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Measure complexity of C source

This file documents Complexity version 0.4. It is a tool designed for analyzing the complexity of “C” program functions. It is very similar to the McCabe scoring, but addresses several issues not considered in that scoring scheme.

This edition documents version 0.4, updated May 2011

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1. Introduction

Complexity measurement tools provide several pieces of information. They help to:

  1. locate suspicious areas in unfamiliar code
  2. get an idea of how much effort may be required to understand that code
  3. get an idea of the effort required to test a code base
  4. provide a reminder to yourself. You may see what you’ve written as obvious, but others may not. It is useful to have a hint about what code may seem harder to understand by others, and then decide if some rework may be in order.

But why another complexity analyzer? Even though the McCabe analysis tool already exists (pmccabe), I think the job it does is too rough for gauging complexity, though it is ideal for gauging the testing effort. Each code path should be tested and the pmccabe program provides a count of code paths. That, however, is not the only issue affecting human comprehension. This program attempts to take into account other factors that affect a human’s ability to understand.

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1.1 Code Length

Since pmccabe does not factor code length into its score, some folks have taken to saying either long functions or a high McCabe score find functions requiring attention. But it means looking at two factors without any visibility into how the length is obfuscating the code.

The technique used by this program is to count 1 for each line that a statement spans, plus the complexity score of control expressions (for, while, and if expressions). The value for a block of code is the sum of these multiplied by a nesting factor (see section nesting-penalty option (-n)). This score is then added to the score of the encompassing block. With all other things equal, a procedure that is twice as long as another will have double the score. pmccabe scores them identically.

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1.2 Switch Statement

pmccabe has changed the scoring of switch statements because they seemed too high. switch statements are now “free” in this new analysis. That’s wrong, too. The code length needs to be counted and the code within a switch statement adds more to the difficulty of comprehension than code at a shallower logic level.

This program will multiply the score of the switch statement content by the See section nesting score factor.

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1.3 Logic Conditions

pmccabe’ does not score logic conditions very well. It overcharges for simple logical operations, it doesn’t charge for comma operators, and it undercharges for mixing assignment operators and relational operators and the and and or logical operators.

For example:

xx = (A && B) || (C && D) || (E && F);

scores as 6. Strictly speaking, there are, indeed, six code paths there. That is a fairly straight forward expression that is not nearly as complicated as this:

  if (A) {
    if (B) {
      if (C) {
        if (D)
      } else if (E) {

and yet this scores exactly the same. This program reduces the cost to very little for a sequence of conditions at the same level. (That is, all and operators or all or operators.) so the raw score for these examples are 4 and 35, respectively (1 and 2 after scaling, see section --scale).

If you nest boolean expressions, there is a little cost, assuming you parenthesize grouped expressions so that and and or operators do not appear at the same parenthesized level. Also assuming that you do not mix assignment and relational and boolean operators all together. If you do not parenthesize these into subexpressions, their small scores get multiplied in ways that sometimes wind up as a much higher score.

The intent here is to encourage easy to understand boolean expressions. This is done by,

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1.4 Personal Experience

I have used pmccabe on a number of occasions. For a first order approximation, it does okay. However, I was interested in zeroing in on the modules that needed the most help and there were a lot of modules needing help. I was finding I was looking at some functions where I ought to have been looking at others. So, I put this together to see if there was a better correlation between what seemed like hard code to me and the score derived by an analysis tool.

This has worked much better. I ran complexity and pmccabe against several million lines of code. I correlated the scores. Where the two tools disagreed noticeably in relative ranking, I took a closer look. I found that ‘complexity’ did, indeed, seem to be more appropriate in its scoring.

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1.5 Rationale Summary

Ultimately, complexity is in the eye of the beholder and, even, the particular mood of the beholder, too. It is difficult to tune a tool to properly accommodate these variables.

complexity will readily score as zero functions that are extremely simple, and code that is long with many levels of logic nesting will wind up scoring much higher than with pmccabe, barring extreme changes to the default values for the tunables.

