GDB's Obsolete Annotations


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GDB Annotations

This document describes the obsolete level two annotation interface implemented in older gdb versions.

Table of Contents


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1 What is an Annotation?

To produce obsolete level two annotations, start gdb with the --annotate=2 option.

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.


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2 Limitations of the Annotation Interface

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.

2.1 Dependant on cli output

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.

2.2 Scalability

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.

2.3 Correctness

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.

2.4 Reliability

The gdb/mi interface includes a dedicated test directory (gdb/gdb.mi), and any addition or fix to gdb/mi must include testsuite changes.

2.5 Maintainability

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.


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3 Migrating to gdb/mi

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.


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4 The Server Prefix

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, use the output command instead of the print command.


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5 Values

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 print and added to the value history, the annotation looks like

     ^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 from the 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 value-history-begin annotation.

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 value-history-begin annotation.

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

     ^Z^Zarray-section-end


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6 Frames

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 backtrace or up, etc.

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

     ^Z^Zframe-end

Between these annotations is the main body of the frame, which can consist of


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7 Displays

Display Annotations have been removed. gdb/mi instead provides Variable Objects.

When gdb is told to display something using the display command, 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.


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8 Annotation for gdb Input

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 post- 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

prompt
When gdb is prompting for a command (the main gdb prompt).


commands
When gdb prompts for a set of commands, like in the commands command. The annotations are repeated for each command which is input.


overload-choice
When gdb wants the user to select between various overloaded functions.


query
When gdb wants the user to confirm a potentially dangerous operation.


prompt-for-continue
When gdb is asking the user to press return to continue. Note: Don't expect this to work well; instead use set height 0 to disable prompting. This is because the counting of lines is buggy in the presence of annotations.


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9 Errors

     ^Z^Zquit

This annotation occurs right before gdb responds to an interrupt.

     ^Z^Zerror

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 error, one cannot expect to receive the matching value-history-end. One 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

     ^Z^Zerror-begin

Any output between that and the quit or error annotation is the error message.

Warning messages are not yet annotated.


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10 Information on Breakpoints

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

     ^Z^Zbreakpoints-table-end


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11 Invalidation Notices

The following annotations say that certain pieces of state may have changed.

^Z^Zframes-invalid
The frames (for example, output from the backtrace command) may have changed.


^Z^Zbreakpoints-invalid
The breakpoints may have changed. For example, the user just added or deleted a breakpoint.


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12 Running the Program

When the program starts executing due to a gdb command such as step or continue,

     ^Z^Zstarting

is output. When the program stops,

     ^Z^Zstopped

is output. Before the stopped annotation, a variety of annotations describe how the program stopped.

^Z^Zexited exit-status
The program exited, and exit-status is the exit status (zero for successful exit, otherwise nonzero).


^Z^Zsignalled
The program exited with a signal. After the ^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 SIGILL or SIGSEGV, and string is the explanation of the signal, such as Illegal Instruction or Segmentation fault. intro-text, middle-text, and end-text are for the user's benefit and have no particular format.


^Z^Zsignal
The syntax of this annotation is just like signalled, but gdb is just saying that the program received the signal, not that it was terminated with it.


^Z^Zbreakpoint number
The program hit breakpoint number number.


^Z^Zwatchpoint number
The program hit watchpoint number number.


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13 Displaying Source

The following annotation is used instead of displaying source code:

     ^Z^Zsource filename:line:character:middle:addr

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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14 GNU Free Documentation License

Version 1.2, November 2002
     Copyright © 2000,2001,2002 Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307, USA
     
     Everyone is permitted to copy and distribute verbatim copies
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    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

14.1 ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  year  your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.