GNU Complexity Command Line Tool Measures Complexity of C Code

GNU complexity is a command line tool that computes a complexity measure of C source code, similar to pmccabe, but with a different method of calculating results with short functions scoring lower than pmccabe and highly nested functionality can score considerably higher. It can be useful to locate suspicious areas in unfamiliar code, get an idea of the efforts required to either understand the code or test it, or self-assess your own code.

Bruce Korb, the maintainer, has just released version 1.5 with some bug fixes, so I’ve given it a quick try.

We’ll need to get the code, build and install it first:

wget ftp://ftp.gnu.org/gnu/complexity/complexity-1.5.tar.gz
tar xvf complexity-1.5.tar.gz
cd complexity-1.5
./configure
make -j8
sudo make install

wget ftp://ftp.gnu.org/gnu/complexity/complexity-1.5.tar.gz

tar xvf complexity-1.5.tar.gz

cd complexity-1.5

./configure

make -j8

sudo make install

The user’s manual provides some insights and an example, which I’ve used against a directory in Linux source code:

cd ~/edev/linux/arch/arm64/kernel
complexity --histogram --score --thresh=3 `ls *.c`
procedure fpsimd_pm_init in fpsimd.c ended before final close bracket
procedure fpsimd_hotplug_init in fpsimd.c ended before final close bracket
Complexity Scores
Score | ln-ct | nc-lns| file-name(line): proc-name
    3      28      17   perf_callchain.c(111): perf_callchain_user
    3      20      18   insn.c(337): aarch64_insn_decode_immediate
    3      21      19   io.c(27): __memcpy_fromio
    3      21      19   io.c(56): __memcpy_toio
    3      30      19   cpufeature.c(860): verify_local_cpu_capabilities
    3      21      20   module.c(74): reloc_data
    3      24      20   setup.c(199): request_standard_resources
    3      27      20   alternative.c(89): __apply_alternatives
    3      29      20   cpu_ops.c(53): cpu_read_enable_method
    3      29      24   insn.c(362): aarch64_insn_encode_immediate
    3      31      24   ptrace.c(76): ptrace_hbptriggered
    3      54      24   kgdb.c(159): kgdb_arch_handle_exception
    3      28      25   armv8_deprecated.c(145): update_insn_emulation_mode
    3      32      25   debug-monitors.c(334): aarch32_break_handler
    3      35      26   smp.c(413): acpi_map_gic_cpu_interface
    3      42      26   process.c(249): copy_thread
    3      58      27   perf_event.c(572): armv8pmu_handle_irq
    3      32      28   hw_breakpoint.c(544): toggle_bp_registers
    3      34      28   insn.c(1019): aarch64_insn_gen_data3
    3      34      29   insn.c(519): aarch64_insn_gen_comp_branch_imm
    3      56      29   setup.c(121): smp_build_mpidr_hash
    3      35      30   insn.c(397): aarch64_insn_encode_register
    3      42      31   signal.c(167): setup_sigframe
    3      37      33   hw_breakpoint.c(349): arch_bp_generic_fields
    3      49      34   setup.c(232): relocate_initrd
    3      41      35   insn.c(708): aarch64_insn_gen_add_sub_imm
    3      63      38   setup.c(292): setup_arch
    3     109      57   cpufeature.c(469): update_cpu_features
    4      46      26   smp.c(477): of_parse_and_init_cpus
    4      38      31   traps.c(335): call_undef_hook
    4      36      32   insn.c(924): aarch64_insn_gen_data1
    4      49      33   armv8_deprecated.c(464): cp15barrier_handler
    4      71      34   stacktrace.c(41): unwind_frame
    4      42      35   ptrace.c(437): hw_break_set
    4      42      37   insn.c(968): aarch64_insn_gen_data2
    4      49      37   smp.c(708): handle_IPI
    4      58      37   hw_breakpoint.c(231): hw_breakpoint_control
    4      47      40   insn.c(757): aarch64_insn_gen_bitfield
    4      54      44   ptrace.c(376): hw_break_get
    4      60      44   armv8_deprecated.c(378): swp_handler
    5      28      27   hw_breakpoint.c(198): hw_breakpoint_slot_setup
    5      45      35   ptrace.c(672): compat_gpr_get
    5      50      36   hw_breakpoint.c(764): reinstall_suspended_bps
    5      59      37   hw_breakpoint.c(476): arch_validate_hwbkpt_settings
    5      48      42   insn.c(811): aarch64_insn_gen_movewide
    5      49      42   insn.c(868): aarch64_insn_gen_add_sub_shifted_reg
    5      60      44   psci.c(36): cpu_psci_cpu_init_idle
    5      49      46   insn.c(277): aarch64_get_imm_shift_mask
    5      95      89   asm-offsets.c(34): main
    6      34      32   cpufeature.c(792): __raw_read_system_reg
    6      67      37   signal.c(332): do_signal
    6      43      39   topology.c(50): parse_core
    6      54      41   ptrace.c(724): compat_gpr_set
    6      74      44   traps.c(147): dump_backtrace
    6      55      49   insn.c(646): aarch64_insn_gen_load_store_pair
    6      76      49   hw_breakpoint.c(393): arch_build_bp_info
    6      71      52   hw_breakpoint.c(584): breakpoint_handler
    6      61      54   insn.c(1062): aarch64_insn_gen_logical_shifted_reg
    6      73      59   ptrace.c(1120): compat_arch_ptrace
    8      50      30   cpuinfo.c(102): c_show
    8      64      50   topology.c(97): parse_cluster
    8      97      65   hw_breakpoint.c(660): watchpoint_handler
    9      44      32   traps.c(59): dump_mem
   11      71      55   signal32.c(129): copy_siginfo_to_user32
   31     198     172   module.c(184): apply_relocate_add