I have included several adjustments so that scores can be tweaked to suit personal taste or gathered experience. (See section nesting score factor, and nested expression scoring factor, but also See section normalization scaling factor, to adjust scores to approximate scores rendered by pmccabe).

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2. Complexity Computation

The principal goal Fundamentally, this program counts lines of non-comment source lines, multiplies by a “nesting factor” for each level of logic nesting and divides by a scaling factor so that the typical results lie roughly in the same range as pmccabe results. That happens to be approximately 20.

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2.1 Parsing Method

The method chosen for parsing the source has an effect on what gets seen (scored) by the program.

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2.1.1 Complexity Measurement Parsing

This program examines the actual source a human looks at when the file is opened, provided it is not pre-processed by unifdef, See section unifdef. This was chosen because uncompiled code adds to the complexity of what a human must understand. However, sometimes the source will contain unbalanced braces a la:

#if FOO
  for (int ix = foo;;) {
  for (int ix = bar;;) {

rendering code that cannot be parsed correctly. unifdef-ing makes it parsable. Unfortunately, because the practice of ifdef-ing unbalanced curly braces is so common, this program cannot rely on finding the correct closing brace.

CAVEAT: for the purposes of this program, procedures end when either a matching closing brace is found or a closing curly brace is found in column 1, whichever comes first. If the closing brace in column one does not match the procedure opening brace, the procedure is considered unscorable.

Fortunately, unscorable procedures are relatively unusual.

CAVEAT2: K&R procedure headers are not recognized. If anything other than an opening curly brace appears after the parameter list will cause the code recognizer to go back into “look for a procedure header” mode. K&R procedures are not just not scored, they are completely ignored.

This should probably get fixed, though.

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2.1.2 Post-PreProcessing Parsing

Another approach would be to use the C compiler and analize the tokens coming out of the preprocessor. The drawbacks are that macro expansions will add to the complexity, even though they do not add to human perceived complexity, and uncompiled code do not add to the complexity measure. The benefit, of course, is that you know for certain where a procedure body starts and ends.

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2.1.3 During PreProcessing Parsing

This would require going into the C preprocessor code and cause macros to not be expanded. Again, the great benefit is that you know for certain you can find the starting and ending braces for every procedure body. The downsides are the extra work and, again, the uncompiled code won’t get counted in the complexity measure.

This might be a useful exercise to do some day, just to see how helpful it might be. Being able to recognize all procedure bodies without fail would be a good thing.

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2.1.4 pmccabe Parsing

The pmccabe parsing actually inspired the method for this program. Thd difference is that pmccabe will always keep scanning until a procedure body’s closing curly brace is found, even if that means counting the code from several following procedure definitions. The consequence of this is that this program’s code will see some procedures that pmccabe will not, and vice versa.

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2.2 Complexity Measurement Algorithm

Fundamentally, this program counts non-comment source lines and examines elements of parenthesized expressions. This score is multiplied by a nesting scoring factor for each layer of code nesting.

A parenthesized expression is scanned for operators. If they are all arithmetic operators, or all arithmetic and one relational operator, the score is zero. If all the operators are boolean ands or they are all ors, then the score is one. An assignment operator with arithmetic operators also scores one. If you mix relational operators and all ands or all ors, the score is the number of boolean elements. If you mix ands and ors at the same parenthetical level, the two counts are multiplied, unless the boolean element count is higher.

Fundamentally, do not use multiple relational or boolean operators at the same parenthetical level, unless they are all boolean ands or they are all boolean ors. If you use boolean operators and relational operators in one expression, you are charged one statement for each boolean element.

After scoring each statement and any parenthesized expressions, the score is multiplied by any encompassing controlled block and added to the score of that block. A “controlled block” is a curly-braced collection of statements controlled by one of the statement controlling statements do, for, else, if, switch, or while. Stand alone blocks for scoping local variables do not trigger the multiplier.

