GDB Internals

Node:User Interface, Next:libgdb, Previous:Algorithms, Up:Top

## User Interface

GDB has several user interfaces. Although the command-line interface is the most common and most familiar, there are others.

### Command Interpreter

The command interpreter in GDB is fairly simple. It is designed to allow for the set of commands to be augmented dynamically, and also has a recursive subcommand capability, where the first argument to a command may itself direct a lookup on a different command list.

For instance, the `set` command just starts a lookup on the `setlist` command list, while `set thread` recurses to the `set_thread_cmd_list`.

To add commands in general, use `add_cmd`. `add_com` adds to the main command list, and should be used for those commands. The usual place to add commands is in the `_initialize_`xyz`` routines at the ends of most source files.

Before removing commands from the command set it is a good idea to deprecate them for some time. Use `deprecate_cmd` on commands or aliases to set the deprecated flag. `deprecate_cmd` takes a `struct cmd_list_element` as it's first argument. You can use the return value from `add_com` or `add_cmd` to deprecate the command immediately after it is created.

The first time a command is used the user will be warned and offered a replacement (if one exists). Note that the replacement string passed to `deprecate_cmd` should be the full name of the command, i.e. the entire string the user should type at the command line.

### UI-Independent Output--the `ui_out` Functions

The `ui_out` functions present an abstraction level for the GDB output code. They hide the specifics of different user interfaces supported by GDB, and thus free the programmer from the need to write several versions of the same code, one each for every UI, to produce output.

#### Overview and Terminology

In general, execution of each GDB command produces some sort of output, and can even generate an input request.

Output can be generated for the following purposes:

- to display a result of an operation;
- to convey info or produce side-effects of a requested operation;
- to provide a notification of an asynchronous event (including progress indication of a prolonged asynchronous operation);
- to display error messages (including warnings);
- to show debug data;
- to query or prompt a user for input (a special case).

This section mainly concentrates on how to build result output, although some of it also applies to other kinds of output.

Generation of output that displays the results of an operation involves one or more of the following:

- output of the actual data
- formatting the output as appropriate for console output, to make it easily readable by humans
- machine oriented formatting-a more terse formatting to allow for easy parsing by programs which read GDB's output
- annotation, whose purpose is to help legacy GUIs to identify interesting parts in the output

The `ui_out` routines take care of the first three aspects. Annotations are provided by separate annotation routines. Note that use of annotations for an interface between a GUI and GDB is deprecated.

Output can be in the form of a single item, which we call a field; a list consisting of identical fields; a tuple consisting of non-identical fields; or a table, which is a tuple consisting of a header and a body. In a BNF-like form:

`<table> ==>`
  `<header> <body>`
`<header> ==>`
  `{ <column> }`
`<column> ==>`
  `<width> <alignment> <title>`
`<body> ==>`
  `{<row>}`

#### General Conventions

Most `ui_out` routines are of type `void`, the exceptions are `ui_out_stream_new` (which returns a pointer to the newly created object) and the `make_cleanup` routines.

The first parameter is always the `ui_out` vector object, a pointer to a `struct ui_out`.

The format parameter is like in `printf` family of functions. When it is present, there must also be a variable list of arguments sufficient used to satisfy the `%` specifiers in the supplied format.

When a character string argument is not used in a `ui_out` function call, a `NULL` pointer has to be supplied instead.

#### Table, Tuple and List Functions

This section introduces `ui_out` routines for building lists, tuples and tables. The routines to output the actual data items (fields) are presented in the next section.

To recap: A tuple is a sequence of fields, each field containing information about an object; a list is a sequence of fields where each field describes an identical object.

Use the table functions when your output consists of a list of rows (tuples) and the console output should include a heading. Use this even when you are listing just one object but you still want the header.

Tables can not be nested. Tuples and lists can be nested up to a maximum of five levels.

