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The `indent` Program

The indent program changes the appearance of a C program by inserting or deleting whitespace. It can be used to make code easier to read. It can also convert from one style of writing C to another.

indent understands a substantial amount about the syntax of C, but it also attempts to cope with incomplete and misformed syntax.

In version 1.2 and more recent versions, the GNU style of indenting is the default.

Invoking `indent`

As of version 1.3, the format of the indent command is:

indent [options] [input-files]

indent [options] [single-input-file] [-o output-file]

This format is different from earlier versions and other versions of indent.

In the first form, one or more input files are specified. indent makes a backup copy of each file, and the original file is replaced with its indented version. See section Backup Files, for an explanation of how backups are made.

In the second form, only one input file is specified. In this case, or when the standard input is used, you may specify an output file after the `-o' option.

To cause indent to write to standard output, use the `-st' option. This is only allowed when there is only one input file, or when the standard input is used.

If no input files are named, the standard input is read for input. Also, if a filename named `-' is specified, then the standard input is read.

As an example, each of the following commands will input the program `slithy_toves.c' and write its indented text to `slithy_toves.out':

indent slithy_toves.c -o slithy_toves.out

indent -st slithy_toves.c > slithy_toves.out

cat slithy_toves.c | indent -o slithy_toves.out

Most other options to indent control how programs are formatted. As of version 1.2, indent also recognizes a long name for each option name. Long options are prefixed by either `--' or `+'.(1) In most of this document, the traditional, short names are used for the sake of brevity. See section Option Summary, for a list of options, including both long and short names.

Here is another example:

indent -br test/metabolism.c -l85

This will indent the program `test/metabolism.c' using the `-br' and `-l85' options, write the output back to `test/metabolism.c', and write the original contents of `test/metabolism.c' to a backup file in the directory `test'.

Equivalent invocations using long option names for this example would be:

indent --braces-on-if-line --line-length185 test/metabolism.c

indent +braces-on-if-line +line-length185 test/metabolism.c

If you find that you often use indent with the same options, you may put those options into a file called `.indent.pro'. indent will first look for `.indent.pro' in the current directory and use that if found. Otherwise, indent will search your home directory for `.indent.pro' and use that file if it is found. This behaviour is different from that of other versions of indent, which load both files if they both exist.

Command line switches are handled after processing `.indent.pro'. Options specified later override arguments specified earlier, with one exception: Explicitly specified options always override background options (see section Common styles). You can prevent indent from reading an `.indent.pro' file by specifying the `-npro' option.

Backup Files

As of version 1.3, GNU indent makes GNU--style backup files, the same way GNU Emacs does. This means that either simple or numbered backup filenames may be made.

Simple backup file names are generated by appending a suffix to the original file name. The default for the this suffix is the one-character string `~' (tilde). Thus, the backup file for `python.c' would be `python.c~'.

Instead of the default, you may specify any string as a suffix by setting the environment variable SIMPLE_BACKUP_SUFFIX to your preferred suffix.

Numbered backup versions of a file `momewraths' look like `momewraths.c.~23~', where 23 is the version of this particular backup. When making a numbered backup of the file `src/momewrath.c', the backup file will be named `src/momewrath.c.~V~', where V is one greater than the highest version currently existing in the directory `src'.

The type of backup file made is controlled by the value of the environment variable VERSION_CONTROL. If it is the string `simple', then only simple backups will be made. If its value is the string `numbered', then numbered backups will be made. If its value is `numbered-existing', then numbered backups will be made if there already exist numbered backups for the file being indented; otherwise, a simple backup is made. If VERSION_CONTROL is not set, then indent assumes the behaviour of `numbered-existing'.

Other versions of indent use the suffix `.BAK' in naming backup files. This behaviour can be emulated by setting SIMPLE_BACKUP_SUFFIX to `.BAK'.

Note also that other versions of indent make backups in the current directory, rather than in the directory of the source file as GNU indent now does.

Common styles

There are several common styles of C code, including the GNU style, the Kernighan & Ritchie style, and the original Berkeley style. A style may be selected with a single background option, which specifies a set of values for all other options. However, explicitly specified options always override options implied by a background option.

