## 17 Misc Guidelines

This chapter covers topics that are lower-level than the major algorithms of gdb.

### 17.1 Cleanups

Cleanups are a structured way to deal with things that need to be done later.

When your code does something (e.g., `xmalloc` some memory, or `open` a file) that needs to be undone later (e.g., `xfree` the memory or `close` the file), it can make a cleanup. The cleanup will be done at some future point: when the command is finished and control returns to the top level; when an error occurs and the stack is unwound; or when your code decides it's time to explicitly perform cleanups. Alternatively you can elect to discard the cleanups you created.

Syntax:

`struct cleanup *`old_chain`;`
  Declare a variable which will hold a cleanup chain handle.

old_chain`= make_cleanup (`function`,` arg`);`
  Make a cleanup which will cause function to be called with arg (a `char *`) later. The result, old_chain, is a handle that can later be passed to `do_cleanups` or `discard_cleanups`. Unless you are going to call `do_cleanups` or `discard_cleanups`, you can ignore the result from `make_cleanup`.

`do_cleanups (`old_chain`);`
  Do all cleanups added to the chain since the corresponding `make_cleanup` call was made.

`discard_cleanups (`old_chain`);`
  Same as `do_cleanups` except that it just removes the cleanups from the chain and does not call the specified functions.

Cleanups are implemented as a chain. The handle returned by `make_cleanups` includes the cleanup passed to the call and any later cleanups appended to the chain (but not yet discarded or performed). E.g.:

         make_cleanup (a, 0);
         {
           struct cleanup *old = make_cleanup (b, 0);
           make_cleanup (c, 0)
           ...
           do_cleanups (old);
         }

will call `c()` and `b()` but will not call `a()`. The cleanup that calls `a()` will remain in the cleanup chain, and will be done later unless otherwise discarded.

Your function should explicitly do or discard the cleanups it creates. Failing to do this leads to non-deterministic behavior since the caller will arbitrarily do or discard your functions cleanups. This need leads to two common cleanup styles.

The first style is try/finally. Before it exits, your code-block calls `do_cleanups` with the old cleanup chain and thus ensures that your code-block's cleanups are always performed. For instance, the following code-segment avoids a memory leak problem (even when `error` is called and a forced stack unwind occurs) by ensuring that the `xfree` will always be called:

         struct cleanup *old = make_cleanup (null_cleanup, 0);
         data = xmalloc (sizeof blah);
         make_cleanup (xfree, data);
         ... blah blah ...
         do_cleanups (old);

The second style is try/except. Before it exits, your code-block calls `discard_cleanups` with the old cleanup chain and thus ensures that any created cleanups are not performed. For instance, the following code segment, ensures that the file will be closed but only if there is an error:

         FILE *file = fopen ("afile", "r");
         struct cleanup *old = make_cleanup (close_file, file);
         ... blah blah ...
         discard_cleanups (old);
         return file;

Some functions, e.g., `fputs_filtered()` or `error()`, specify that they “should not be called when cleanups are not in place”. This means that any actions you need to reverse in the case of an error or interruption must be on the cleanup chain before you call these functions, since they might never return to your code (they ‘`longjmp`’ instead).

### 17.2 Per-architecture module data

The multi-arch framework includes a mechanism for adding module specific per-architecture data-pointers to the `struct gdbarch` architecture object.

A module registers one or more per-architecture data-pointers using:

— Architecture Function: struct gdbarch_data * gdbarch_data_register_pre_init (gdbarch_data_pre_init_ftype *pre_init)

> pre_init is used to, on-demand, allocate an initial value for a per-architecture data-pointer using the architecture's obstack (passed in as a parameter). Since pre_init can be called during architecture creation, it is not parameterized with the architecture. and must not call modules that use per-architecture data.

— Architecture Function: struct gdbarch_data * gdbarch_data_register_post_init (gdbarch_data_post_init_ftype *post_init)

> post_init is used to obtain an initial value for a per-architecture data-pointer after. Since post_init is always called after architecture creation, it both receives the fully initialized architecture and is free to call modules that use per-architecture data (care needs to be taken to ensure that those other modules do not try to call back to this module as that will create in cycles in the initialization call graph).

These functions return a `struct gdbarch_data` that is used to identify the per-architecture data-pointer added for that module.

The per-architecture data-pointer is accessed using the function:

— Architecture Function: void * gdbarch_data (struct gdbarch *gdbarch, struct gdbarch_data *data_handle)

> Given the architecture arch and module data handle data_handle (returned by `gdbarch_data_register_pre_init` or `gdbarch_data_register_post_init`), this function returns the current value of the per-architecture data-pointer. If the data pointer is `NULL`, it is first initialized by calling the corresponding pre_init or post_init method.

The examples below assume the following definitions:

         struct nozel { int total; };
         static struct gdbarch_data *nozel_handle;

A module can extend the architecture vector, adding additional per-architecture data, using the pre_init method. The module's per-architecture data is then initialized during architecture creation.