The overall structure of the table output code is something like this:

           ui_out_table_begin
             ui_out_table_header
             ...
             ui_out_table_body
               ui_out_tuple_begin
                 ui_out_field_*
                 ...
               ui_out_tuple_end
               ...
           ui_out_table_end

Here is the description of table-, tuple- and list-related `ui_out` functions:

void ui_out_table_begin (struct ui_out *uiout, int nbrofcols, int nr_rows, const char *tblid) | Function

The function `ui_out_table_begin` marks the beginning of the output of a table. It should always be called before any other `ui_out` function for a given table. nbrofcols is the number of columns in the table. nr_rows is the number of rows in the table. tblid is an optional string identifying the table. The string pointed to by tblid is copied by the implementation of `ui_out_table_begin`, so the application can free the string if it was `malloc`ed.

The companion function `ui_out_table_end`, described below, marks the end of the table's output.

void ui_out_table_header (struct ui_out *uiout, int width, enum ui_align alignment, const char *colhdr) | Function

`ui_out_table_header` provides the header information for a single table column. You call this function several times, one each for every column of the table, after `ui_out_table_begin`, but before `ui_out_table_body`. The value of width gives the column width in characters. The value of alignment is one of `left`, `center`, and `right`, and it specifies how to align the header: left-justify, center, or right-justify it. colhdr points to a string that specifies the column header; the implementation copies that string, so column header strings in `malloc`ed storage can be freed after the call.

void ui_out_table_body (struct ui_out *uiout) | Function

This function delimits the table header from the table body.

void ui_out_table_end (struct ui_out *uiout) | Function

This function signals the end of a table's output. It should be called after the table body has been produced by the list and field output functions. There should be exactly one call to `ui_out_table_end` for each call to `ui_out_table_begin`, otherwise the `ui_out` functions will signal an internal error.

The output of the tuples that represent the table rows must follow the call to `ui_out_table_body` and precede the call to `ui_out_table_end`. You build a tuple by calling `ui_out_tuple_begin` and `ui_out_tuple_end`, with suitable calls to functions which actually output fields between them.

void ui_out_tuple_begin (struct ui_out *uiout, const char *id) | Function

This function marks the beginning of a tuple output. id points to an optional string that identifies the tuple; it is copied by the implementation, and so strings in `malloc`ed storage can be freed after the call.

void ui_out_tuple_end (struct ui_out *uiout) | Function

This function signals an end of a tuple output. There should be exactly one call to `ui_out_tuple_end` for each call to `ui_out_tuple_begin`, otherwise an internal GDB error will be signaled.

struct cleanup *make_cleanup_ui_out_tuple_begin_end (struct ui_out *uiout, const char *id) | Function

This function first opens the tuple and then establishes a cleanup (see Cleanups) to close the tuple. It provides a convenient and correct implementation of the non-portable1 code sequence:

              struct cleanup *old_cleanup;
              ui_out_tuple_begin (uiout, "...");
              old_cleanup = make_cleanup ((void(*)(void *)) ui_out_tuple_end,
                                          uiout);

void ui_out_list_begin (struct ui_out *uiout, const char *id) | Function

This function marks the beginning of a list output. id points to an optional string that identifies the list; it is copied by the implementation, and so strings in `malloc`ed storage can be freed after the call.

void ui_out_list_end (struct ui_out *uiout) | Function

This function signals an end of a list output. There should be exactly one call to `ui_out_list_end` for each call to `ui_out_list_begin`, otherwise an internal GDB error will be signaled.

struct cleanup *make_cleanup_ui_out_list_begin_end (struct ui_out *uiout, const char *id) | Function

Similar to `make_cleanup_ui_out_tuple_begin_end`, this function opens a list and then establishes cleanup (see Cleanups) that will close the list.list.

#### Item Output Functions

The functions described below produce output for the actual data items, or fields, which contain information about the object.