You may trace the scores of parenthesized expressions and code blocks (see section trace output file). You will see the raw score of the code block or expression.

The final score is the outermost score divided by the “scaling factor”, See section complexity scaling factor.

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2.3 Complexity Scores

The “Complexity Scores” table shows the score of each procedure identified that also exceeded the threshold score, See section —threshold. The entries on each line are:

The output is sorted by the score and then the number of non-comment lines. Procedures with scores below the threshold are not displayed.

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2.4 Complexity Statistics

The statistics are displayed both as a table and as a histogram, See section Example Output. It is under the control of the —histogram option. The statistics are for each non-comment source line and each source line is given the score of its encompassing procedure. This way, larger procedures are given proportionally more weight than one line procedures.

The histogram is broken up into three ranges. Scores of 0 through 99 are displayed in 10 point groupings, 100 through 999 in 100 point groupings and 1000 and above (good grief!!, but they exist) are in 1000 point groupings. The number of asterisks represent the number of lines of code that are in procedures that score in the specified range.

The tabular statistics are also based on lines, not procedures.

Average line score

This is the procedure score times the non-comment line count, all added up and divided by the total non-comment source lines found.

25%-ile score
50%-ile score
75%-ile score
Highest score

Since the distribution of scores is nothing like a bell curve, the mean and standard deviation do not give a very clear picture of the distribution of the scores. Typically, the standard deviation is larger than the average score. So, instead the program prints the the four quartile scores. The score for which 25, 50, and 75 percent of code is scored less than, plus the highest scoring procedure (100 percent of code scores less than or equal to that score).

Unscored procedures

If any procedures were found that could not be scored, the number of such procedures is printed.

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2.5 Scoring Adjustments

Scores can be adjusted with three different options:


See section —nesting-penalty.


See section —demi-nesting-penalty.


See section —scale.

The raw score is the number of lines or statements, whichever is greater, adjusted by a factor for the depth of the logic. Statements are nested when they are inside of a block of statements for a “block” statement (viz., “do”, “for”, “if”, “switch” or “while”). Statements within blocks used to constrain the scope of variables (not controlled by a block statement) are not multiplied by this factor.

Expressions are nested when contained within parentheses. The cost of these is different. Block level nesting multiplies the score for the block by the --nesting-penalty factor (2.0 by default). Nested expressions are multiplied by the --demi-nesting-penalty, the square root of --nesting-penalty by default.

Some attempt is made to judge the complexity of an expression. A complicated expression is one that contains an assignment operator, more than one relation operator, or a mixture of “and” and “or” operators with any other different kind of non-arithmetic operator. Expression scores are minimized by:

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3. Example Output

This is a self-referential example. This output was obtained by going into the complexity source directory and running the command:

complexity --histogram --score --thresh=3 '*.c'

The --threshold is set to three because all of the functions score below the default threshold of 30. It is not zero because there are too many trivial (0, 1 or 2) functions for a short example.

This results in:

Complexity Scores
Score | ln-ct | nc-lns| file-name(line): proc-name
    3      13      12   tokenize.c(507): skip_params
    3      15      13   score.c(121): handle_stmt_block
    3      22      17   tokenize.c(64): check_quote
    3      34      27   score.c(736): score_proc
    3      37      27   complexity.c(72): initialize
    3      43      35   score.c(513): handle_TKN_KW_DO
    4      15      13   tokenize.c(28): skip_comment
    4      20      16   tokenize.c(405): next_nonblank
    4      22      18   tokenize.c(528): skip_to_semi
    4      35      28   tokenize.c(335): keyword_check
    4      52      40   complexity.c(360): load_file
    5      33      28   score.c(466): handle_TKN_KW_CASE
    5      35      30   score.c(315): handle_parms
    5      58      41   complexity.c(297): popen_unifdef
    5      59      48   score.c(160): fiddle_subexpr_score
    5      78      58   score.c(561): handle_TKN_KW_IF
    6      45      30   opts.c(776): translate_option_strings
    6      67      44   opts.c(670): main
    7      49      40   score.c(644): handle_TKN_KW_FOR
   10      84      65   complexity.c(441): complex_eval
   11      57      50   tokenize.c(555): find_proc_start
   11      88      65   complexity.c(123): print_histogram
   12      81      60   score.c(224): handle_subexpr
   12      92      68   score.c(355): handle_expression
   15      64      51   tokenize.c(103): hash_check
   25      72      60   tokenize.c(430): next_token

