pahole man page

pahole — Shows and manipulates data structure layout.

Synopsis

pahole [options] files

Description

pahole shows data structure layouts encoded in debugging information formats, DWARF and CTF being supported.

This is useful for, among other things: optimizing important data structures by reducing its size, figuring out what is the field sitting at an offset from the start of a data structure, investigating ABI changes and more generally understanding a new codebase you have to work with.

The files must have associated debugging information.  This information may be inside the file itself, in ELF sections, or in another file.

One way to have this information is to specify the -g option to the compiler when building it. When this is done the information will be stored in an ELF section. For the DWARF debugging information format this, adds, among others, the .debug_info ELF section. For CTF it is found in just one ELF section, .SUNW_ctf.

The debuginfo packages available in most Linux distributions are also supported by pahole, where the debugging information is available in a separate file.

By default, pahole shows the layout of all named structs in the files specified.

Options

pahole supports the following options.

-C, --class_name=CLASS_NAMES

Show just these classes. This can be a comma separated list of class names or file URLs (e.g.: file://class_list.txt)

-c, --cacheline_size=SIZE

Set cacheline size to SIZE bytes.

-E, --expand_types

Expand class members. Useful to find in what member of inner structs where an offset from the beginning of a struct is.

-F, --format_path

Allows specifying a list of debugging formats to try, in order. Right now this includes "ctf" and "dwarf". The default format path used is equivalent to "-F dwarf,ctf".

-r, --rel_offset

Show relative offsets of members in inner structs.

-p, --expand_pointers

Expand class pointer members.

-R, --reorganize

Reorganize struct, demoting and combining bitfields, moving members to remove alignment holes and padding.

-S, --show_reorg_steps

Show the struct layout at each reorganization step.

-i, --contains=CLASS_NAME

Show classes that contains CLASS_NAME.

-a, --anon_include

Include anonymous classes.

-A, --nested_anon_include

Include nested (inside other structs) anonymous classes.

-B, --bit_holes=NR_HOLES

Show only structs at least NR_HOLES bit holes.

-d, --recursive

Recursive mode, affects several other flags.

-D, --decl_exclude=PREFIX

exclude classes declared in files with PREFIX.

-f, --find_pointers_to=CLASS_NAME

Find pointers to CLASS_NAME.

-H, --holes=NR_HOLES

Show only structs with at least NR_HOLES holes.

-I, --show_decl_info

Show the file and line number where the tags were defined, if available in the debugging information.

-l, --show_first_biggest_size_base_type_member

Show first biggest size base_type member.

-m, --nr_methods

Show number of methods.

-M, --show_only_data_members

Show only the members that use space in the class layout. C++ methods will be suppressed.

-n, --nr_members

Show number of members.

-N, --class_name_len

Show size of classes.

-O, --dwarf_offset=OFFSET

Show tag with DWARF OFFSET.

-P, --packable

Show only structs that has holes that can be packed if members are reorganized, for instance when using the --reorganize option.

-q, --quiet

Be quieter.

-s, --sizes

Show size of classes.

-t, --separator=SEP

Use SEP as the field separator.

-T, --nr_definitions

Show how many times struct was defined.

-u, --defined_in

Show CUs where CLASS_NAME (-C) is defined.

--flat_arrays

Flatten arrays, so that array[10][2] becomes array[20]. Useful when generating from both CTF and DWARF encodings for the same binary for testing purposes.

--fixup_silly_bitfields

Converts silly bitfields such as "int foo:32" to plain "int foo".

-V, --verbose

be verbose

-w, --word_size=WORD_SIZE

Change the arch word size to WORD_SIZE.

-x, --exclude=PREFIX

Exclude PREFIXed classes.

-X, --cu_exclude=PREFIX

Exclude PREFIXed compilation units.

-y, --prefix_filter=PREFIX

Include PREFIXed classes.

-z, --hole_size_ge=HOLE_SIZE

Show only structs with at least one hole greater or equal to HOLE_SIZE.

Notes

To enable the generation of debugging information in the Linux kernel build process select CONFIG_DEBUG_INFO. This can be done using make menuconfig by this path: "Kernel Hacking" -> "Kernel Debugging" -> "Compile the kernel with debug info".

Many distributions also come with debuginfo packages, so just enable it in your package manager repository configuration and install the kernel-debuginfo, or any other userspace program written in a language that the compiler generates debuginfo (C, C++, for instance).

See Also

eu-readelf(1), readelf(1), objdump(1).

http://oops.ghostprotocols.net:81/acme/7dwarves.pdf.

Author

pahole was written by Arnaldo Carvalho de Melo <acme@ghostprotocols.net>.

Please send bug reports to <dwarves@vger.kernel.org>.

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Info

February 13, 2009 dwarves