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18.5 Index Files Speed Up GDB

When GDB finds a symbol file, it scans the symbols in the file in order to construct an internal symbol table. This lets most GDB operations work quickly—at the cost of a delay early on. For large programs, this delay can be quite lengthy, so GDB provides a way to build an index, which speeds up startup.

For convenience, GDB comes with a program, gdb-add-index, which can be used to add the index to a symbol file. It takes the symbol file as its only argument:

$ gdb-add-index symfile

See gdb-add-index.

It is also possible to do the work manually. Here is what gdb-add-index does behind the curtains.

The index is stored as a section in the symbol file. GDB can write the index to a file, then you can put it into the symbol file using objcopy.

To create an index file, use the save gdb-index command:

save gdb-index [-dwarf-5] directory

Create index files for all symbol files currently known by GDB. For each known symbol-file, this command by default creates it produces a single file symbol-file.gdb-index. If you invoke this command with the -dwarf-5 option, it produces 2 files: symbol-file.debug_names and symbol-file.debug_str. The files are created in the given directory.

Once you have created an index file you can merge it into your symbol file, here named symfile, using objcopy:

$ objcopy --add-section .gdb_index=symfile.gdb-index \
    --set-section-flags .gdb_index=readonly symfile symfile

Or for -dwarf-5:

$ objcopy --dump-section .debug_str=symfile.debug_str.new symfile
$ cat symfile.debug_str >>symfile.debug_str.new
$ objcopy --add-section .debug_names=symfile.gdb-index \
    --set-section-flags .debug_names=readonly \
    --update-section .debug_str=symfile.debug_str.new symfile symfile

GDB will normally ignore older versions of .gdb_index sections that have been deprecated. Usually they are deprecated because they are missing a new feature or have performance issues. To tell GDB to use a deprecated index section anyway specify set use-deprecated-index-sections on. The default is off. This can speed up startup, but may result in some functionality being lost. See Index Section Format.

Warning: Setting use-deprecated-index-sections to on must be done before gdb reads the file. The following will not work:

$ gdb -ex "set use-deprecated-index-sections on" <program>

Instead you must do, for example,

$ gdb -iex "set use-deprecated-index-sections on" <program>

There are currently some limitation on indices. They only work when using DWARF debugging information, not stabs. And, only the -dwarf-5 index works for programs using Ada.

18.5.1 Automatic symbol index cache

It is possible for GDB to automatically save a copy of this index in a cache on disk and retrieve it from there when loading the same binary in the future. This feature can be turned on with set index-cache on. The following commands can be used to tweak the behavior of the index cache.

set index-cache on
set index-cache off

Enable or disable the use of the symbol index cache.

set index-cache directory directory
show index-cache directory

Set/show the directory where index files will be saved.

The default value for this directory depends on the host platform. On most systems, the index is cached in the gdb subdirectory of the directory pointed to by the XDG_CACHE_HOME environment variable, if it is defined, else in the .cache/gdb subdirectory of your home directory. However, on some systems, the default may differ according to local convention.

There is no limit on the disk space used by index cache. It is perfectly safe to delete the content of that directory to free up disk space.

show index-cache stats

Print the number of cache hits and misses since the launch of GDB.


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