Complexity Histogram
Score-Range  Lin-Ct
    0-9         565 **********************************************
   10-19        359 **************************************
   20-29         60 ******

Scored procedure ct:       26
Non-comment line ct:      984
Average line score:         8
25%-ile score:              4 (75% in higher score procs)
50%-ile score:              6 (half in higher score procs)
75%-ile score:             11 (25% in higher score procs)
Highest score:             25 (next_token() in tokenize.c)

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4. Invoking complexity

Compute the complexity of source code not just with a path-through-the-code count, but also amplifying line counts by logic level nesting. complexity ignores all cpp preprocessor directives - calculating the complexity of the appearance of the code, rather than the complexity after the preprocessor manipulates the code. getchar(3), for example, will expand into quite complicated code.

This chapter was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the complexity program.

This software is released under the GNU General Public License.

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4.1 complexity usage help (-?)

This is the automatically generated usage text for complexity:

complexity (GNU Complexity) - Measure complexity of C source - Ver. 0.4
USAGE:  complexity [ -<flag> [<val>] | --<name>[{=| }<val>] ]... \
                [ <file-name> ... ]

   -t, --threshold=num        Reporting threshold
       --horrid-threshold=num zero exit threshold
   -n, --nesting-penalty=str  score multiplier for nested code
       --demi-nesting-penalty=str score multiplier for nested expressions
   -s, --scale=num            complexity scaling factor
   -h, --histogram            Display histogram of complexity numbers
                                - disabled as --no-histogram
                                - may not be preset
   -c, --scores               Display the score for each procedure
                                - disabled as --no-scores
                                - may not be preset
   -I, --ignore=str           procedure name to be ignored
                                - may appear multiple times
   -H, --no-header            do not print scoring header
                                - may not be preset
   -u, --unifdef=str          Run the source(s) through unifdef(1BSD)
                                - may appear multiple times
       --unif-exe=str         Specify the unifdef program
   -i, --input=str            file of file list
       --trace=str            trace output file
   -v, --version[=arg]        Output version information and exit
   -?, --help                 Display extended usage information and exit
   ->, --save-opts[=arg]      Save the option state to a config file
   -<, --load-opts=str        Load options from a config file
                                - disabled as --no-load-opts
                                - may appear multiple times

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Compute the complexity of source code not just with a
path-through-the-code count, but also amplifying line counts by logic
level nesting.
If no arguments are provided, input arguments are read from stdin,
one per line; blank and '#'-prefixed lines are comments.
'stdin' may not be a terminal (tty).

The following option preset mechanisms are supported:
 - reading file $@/complex.conf
 - reading file $HOME/.complexityrc
 - reading file $PROJECT_ROOT/complex.conf
 - reading file ./.complexityrc
 - examining environment variables named COMPLEXITY_*

``complexity'' ignores all cpp preprocessor directives - calculating
the complexity of the appearance of the code, rather than the complexity
after the preprocessor manipulates the code.  ``getchar(3)'', for example,
will expand into quite complicated code.

please send bug reports to:  bkorb@gnu.org

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4.2 demi-nesting-penalty option

This is the “score multiplier for nested expressions” option. By default, this value is halfway between 1.0 and the nesting penalty (specifically, the square root of the nesting penalty). It refers to a parenthesized sub-expression. e.g.

