pahole man page

pahole — Shows and manipulates data structure layout.


pahole [options] files


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.


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.
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.
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.


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).



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

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

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