Complexity Histogram
Score-Range  Lin-Ct
    0-9        2206 ************************************************************
   10-19         55 *
   20-29          0
   30-39        172 *****

Scored procedure ct:       65
Non-comment line ct:     2433
Average line score:         7
25%-ile score:              3 (75% in higher score procs)
50%-ile score:              5 (half in higher score procs)
75%-ile score:              6 (25% in higher score procs)
Highest score:             31 (apply_relocate_add() in module.c)

cd ~/edev/linux/arch/arm64/kernel

complexity --histogram --score --thresh=3 `ls *.c`

procedure fpsimd_pm_init in fpsimd.c ended before final close bracket

procedure fpsimd_hotplug_init in fpsimd.c ended before final close bracket

Complexity Scores

Score | ln-ct | nc-lns| file-name(line): proc-name

3 28 17 perf_callchain.c(111): perf_callchain_user

3 20 18 insn.c(337): aarch64_insn_decode_immediate

3 21 19 io.c(27): __memcpy_fromio

3 21 19 io.c(56): __memcpy_toio

3 30 19 cpufeature.c(860): verify_local_cpu_capabilities

3 21 20 module.c(74): reloc_data

3 24 20 setup.c(199): request_standard_resources

3 27 20 alternative.c(89): __apply_alternatives

3 29 20 cpu_ops.c(53): cpu_read_enable_method

3 29 24 insn.c(362): aarch64_insn_encode_immediate

3 31 24 ptrace.c(76): ptrace_hbptriggered

3 54 24 kgdb.c(159): kgdb_arch_handle_exception

3 28 25 armv8_deprecated.c(145): update_insn_emulation_mode

3 32 25 debug-monitors.c(334): aarch32_break_handler

3 35 26 smp.c(413): acpi_map_gic_cpu_interface

3 42 26 process.c(249): copy_thread

3 58 27 perf_event.c(572): armv8pmu_handle_irq

3 32 28 hw_breakpoint.c(544): toggle_bp_registers

3 34 28 insn.c(1019): aarch64_insn_gen_data3

3 34 29 insn.c(519): aarch64_insn_gen_comp_branch_imm

3 56 29 setup.c(121): smp_build_mpidr_hash

3 35 30 insn.c(397): aarch64_insn_encode_register

3 42 31 signal.c(167): setup_sigframe

3 37 33 hw_breakpoint.c(349): arch_bp_generic_fields

3 49 34 setup.c(232): relocate_initrd

3 41 35 insn.c(708): aarch64_insn_gen_add_sub_imm

3 63 38 setup.c(292): setup_arch

3 109 57 cpufeature.c(469): update_cpu_features

4 46 26 smp.c(477): of_parse_and_init_cpus

4 38 31 traps.c(335): call_undef_hook

4 36 32 insn.c(924): aarch64_insn_gen_data1

4 49 33 armv8_deprecated.c(464): cp15barrier_handler

4 71 34 stacktrace.c(41): unwind_frame

4 42 35 ptrace.c(437): hw_break_set

4 42 37 insn.c(968): aarch64_insn_gen_data2

4 49 37 smp.c(708): handle_IPI

4 58 37 hw_breakpoint.c(231): hw_breakpoint_control

4 47 40 insn.c(757): aarch64_insn_gen_bitfield

4 54 44 ptrace.c(376): hw_break_get