Choose the appropriate function accordingly to your particular needs.

void ui_out_field_fmt (struct ui_out *uiout, char *fldname, char *format, ...) | Function

This is the most general output function. It produces the representation of the data in the variable-length argument list according to formatting specifications in format, a `printf`-like format string. The optional argument fldname supplies the name of the field. The data items themselves are supplied as additional arguments after format.

This generic function should be used only when it is not possible to use one of the specialized versions (see below).

void ui_out_field_int (struct ui_out *uiout, const char *fldname, int value) | Function

This function outputs a value of an `int` variable. It uses the `"%d"` output conversion specification. fldname specifies the name of the field.

void ui_out_field_core_addr (struct ui_out *uiout, const char *fldname, CORE_ADDR address) | Function

This function outputs an address.

void ui_out_field_string (struct ui_out *uiout, const char *fldname, const char *string) | Function

This function outputs a string using the `"%s"` conversion specification.

Sometimes, there's a need to compose your output piece by piece using functions that operate on a stream, such as `value_print` or `fprintf_symbol_filtered`. These functions accept an argument of the type `struct ui_file *`, a pointer to a `ui_file` object used to store the data stream used for the output. When you use one of these functions, you need a way to pass their results stored in a `ui_file` object to the `ui_out` functions. To this end, you first create a `ui_stream` object by calling `ui_out_stream_new`, pass the `stream` member of that `ui_stream` object to `value_print` and similar functions, and finally call `ui_out_field_stream` to output the field you constructed. When the `ui_stream` object is no longer needed, you should destroy it and free its memory by calling `ui_out_stream_delete`.

struct ui_stream *ui_out_stream_new (struct ui_out *uiout) | Function

This function creates a new `ui_stream` object which uses the same output methods as the `ui_out` object whose pointer is passed in uiout. It returns a pointer to the newly created `ui_stream` object.

void ui_out_stream_delete (struct ui_stream *streambuf) | Function

This functions destroys a `ui_stream` object specified by streambuf.

void ui_out_field_stream (struct ui_out *uiout, const char *fieldname, struct ui_stream *streambuf) | Function

This function consumes all the data accumulated in `streambuf->stream` and outputs it like `ui_out_field_string` does. After a call to `ui_out_field_stream`, the accumulated data no longer exists, but the stream is still valid and may be used for producing more fields.

Important: If there is any chance that your code could bail out before completing output generation and reaching the point where `ui_out_stream_delete` is called, it is necessary to set up a cleanup, to avoid leaking memory and other resources. Here's a skeleton code to do that:

          struct ui_stream *mybuf = ui_out_stream_new (uiout);
          struct cleanup *old = make_cleanup (ui_out_stream_delete, mybuf);
          ...
          do_cleanups (old);

If the function already has the old cleanup chain set (for other kinds of cleanups), you just have to add your cleanup to it:

           mybuf = ui_out_stream_new (uiout);
           make_cleanup (ui_out_stream_delete, mybuf);

Note that with cleanups in place, you should not call `ui_out_stream_delete` directly, or you would attempt to free the same buffer twice.

#### Utility Output Functions

void ui_out_field_skip (struct ui_out *uiout, const char *fldname) | Function

This function skips a field in a table. Use it if you have to leave an empty field without disrupting the table alignment. The argument fldname specifies a name for the (missing) filed.

void ui_out_text (struct ui_out *uiout, const char *string) | Function

This function outputs the text in string in a way that makes it easy to be read by humans. For example, the console implementation of this method filters the text through a built-in pager, to prevent it from scrolling off the visible portion of the screen.