As of version 1.2, the default style of GNU indent is the GNU style. Thus, it is no longer neccessary to specify the option `-gnu' to obtain this format, although doing so will not cause an error. Option settings which correspond to the GNU style are:

-nbad -bap -nbbb -nbc -bl -bli2 -c33 -cd33 -ncdb -nce
-cli0 -cp1 -di2 -nfc1 -nfca -i2 -ip5 -lp -pcs -psl -cs
-nsc -nsob -nss -ts8 -d0 -ci0 -l78

The GNU coding style is that preferred by the GNU project. It is the style that the GNU Emacs C mode encourages and which is used in the C portions of GNU Emacs. (People interested in writing programs for Project GNU should get a copy of The GNU Coding Standards, which also covers semantic and portability issues such as memory usage, the size of integers, etc.)

The Kernighan & Ritchie style is used throughout their well-known book The C Programming Language. It is enabled with the `-kr' option. The Kernighan & Ritchie style corresponds to the following set of options:

-nbad -bap -nbbb -nbc -br -c33 -cd33 -ncdb -ce -ci4
-cli0 -cp33 -d0 -di1 -nfc1 -nfca -i4 -ip0 -l75 -lp
-npcs -npsl -nsc -nsob -nss -ts8 -cs

Kernighan & Ritchie style does not put comments to the right of code in the same column at all times (nor does it use only one space to the right of the code), so for this style indent has arbitrarily chosen column 33.

The style of the original Berkeley indent may be obtained by specifying `-orig' (or by specifyfying `--original', using the long option name). This style is equivalent to the following settings:

-nbap -nbad -nbbb -bc -br -c33 -cd33 -cdb -ce -ci4
-cli0 -cp33 -d4 -di16 -fc1 -fca -i4 -ip4 -l75 -lp 
-npcs -psl -sc -nsob -nss -ts8 -ncs

Blank lines

Various programming styles use blank lines in different places. indent has a number of options to insert or delete blank lines in specific places.

The `-bad' option causes indent to force a blank line after every block of declarations. The `-nbad' option causes indent not to force such blank lines.

The `-bap' option forces a blank line after every procedure body. The `-nbap' option forces no such blank line.

The `-bbb' option forces a blank line before every block comment. A block comment is one which starts in column one when formatting of such comments is disabled, or one with - or * immediately following the /*. The `-nbbb' option does not force such blank lines.

The `-sob' option causes indent to swallow optional blank lines (that is, any optional blank lines present in the input will be removed from the output). If the `-nsob' is specified, any blank lines present in the input file will be copied to the output file.

For example, given the input

char *foo;
char *bar;
/* This separates blocks of declarations.  */
int baz;

indent -bad produces

char *foo;
char *bar;

/* This separates blocks of declarations.  */
int baz;

and indent -nbad produces

char *foo;
char *bar;
/* This separates blocks of declarations.  */
int baz;

The `-bap' option forces a blank line after every procedure body.

For example, given the input

int
foo ()
{
  puts("Hi");
}
/* The procedure bar is even less interesting.  */
char *
bar ()
{
  puts("Hello");
}

indent -bap produces

int
foo ()
{
  puts ("Hi");
}

/* The procedure bar is even less interesting.  */
char *
bar ()
{
  puts ("Hello");
}

and indent -nbap produces

int
foo ()
{
  puts ("Hi");
}
/* The procedure bar is even less interesting.  */
char *
bar ()
{
  puts ("Hello");
}

No blank line will be added after the procedure foo.

Comments

Comments are no longer formatted by default as of version 1.2. This can be enabled with the `-fca' option. Doing so will cause newlines in the comment text to be ignored and the line will be filled up to the length of a line (which can be modified with `-l'). When formatting is enabled, blank lines indicate paragraph breaks.

The `-fc1' option enables the formatting of comments which begin in the first column. The `-nfc1' option disables the formatting of first column comments. When comment formatting is disabled, overall comment indentation may still be adjusted.

The indentation of comments which do not appear to the right of code is set by the `-d' option, which specifies the number of spaces to the left of the surrounding code that the comment appears. For example, `-d2' places comments two spaces to the left of code; `-d0' lines up comments with the code. The `-cdb' option controls whether the `/*' and `*/' are placed on blank lines. With `-cdb', comments look like this:

/*
 * this is a comment
 */

With `-ncdb', comments look like this:

/* this is a comment */

`-cdb' only affects block comments, not comments to the right of code. The default is `-ncdb'.

Comments which appear on the same line as code are placed to the right. The column in which comments on code start is controlled by the `-c' option. The column in which comments to the right of declarations start is controlled by the `-cd' option. By default, they start in the same column as comments to the right of code, which is column 33. The column number for comments to the right of `#else' and `#endif' statements is controlled by the `-cp' option. If the code on a line extends past the comment column, the comment starts further to the right, and the right margin may be automatically extended in extreme cases.