4 60 44 armv8_deprecated.c(378): swp_handler

5 28 27 hw_breakpoint.c(198): hw_breakpoint_slot_setup

5 45 35 ptrace.c(672): compat_gpr_get

5 50 36 hw_breakpoint.c(764): reinstall_suspended_bps

5 59 37 hw_breakpoint.c(476): arch_validate_hwbkpt_settings

5 48 42 insn.c(811): aarch64_insn_gen_movewide

5 49 42 insn.c(868): aarch64_insn_gen_add_sub_shifted_reg

5 60 44 psci.c(36): cpu_psci_cpu_init_idle

5 49 46 insn.c(277): aarch64_get_imm_shift_mask

5 95 89 asm-offsets.c(34): main

6 34 32 cpufeature.c(792): __raw_read_system_reg

6 67 37 signal.c(332): do_signal

6 43 39 topology.c(50): parse_core

6 54 41 ptrace.c(724): compat_gpr_set

6 74 44 traps.c(147): dump_backtrace

6 55 49 insn.c(646): aarch64_insn_gen_load_store_pair

6 76 49 hw_breakpoint.c(393): arch_build_bp_info

6 71 52 hw_breakpoint.c(584): breakpoint_handler

6 61 54 insn.c(1062): aarch64_insn_gen_logical_shifted_reg

6 73 59 ptrace.c(1120): compat_arch_ptrace

8 50 30 cpuinfo.c(102): c_show

8 64 50 topology.c(97): parse_cluster

8 97 65 hw_breakpoint.c(660): watchpoint_handler

9 44 32 traps.c(59): dump_mem

11 71 55 signal32.c(129): copy_siginfo_to_user32

31 198 172 module.c(184): apply_relocate_add

Complexity Histogram

Score-Range Lin-Ct

0-9 2206 ************************************************************

10-19 55 *

20-29 0

30-39 172 *****

Scored procedure ct: 65

Non-comment line ct: 2433

Average line score: 7

25%-ile score: 3 (75% in higher score procs)

50%-ile score: 5 (half in higher score procs)

75%-ile score: 6 (25% in higher score procs)

Highest score: 31 (apply_relocate_add() in module.c)

The resulting table shows six information per line: the computed score, the number of lines between the opening and closing curly braces (ln-ct), the number of non-comment, non-blank lines found there (nc-lns), the name of the source file, the line number of the opening curly brace, and the name of the procedure. The higher the score the more complex is the procedure, and thres parameter is used to remove the lowest complexity function from the list. The ranking goes as follows:

‘0-9’ – Easily maintained code.
‘10-19’ – Maintained with little trouble.
‘20-29’ – Maintained with some effort.
‘30-39’ – Difficult to maintain code.
‘40-49’ – Hard to maintain code.
‘50-99’ – Unmaintainable code.
‘100-199’ – Crazy making difficult code.
‘200+’ – I only wish I were kidding.

Have fun.

Jean-Luc Aufranc (CNXSoft)

Jean-Luc started CNX Software in 2010 as a part-time endeavor, before quitting his job as a software engineering manager, and starting to write daily news, and reviews full time later in 2011.