((a > b) && (c > d))

contains two parenthesized sub-expressions. This would count 3.5 points. On the other hand, this:

(a > b && c > d)

contains two relation operators and a logical operator at the same level. These nested counts will be multiplied together and yield 2.5 * 2.5, or 6.25. Don’t do that. It gets even worse if you have logical ands and ors at the same level.

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4.3 histogram option (-h)

This is the “display histogram of complexity numbers” option.

This option has some usage constraints. It:

Instead of printing out each function’s score, a summary is printed at the end showing how many functions had particular ranges of scores. Unless --scores is specifically called out, the scores will not print with this option specified. The minimum scoring threshold will also be reduced to zero (0), unless --threshold is specified.

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4.4 horrid-threshold option

This is the “zero exit threshold” option. If any procedures score higher than this threshold, then the program will exit non-zero. (4/COMPLEX_EXIT_HORRID_FUNCTION, if no other problems are encountered.) By default, this program exits zero unless one function exceeds the horrid score of 100.

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4.5 ignore option (-I)

This is the “procedure name to be ignored” option.

This option has some usage constraints. It:

Some code has macros defined that confuse the lexical analysis. This will cause them to be ignored. Other ways to cause functions to be ignored are:

  1. Use K&R syntax for a procedure header.
  2. Use a preprocessing macro to assemble the procedure header.
  3. Simplify your code.

Generally speaking, anything you do that alters normal C syntax will confuse the lexical analysis. If a procedure is not seen, then it will not get counted. If code within a procedure is incomprehensible, you will likely get inappropriate results.

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4.6 input option (-i)

This is the “file of file list” option. Instead of either a command line list of input files or reading them from standard input, read the list of files from this file.

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4.7 nesting-penalty option (-n)

This is the “score multiplier for nested code” option. Linguistic constructs weigh more heavily the more deeply nested they are. By default, each layer penalizes by a factor of 2.0. The option argument is a floating point number. The penalty may be 1, but not less.

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4.8 no-header option (-H)

This is the “do not print scoring header” option.

This option has some usage constraints. It:

If a script is going to process the scoring output, parsing is easier without a header. The histogram output will always have a header.

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4.9 scale option (-s)

This is the “complexity scaling factor” option. By default, the scaling is 20 which divides the raw score by 20. This was normalized to roughly correspond to the pmccabe scores:


Easily maintained code.


Maintained with little trouble.


Maintained with some effort.


Difficult to maintain code.


Hard to maintain code.


Unmaintainable code.


Crazy making difficult code.


I only wish I were kidding.

Score | ln-ct | nc-lns| file-name(line): proc-name
 4707    3815    2838   lib/vasnprintf.c(1747): VASNPRINTF

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4.10 scores option (-c)

This is the “display the score for each procedure” option.

This option has some usage constraints. It:

If you specify --histogram, individual scores will not be displayed, unless this option is specified.

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4.11 threshold option (-t)

This is the “reporting threshold” option. Ignore any procedures with a complexity measure below this threshold. By default, a complexity score of under 30 is not printed. However, if a histogram and statistics are to be printed, but not individual procedure scores, then the default is set to zero. Procedures below this limit are not counted in the statistics.

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4.12 trace option

This is the “trace output file” option. Print intermediate scores to a trace file.

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4.13 unif-exe option

This is the “specify the unifdef program” option. Alternate program to use for unifdef-ing the input.

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4.14 unifdef option (-u)

This is the “run the source(s) through unifdef(1bsd)” option.

This option has some usage constraints. It:

Strip out sections of code surrounded by #if/#endif directives. The option argument is passed as an argument to the ‘unifdef(1BSD)’ program. For example:

complexity -u-Dsymbol

would cause symbol to be defined and remove sections of code preceded by #ifndef symbol directives.

Please see the ‘unifdef’ documentation for more information.

