23.3.2.29 Accessing blocks from Python

In GDB, symbols are stored in blocks. A block corresponds roughly to a scope in the source code. Blocks are organized hierarchically, and are represented individually in Python as a gdb.Block. Blocks rely on debugging information being available.

A frame has a block. Please see Frames In Python, for a more in-depth discussion of frames.

The outermost block is known as the global block. The global block typically holds public global variables and functions.

The block nested just inside the global block is the static block. The static block typically holds file-scoped variables and functions.

GDB provides a method to get a block’s superblock, but there is currently no way to examine the sub-blocks of a block, or to iterate over all the blocks in a symbol table (see Symbol Tables In Python).

Here is a short example that should help explain blocks:

/* This is in the global block.  */
int global;

/* This is in the static block.  */
static int file_scope;

/* 'function' is in the global block, and 'argument' is
   in a block nested inside of 'function'.  */
int function (int argument)
{
  /* 'local' is in a block inside 'function'.  It may or may
     not be in the same block as 'argument'.  */
  int local;

  {
     /* 'inner' is in a block whose superblock is the one holding
        'local'.  */
     int inner;

     /* If this call is expanded by the compiler, you may see
        a nested block here whose function is 'inline_function'
        and whose superblock is the one holding 'inner'.  */
     inline_function ();
  }
}

A gdb.Block is iterable. The iterator returns the symbols (see Symbols In Python) local to the block. Python programs should not assume that a specific block object will always contain a given symbol, since changes in GDB features and infrastructure may cause symbols move across blocks in a symbol table. You can also use Python’s dictionary syntax to access variables in this block, e.g.:

symbol = some_block['variable']  # symbol is of type gdb.Symbol

The following block-related functions are available in the gdb module:

Function: gdb.block_for_pc (pc)

Return the innermost gdb.Block containing the given pc value. If the block cannot be found for the pc value specified, the function will return None. This is identical to gdb.current_progspace().block_for_pc(pc) and is included for historical compatibility.

A gdb.Block object has the following methods:

Function: Block.is_valid ()

Returns True if the gdb.Block object is valid, False if not. A block object can become invalid if the block it refers to doesn’t exist anymore in the inferior. All other gdb.Block methods will throw an exception if it is invalid at the time the method is called. The block’s validity is also checked during iteration over symbols of the block.

A gdb.Block object has the following attributes:

Variable: Block.start

The start address of the block. This attribute is not writable.

Variable: Block.end

One past the last address that appears in the block. This attribute is not writable.

Variable: Block.function

The name of the block represented as a gdb.Symbol. If the block is not named, then this attribute holds None. This attribute is not writable.

For ordinary function blocks, the superblock is the static block. However, you should note that it is possible for a function block to have a superblock that is not the static block – for instance this happens for an inlined function.

Variable: Block.superblock

The block containing this block. If this parent block does not exist, this attribute holds None. This attribute is not writable.

Variable: Block.global_block

The global block associated with this block. This attribute is not writable.

Variable: Block.static_block

The static block associated with this block. This attribute is not writable.

Variable: Block.is_global

True if the gdb.Block object is a global block, False if not. This attribute is not writable.

Variable: Block.is_static

True if the gdb.Block object is a static block, False if not. This attribute is not writable.