Use this function for printing relatively long chunks of text around the actual field data: the text it produces is not aligned according to the table's format. Use `ui_out_field_string` to output a string field, and use `ui_out_message`, described below, to output short messages.

void ui_out_spaces (struct ui_out *uiout, int nspaces) | Function

This function outputs nspaces spaces. It is handy to align the text produced by `ui_out_text` with the rest of the table or list.

void ui_out_message (struct ui_out *uiout, int verbosity, const char *format, ...) | Function

This function produces a formatted message, provided that the current verbosity level is at least as large as given by verbosity. The current verbosity level is specified by the user with the `set verbositylevel` command.2

void ui_out_wrap_hint (struct ui_out *uiout, char *indent) | Function

This function gives the console output filter (a paging filter) a hint of where to break lines which are too long. Ignored for all other output consumers. indent, if non-`NULL`, is the string to be printed to indent the wrapped text on the next line; it must remain accessible until the next call to `ui_out_wrap_hint`, or until an explicit newline is produced by one of the other functions. If indent is `NULL`, the wrapped text will not be indented.

void ui_out_flush (struct ui_out *uiout) | Function

This function flushes whatever output has been accumulated so far, if the UI buffers output.

#### Examples of Use of `ui_out` functions

This section gives some practical examples of using the `ui_out` functions to generalize the old console-oriented code in GDB. The examples all come from functions defined on the `breakpoints.c` file.

This example, from the `breakpoint_1` function, shows how to produce a table.

The original code was:

          if (!found_a_breakpoint++)
            {
              annotate_breakpoints_headers ();

              annotate_field (0);
              printf_filtered ("Num ");
              annotate_field (1);
              printf_filtered ("Type           ");
              annotate_field (2);
              printf_filtered ("Disp ");
              annotate_field (3);
              printf_filtered ("Enb ");
              if (addressprint)
                {
                  annotate_field (4);
                  printf_filtered ("Address    ");
                }
              annotate_field (5);
              printf_filtered ("What\n");

              annotate_breakpoints_table ();
            }

Here's the new version:

           nr_printable_breakpoints = ...;

           if (addressprint)
             ui_out_table_begin (ui, 6, nr_printable_breakpoints, "BreakpointTable");
           else
             ui_out_table_begin (ui, 5, nr_printable_breakpoints, "BreakpointTable");

           if (nr_printable_breakpoints > 0)
             annotate_breakpoints_headers ();
           if (nr_printable_breakpoints > 0)
             annotate_field (0);
           ui_out_table_header (uiout, 3, ui_left, "number", "Num");		/* 1 */
           if (nr_printable_breakpoints > 0)
             annotate_field (1);
           ui_out_table_header (uiout, 14, ui_left, "type", "Type");		/* 2 */
           if (nr_printable_breakpoints > 0)
             annotate_field (2);
           ui_out_table_header (uiout, 4, ui_left, "disp", "Disp");		/* 3 */
           if (nr_printable_breakpoints > 0)
             annotate_field (3);
           ui_out_table_header (uiout, 3, ui_left, "enabled", "Enb");	/* 4 */
           if (addressprint)
             {
              if (nr_printable_breakpoints > 0)
                annotate_field (4);
              if (TARGET_ADDR_BIT <= 32)
                ui_out_table_header (uiout, 10, ui_left, "addr", "Address");/* 5 */
              else
                ui_out_table_header (uiout, 18, ui_left, "addr", "Address");/* 5 */
             }
           if (nr_printable_breakpoints > 0)
             annotate_field (5);
           ui_out_table_header (uiout, 40, ui_noalign, "what", "What");	/* 6 */
           ui_out_table_body (uiout);
           if (nr_printable_breakpoints > 0)
             annotate_breakpoints_table ();

This example, from the `print_one_breakpoint` function, shows how to produce the actual data for the table whose structure was defined in the above example. The original code was:

            annotate_record ();
            annotate_field (0);
            printf_filtered ("%-3d ", b->number);
            annotate_field (1);
            if ((int)b->type > (sizeof(bptypes)/sizeof(bptypes[0]))
                || ((int) b->type != bptypes[(int) b->type].type))
              internal_error ("bptypes table does not describe type #%d.",
                              (int)b->type);
            printf_filtered ("%-14s ", bptypes[(int)b->type].description);
            annotate_field (2);
            printf_filtered ("%-4s ", bpdisps[(int)b->disposition]);
            annotate_field (3);
            printf_filtered ("%-3c ", bpenables[(int)b->enable]);
            ...