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4.15 presetting/configuring complexity

Any option that is not marked as not presettable may be preset by loading values from configuration ("rc" or "ini") files, and values from environment variables named COMPLEXITY and COMPLEXITY_<OPTION_NAME>. “<OPTION_NAME>” must be one of the options listed above in upper case and segmented with underscores. The COMPLEXITY variable will be tokenized and parsed like the command line. The remaining variables are tested for existence and their values are treated like option arguments.

libopts will search in 4 places for configuration files:

The value for $(pkgdatadir) is recorded at package configure time and replaced by ‘libopts’ when ‘complexity’ runs. The environment variables HOME, PROJECT_ROOT, and PWD are expanded and replaced when ‘complexity’ runs. For any of these that are plain files, they are simply processed. For any that are directories, then a file named ‘.complexityrc’ is searched for within that directory and processed.

Configuration files may be in a wide variety of formats. The basic format is an option name followed by a value (argument) on the same line. Values may be separated from the option name with a colon, equal sign or simply white space. Values may be continued across multiple lines by escaping the newline with a backslash.

Multiple programs may also share the same initialization file. Common options are collected at the top, followed by program specific segments. The segments are separated by lines like:


or by

<?program complexity>

Do not mix these within one configuration file.

Compound values and carefully constructed string values may also be specified using XML syntax:


yielding an option-name.sub-opt string value of


AutoOpts does not track suboptions. You simply note that it is a hierarchicly valued option. AutoOpts does provide a means for searching the associated name/value pair list (see: optionFindValue).

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4.16 complexity exit codes

One of the following exit values will be returned:


Successful program execution.


The operation failed or the command syntax was not valid.


insufficient memory to run program


One or more functions scored over 100


No qualifying procedures were found.


one or more input files were unreadable or empty.

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4.17 complexity Description

The weight of each statement is the number of lines the statement uses. This value is multiplied by the nested logic weighting (2.0 by default) for each layer of logic. For example, this snippet:

if (foo) {
  if (bar) {
    bumble; baz;

will score 11. This score is then scaled to approximate pmccabe results by dividing by 20 and rounding. This scores "1" at the end. pmccabe scores higher on simple procedures and complexity scores higher with more deeply nested logic.

The scoring can be tweaked by adjusting the --nesting-penalty and --scale-ing factors. The default values were calibrated by comparing the average results of millions of lines of code with the results of pmccabe.

For the purposes of this program, a procedure is identified by a name followed by a parenthesized expression and then an opening curly brace. It ends with a closing curly brace in column 1.

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4.18 complexity Bugs

This program does not recognize K&R procedure headers.

Some procedures still get missed. Usually, these are procedures that use the C pre-processor to extend the C language in some way.

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A. Copying This Manual

You may copy this manual under the terms of the FDL (the GNU Free Documentation License).

Version 1.3, 3 November 2008

Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

    The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

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ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

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If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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Concept Index

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Index Entry Section

complexity4. Invoking complexity
Complexity Computation2. Complexity Computation
complexity usage4.1 complexity usage help (-?)
complexity-demi-nesting-penalty4.2 demi-nesting-penalty option
complexity-histogram4.3 histogram option (-h)
complexity-horrid-threshold4.4 horrid-threshold option
complexity-ignore4.5 ignore option (-I)
complexity-input4.6 input option (-i)
complexity-nesting-penalty4.7 nesting-penalty option (-n)
complexity-no-header4.8 no-header option (-H)
complexity-scale4.9 scale option (-s)
complexity-scores4.10 scores option (-c)
complexity-threshold4.11 threshold option (-t)
complexity-trace4.12 trace option
complexity-unif-exe4.13 unif-exe option
complexity-unifdef4.14 unifdef option (-u)

Example Output3. Example Output

FDL, GNU Free Documentation LicenseA. Copying This Manual

Introduction1. Introduction

Measure complexity of C source4. Invoking complexity

parsing2.1 Parsing Method

scores2.3 Complexity Scores
scores2.5 Scoring Adjustments
statistics2.4 Complexity Statistics

tuning2.5 Scoring Adjustments

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Table of Contents

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About This Document

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