's benefit
(such as `='), and value-flags and the-value have the
same meanings as in a
When printing an array, gdb annotates it as follows:
^Z^Zarray-section-begin array-index value-flags
where array-index is the index of the first element being
annotated and value-flags has the same meaning as in a
value-history-begin annotation. This is followed by any number
of elements, where is element can be either a single element:
`,' whitespace ; omitted for the first element the-value ^Z^Zelt
or a repeated element
`,' whitespace ; omitted for the first element the-value ^Z^Zelt-rep number-of-repetitions repetition-string ^Z^Zelt-rep-end
In both cases, the-value is the output for the value of the element and whitespace can contain spaces, tabs, and newlines. In the repeated case, number-of-repetitions is the number of consecutive array elements which contain that value, and repetition-string is a string which is designed to convey to the user that repetition is being depicted.
Once all the array elements have been output, the array annotation is ended with
Value Annotations have been removed. gdb/mi instead provides a number of frame commands.
Frame annotations are no longer available. The gdb/mi provides `-stack-list-arguments', `-stack-list-locals', and `-stack-list-frames' commands.
Whenever gdb prints a frame, it annotates it. For example, this applies
to frames printed when gdb stops, output from commands such as
The frame annotation begins with
^Z^Zframe-begin level address level-string
where level is the number of the frame (0 is the innermost frame, and other frames have positive numbers), address is the address of the code executing in that frame, and level-string is a string designed to convey the level to the user. address is in the form `0x' followed by one or more lowercase hex digits (note that this does not depend on the language). The frame ends with
Between these annotations is the main body of the frame, which can consist of
where function-call-string is text designed to convey to the user that this frame is associated with a function call made by gdb to a function in the program being debugged.
where signal-handler-caller-string is text designed to convey to the user that this frame is associated with whatever mechanism is used by this operating system to call a signal handler (it is the frame which calls the signal handler, not the frame for the signal handler itself).
^Z^Zframe-address address ^Z^Zframe-address-end separator-string
where address is the address executing in the frame (the same
address as in the
frame-begin annotation, but printed in a form
which is intended for user consumption—in particular, the syntax varies
depending on the language), and separator-string is a string
intended to separate this address from what follows for the user's
^Z^Zframe-function-name function-name ^Z^Zframe-args arguments
where function-name is the name of the function executing in the frame, or `??' if not known, and arguments are the arguments to the frame, with parentheses around them (each argument is annotated individually as well, see Value Annotations).
^Z^Zframe-source-begin source-intro-string ^Z^Zframe-source-file filename ^Z^Zframe-source-file-end : ^Z^Zframe-source-line line-number ^Z^Zframe-source-end
where source-intro-string separates for the user's benefit the reference from the text which precedes it, filename is the name of the source file, and line-number is the line number within that file (the first line is line 1).
Then, if source is to actually be displayed for this frame (for example,
this is not true for output from the
backtrace command), then a
source annotation (see Source Annotations) is displayed. Unlike
most annotations, this is output instead of the normal text which would be
output, not in addition.
Display Annotations have been removed. gdb/mi instead provides Variable Objects.
When gdb is told to display something using the
the results of the display are annotated:
^Z^Zdisplay-begin number ^Z^Zdisplay-number-end number-separator ^Z^Zdisplay-format format ^Z^Zdisplay-expression expression ^Z^Zdisplay-expression-end expression-separator ^Z^Zdisplay-value value ^Z^Zdisplay-end
where number is the number of the display, number-separator is intended to separate the number from what follows for the user, format includes information such as the size, format, or other information about how the value is being displayed, expression is the expression being displayed, expression-separator is intended to separate the expression from the text that follows for the user, and value is the actual value being displayed.
When gdb prompts for input, it annotates this fact so it is possible to know when to send output, when the output from a given command is over, etc.
Different kinds of input each have a different input type. Each
input type has three annotations: a
pre- annotation, which
denotes the beginning of any prompt which is being output, a plain
annotation, which denotes the end of the prompt, and then a
annotation which denotes the end of any echo which may (or may not) be
associated with the input. For example, the
prompt input type
features the following annotations:
^Z^Zpre-prompt ^Z^Zprompt ^Z^Zpost-prompt
The input types are
commandscommand. The annotations are repeated for each command which is input.
set height 0to disable prompting. This is because the counting of lines is buggy in the presence of annotations.
This annotation occurs right before gdb responds to an interrupt.
This annotation occurs right before gdb responds to an error.
Quit and error annotations indicate that any annotations which gdb was
in the middle of may end abruptly. For example, if a
value-history-begin annotation is followed by a
cannot expect to receive the matching
cannot expect not to receive it either, however; an error annotation
does not necessarily mean that gdb is immediately returning all the way
to the top level.
A quit or error annotation may be preceded by
Any output between that and the quit or error annotation is the error message.
Warning messages are not yet annotated.
Breakpoint Annotations have been removed. gdb/mi instead provides breakpoint commands.
The output from the
info breakpoints command is annotated as follows:
^Z^Zbreakpoints-headers header-entry ^Z^Zbreakpoints-table
where header-entry has the same syntax as an entry (see below) but instead of containing data, it contains strings which are intended to convey the meaning of each field to the user. This is followed by any number of entries. If a field does not apply for this entry, it is omitted. Fields may contain trailing whitespace. Each entry consists of:
^Z^Zrecord ^Z^Zfield 0 number ^Z^Zfield 1 type ^Z^Zfield 2 disposition ^Z^Zfield 3 enable ^Z^Zfield 4 address ^Z^Zfield 5 what ^Z^Zfield 6 frame ^Z^Zfield 7 condition ^Z^Zfield 8 ignore-count ^Z^Zfield 9 commands
Note that address is intended for user consumption—the syntax varies depending on the language.
The output ends with
The following annotations say that certain pieces of state may have changed.
backtracecommand) may have changed.
When the program starts executing due to a gdb command such as
is output. When the program stops,
is output. Before the
stopped annotation, a variety of
annotations describe how the program stopped.
^Z^Zsignalled, the annotation continues:
intro-text ^Z^Zsignal-name name ^Z^Zsignal-name-end middle-text ^Z^Zsignal-string string ^Z^Zsignal-string-end end-text
where name is the name of the signal, such as
SIGSEGV, and string is the explanation of the signal, such
Illegal Instruction or
intro-text, middle-text, and end-text are for the
user's benefit and have no particular format.
signalled, but gdb is just saying that the program received the signal, not that it was terminated with it.
The following annotation is used instead of displaying source code:
where filename is an absolute file name indicating which source file, line is the line number within that file (where 1 is the first line in the file), character is the character position within the file (where 0 is the first character in the file) (for most debug formats this will necessarily point to the beginning of a line), middle is `middle' if addr is in the middle of the line, or `beg' if addr is at the beginning of the line, and addr is the address in the target program associated with the source which is being displayed. addr is in the form `0x' followed by one or more lowercase hex digits (note that this does not depend on the language).
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