In the below, the module's per-architecture nozel is added. An architecture can specify its nozel by calling `set_gdbarch_nozel` from `gdbarch_init`.

         static void *
         nozel_pre_init (struct obstack *obstack)
         {
           struct nozel *data = OBSTACK_ZALLOC (obstack, struct nozel);
           return data;
         }

         extern void
         set_gdbarch_nozel (struct gdbarch *gdbarch, int total)
         {
           struct nozel *data = gdbarch_data (gdbarch, nozel_handle);
           data->total = nozel;
         }

A module can on-demand create architecture dependent data structures using `post_init`.

In the below, the nozel's total is computed on-demand by `nozel_post_init` using information obtained from the architecture.

         static void *
         nozel_post_init (struct gdbarch *gdbarch)
         {
           struct nozel *data = GDBARCH_OBSTACK_ZALLOC (gdbarch, struct nozel);
           nozel->total = gdbarch... (gdbarch);
           return data;
         }

         extern int
         nozel_total (struct gdbarch *gdbarch)
         {
           struct nozel *data = gdbarch_data (gdbarch, nozel_handle);
           return data->total;
         }

### 17.3 Wrapping Output Lines

Output that goes through `printf_filtered` or `fputs_filtered` or `fputs_demangled` needs only to have calls to `wrap_here` added in places that would be good breaking points. The utility routines will take care of actually wrapping if the line width is exceeded.

The argument to `wrap_here` is an indentation string which is printed only if the line breaks there. This argument is saved away and used later. It must remain valid until the next call to `wrap_here` or until a newline has been printed through the `*_filtered` functions. Don't pass in a local variable and then return!

It is usually best to call `wrap_here` after printing a comma or space. If you call it before printing a space, make sure that your indentation properly accounts for the leading space that will print if the line wraps there.

Any function or set of functions that produce filtered output must finish by printing a newline, to flush the wrap buffer, before switching to unfiltered (`printf`) output. Symbol reading routines that print warnings are a good example.

### 17.4 Memory Management

gdb does not use the functions `malloc`, `realloc`, `calloc`, `free` and `asprintf`.

gdb uses the functions `xmalloc`, `xrealloc` and `xcalloc` when allocating memory. Unlike `malloc` et.al. these functions do not return when the memory pool is empty. Instead, they unwind the stack using cleanups. These functions return `NULL` when requested to allocate a chunk of memory of size zero.

Pragmatics: By using these functions, the need to check every memory allocation is removed. These functions provide portable behavior.

gdb does not use the function `free`.

gdb uses the function `xfree` to return memory to the memory pool. Consistent with ISO-C, this function ignores a request to free a `NULL` pointer.

Pragmatics: On some systems `free` fails when passed a `NULL` pointer.

gdb can use the non-portable function `alloca` for the allocation of small temporary values (such as strings).

Pragmatics: This function is very non-portable. Some systems restrict the memory being allocated to no more than a few kilobytes.

gdb uses the string function `xstrdup` and the print function `xstrprintf`.

Pragmatics: `asprintf` and `strdup` can fail. Print functions such as `sprintf` are very prone to buffer overflow errors.

### 17.5 Compiler Warnings

With few exceptions, developers should avoid the configuration option ‘`--disable-werror`’ when building gdb. The exceptions are listed in the file `gdb/MAINTAINERS`. The default, when building with gcc, is ‘`--enable-werror`’.

This option causes gdb (when built using GCC) to be compiled with a carefully selected list of compiler warning flags. Any warnings from those flags are treated as errors.

The current list of warning flags includes:

‘`-Wall`’
  Recommended gcc warnings.
‘`-Wdeclaration-after-statement`’
  gcc 3.x (and later) and c99 allow declarations mixed with code, but gcc 2.x and c89 do not.
‘`-Wpointer-arith`’
‘`-Wformat-nonliteral`’
  Non-literal format strings, with a few exceptions, are bugs - they might contain unintended user-supplied format specifiers. Since gdb uses the `format printf` attribute on all `printf` like functions this checks not just `printf` calls but also calls to functions such as `fprintf_unfiltered`.
‘`-Wno-pointer-sign`’
  In version 4.0, GCC began warning about pointer argument passing or assignment even when the source and destination differed only in signedness. However, most gdb code doesn't distinguish carefully between `char` and `unsigned char`. In early 2006 the gdb developers decided correcting these warnings wasn't worth the time it would take.
‘`-Wno-unused-parameter`’
  Due to the way that gdb is implemented many functions have unused parameters. Consequently this warning is avoided. The macro `ATTRIBUTE_UNUSED` is not used as it leads to false negatives — it is not an error to have `ATTRIBUTE_UNUSED` on a parameter that is being used.
‘`-Wno-unused`’
‘`-Wno-switch`’
‘`-Wno-char-subscripts`’
  These are warnings which might be useful for gdb, but are currently too noisy to enable with ‘`-Werror`’.

### 17.6 Internal Error Recovery

During its execution, gdb can encounter two types of errors. User errors and internal errors. User errors include not only a user entering an incorrect command but also problems arising from corrupt object files and system errors when interacting with the target. Internal errors include situations where gdb has detected, at run time, a corrupt or erroneous situation.