This is the new version:

            annotate_record ();
            ui_out_tuple_begin (uiout, "bkpt");
            annotate_field (0);
            ui_out_field_int (uiout, "number", b->number);
            annotate_field (1);
            if (((int) b->type > (sizeof (bptypes) / sizeof (bptypes[0])))
                || ((int) b->type != bptypes[(int) b->type].type))
              internal_error ("bptypes table does not describe type #%d.",
                              (int) b->type);
            ui_out_field_string (uiout, "type", bptypes[(int)b->type].description);
            annotate_field (2);
            ui_out_field_string (uiout, "disp", bpdisps[(int)b->disposition]);
            annotate_field (3);
            ui_out_field_fmt (uiout, "enabled", "%c", bpenables[(int)b->enable]);
            ...

This example, also from `print_one_breakpoint`, shows how to produce a complicated output field using the `print_expression` functions which requires a stream to be passed. It also shows how to automate stream destruction with cleanups. The original code was:

             annotate_field (5);
             print_expression (b->exp, gdb_stdout);

The new version is:

           struct ui_stream *stb = ui_out_stream_new (uiout);
           struct cleanup *old_chain = make_cleanup_ui_out_stream_delete (stb);
           ...
           annotate_field (5);
           print_expression (b->exp, stb->stream);
           ui_out_field_stream (uiout, "what", local_stream);

This example, also from `print_one_breakpoint`, shows how to use `ui_out_text` and `ui_out_field_string`. The original code was:

           annotate_field (5);
           if (b->dll_pathname == NULL)
             printf_filtered ("<any library> ");
           else
             printf_filtered ("library \"%s\" ", b->dll_pathname);

It became:

           annotate_field (5);
           if (b->dll_pathname == NULL)
             {
               ui_out_field_string (uiout, "what", "<any library>");
               ui_out_spaces (uiout, 1);
             }
           else
             {
               ui_out_text (uiout, "library \"");
               ui_out_field_string (uiout, "what", b->dll_pathname);
               ui_out_text (uiout, "\" ");
             }

The following example from `print_one_breakpoint` shows how to use `ui_out_field_int` and `ui_out_spaces`. The original code was:

           annotate_field (5);
           if (b->forked_inferior_pid != 0)
             printf_filtered ("process %d ", b->forked_inferior_pid);

It became:

           annotate_field (5);
           if (b->forked_inferior_pid != 0)
             {
               ui_out_text (uiout, "process ");
               ui_out_field_int (uiout, "what", b->forked_inferior_pid);
               ui_out_spaces (uiout, 1);
             }

Here's an example of using `ui_out_field_string`. The original code was:

           annotate_field (5);
           if (b->exec_pathname != NULL)
             printf_filtered ("program \"%s\" ", b->exec_pathname);

It became:

           annotate_field (5);
           if (b->exec_pathname != NULL)
             {
               ui_out_text (uiout, "program \"");
               ui_out_field_string (uiout, "what", b->exec_pathname);
               ui_out_text (uiout, "\" ");
             }

Finally, here's an example of printing an address. The original code:

           annotate_field (4);
           printf_filtered ("%s ",
                 local_hex_string_custom ((unsigned long) b->address, "08l"));

It became:

           annotate_field (4);
           ui_out_field_core_addr (uiout, "Address", b->address);

### Console Printing

### TUI

#### Footnotes

1. The function cast is not portable ISO-C.

2. As of this writing (April 2001), setting verbosity level is not yet implemented, and is always returned as zero. So calling `ui_out_message` with a verbosity argument more than zero will cause the message to never be printed.