When reporting an internal error, gdb uses `internal_error` and `gdb_assert`.

gdb must not call `abort` or `assert`.

Pragmatics: There is no `internal_warning` function. Either the code detected a user error, recovered from it and issued a `warning` or the code failed to correctly recover from the user error and issued an `internal_error`.

### 17.7 Command Names

GDB U/I commands are written ‘`foo-bar`’, not ‘`foo_bar`’.

### 17.8 Clean Design and Portable Implementation

In addition to getting the syntax right, there's the little question of semantics. Some things are done in certain ways in gdb because long experience has shown that the more obvious ways caused various kinds of trouble.

You can't assume the byte order of anything that comes from a target (including values, object files, and instructions). Such things must be byte-swapped using `SWAP_TARGET_AND_HOST` in gdb, or one of the swap routines defined in `bfd.h`, such as `bfd_get_32`.

You can't assume that you know what interface is being used to talk to the target system. All references to the target must go through the current `target_ops` vector.

You can't assume that the host and target machines are the same machine (except in the “native” support modules). In particular, you can't assume that the target machine's header files will be available on the host machine. Target code must bring along its own header files – written from scratch or explicitly donated by their owner, to avoid copyright problems.

Insertion of new `#ifdef`'s will be frowned upon. It's much better to write the code portably than to conditionalize it for various systems.

New `#ifdef`'s which test for specific compilers or manufacturers or operating systems are unacceptable. All `#ifdef`'s should test for features. The information about which configurations contain which features should be segregated into the configuration files. Experience has proven far too often that a feature unique to one particular system often creeps into other systems; and that a conditional based on some predefined macro for your current system will become worthless over time, as new versions of your system come out that behave differently with regard to this feature.

Adding code that handles specific architectures, operating systems, target interfaces, or hosts, is not acceptable in generic code.

One particularly notorious area where system dependencies tend to creep in is handling of file names. The mainline gdb code assumes Posix semantics of file names: absolute file names begin with a forward slash `/`, slashes are used to separate leading directories, case-sensitive file names. These assumptions are not necessarily true on non-Posix systems such as MS-Windows. To avoid system-dependent code where you need to take apart or construct a file name, use the following portable macros:

`HAVE_DOS_BASED_FILE_SYSTEM`
  This preprocessing symbol is defined to a non-zero value on hosts whose filesystems belong to the MS-DOS/MS-Windows family. Use this symbol to write conditional code which should only be compiled for such hosts.

`IS_DIR_SEPARATOR (`c`)`
  Evaluates to a non-zero value if c is a directory separator character. On Unix and GNU/Linux systems, only a slash `/` is such a character, but on Windows, both `/` and `\` will pass.

`IS_ABSOLUTE_PATH (`file`)`
  Evaluates to a non-zero value if file is an absolute file name. For Unix and GNU/Linux hosts, a name which begins with a slash `/` is absolute. On DOS and Windows, `d:/foo` and `x:\bar` are also absolute file names.

`FILENAME_CMP (`f1`,` f2`)`
  Calls a function which compares file names f1 and f2 as appropriate for the underlying host filesystem. For Posix systems, this simply calls `strcmp`; on case-insensitive filesystems it will call `strcasecmp` instead.

`DIRNAME_SEPARATOR`
  Evaluates to a character which separates directories in `PATH`-style lists, typically held in environment variables. This character is ‘`:`’ on Unix, ‘`;`’ on DOS and Windows.

`SLASH_STRING`
  This evaluates to a constant string you should use to produce an absolute filename from leading directories and the file's basename. `SLASH_STRING` is `"/"` on most systems, but might be `"\\"` for some Windows-based ports.

In addition to using these macros, be sure to use portable library functions whenever possible. For example, to extract a directory or a basename part from a file name, use the `dirname` and `basename` library functions (available in `libiberty` for platforms which don't provide them), instead of searching for a slash with `strrchr`.

Another way to generalize gdb along a particular interface is with an attribute struct. For example, gdb has been generalized to handle multiple kinds of remote interfaces—not by `#ifdef`s everywhere, but by defining the `target_ops` structure and having a current target (as well as a stack of targets below it, for memory references). Whenever something needs to be done that depends on which remote interface we are using, a flag in the current target_ops structure is tested (e.g., `target_has_stack`), or a function is called through a pointer in the current target_ops structure. In this way, when a new remote interface is added, only one module needs to be touched—the one that actually implements the new remote interface. Other examples of attribute-structs are BFD access to multiple kinds of object file formats, or gdb's access to multiple source languages.

Please avoid duplicating code. For example, in gdb 3.x all the code interfacing between `ptrace` and the rest of gdb was duplicated in `*-dep.c`, and so changing something was very painful. In gdb 4.x, these have all been consolidated into `infptrace.c`. `infptrace.c` can deal with variations between systems the same way any system-independent file would (hooks, `#if defined`, etc.), and machines which are radically different don't need to use `infptrace.c` at all.

All debugging code must be controllable using the ‘`set debug module`’ command. Do not use `printf` to print trace messages. Use `fprintf_unfiltered(gdb_stdlog, ...`. Do not use `#ifdef DEBUG`.