If the `-sc' option is specified, `*' is placed at the left edge of all comments. For example:

/* This is a comment which extends from one line 
 * onto the next line, thus causing us to consider 
 * how it should continue.  */

instead of

/* This is a comment which extends from one line
   onto the next line, thus causing us to consider
   how it should continue.  */

Statements

The `-br' or `-bl' option specifies how to format braces.

The `-br' option formats braces like this:

if (x > 0) {
  x--;
}

The `-bl' option formats them like this:

if (x > 0)
  {
    x--;
  }

If you use the `-bl' option, you may also want to specify the `-bli' option. This option specifies the number of spaces by which braces are indented. `-bli2', the default, gives the result shown above. `-bli0' results in the following:

if (x > 0)
{
  x--;
}

If you are using the `-br' option, you probably want to also use the `-ce' option. This causes the else in an if-then-else construct to cuddle up to the immediately preceding `}'. For example, with `-br -ce' you get the following:

if (x > 0) {
  x--;
} else {
  fprintf (stderr, "...something wrong?\n");
}

With `-br -nce' that code would appear as

if (x > 0) {
  x--;
}
else {
  fprintf (stderr, "...something wrong?\n");
}

The `-cli' option specifies the number of spaces that case labels should be indented to the right of the containing `switch' statement.

If a semicolon is on the same line as a for or while statement, the `-ss' option will cause a space to be placed before the semicolon. This emphasizes the semicolon, making it clear that the body of the for or while statement is an empty statement. -nss disables this feature.

The `-pcs' option causes a space to be placed between the name of the procedure being called and the `(' (for example, puts ("Hi");. The `-npcs' option would give puts("Hi");).

If the `-cs' option is specified, indent puts a space after a cast operator.

The `-bs' option ensures that there is a space between the keyword sizeof and its argument. In some versions, this is known as the `Bill_Shannon' option.

Declarations

By default indent will line up identifiers, in the column specified by the `-di' option. For example, `-di16' makes things look like:

int             foo;
char           *bar;

Using a small value (such as one or two) for the `-di' option can be used to cause the indentifiers to be placed in the first available position, for example

int foo;
char *bar;

The value given to the `-di' option will still affect variables which are put on separate lines from their types, for example `-di2' will lead to

int
  foo;

If the `-bc' option is specified, a newline is forced after each comma in a declaration. For example,

int a,
  b,
  c;

With the `-nbc' option this would look like

int a, b, c;

The `-psl' option causes the type of a procedure being defined to be placed on the line before the name of the procedure. This style is required for the etags program to work correctly, as well as some of the c-mode functions of Emacs.

If you are not using the `-di1' option to place variables being declared immediately after their type, you need to use the `-T' option to tell indent the name of all the typenames in your program that are defined by typedef. `-T' can be specified more than once, and all names specified are used. For example, if your program contains

typedef unsigned long CODE_ADDR;
typedef enum {red, blue, green} COLOR;

you would use the options `-T CODE_ADDR -T COLOR'.

Indentation

One issue in the formatting of code is how far each line should be indented from the left margin. When the beginning of a statement such as if or for is encountered, the indentation level is increased by the value specified by the `-i' option. For example, use `-i8' to specify an eight character indentation for each level. When a statement is continued from a previous line, it is indented by a number of additional spaces specified by the `-ci' option. `-ci' defaults to 0. However, if the `-lp' option is specified, and a line has a left parenthesis which is not closed on that line, then continuation lines will be lined up to start at the character position just after the left parenthesis. This processing also applies to `[' and applies to `{' when it occurs in initialization lists. For example, a piece of continued code might look like this with `-nlp -ci3' in effect:

  p1 = first_procedure (second_procedure (p2, p3),
     third_procedure (p4, p5));

With `-lp' in effect the code looks somewhat clearer:

  p1 = first_procedure (second_procedure (p2, p3),
                        third_procedure (p4, p5));

indent assumes that tabs are placed at regular intervals of both input and output character streams. These intervals are by default 8 columns wide, but (as of version 1.2) may be changed by the `-ts' option. Tabs are treated as the equivalent number of spaces.

The indentation of type declarations in old-style function definitions is controlled by the `-ip' parameter. This is a numeric parameter specifying how many spaces to indent type declarations. For example, the default `-ip5' makes definitions look like this:

char *
create_world (x, y, scale)
     int x;
     int y;
     float scale;
{
  . . .
}

For compatibility with other versions of indent, the option `-nip' is provided, which is equivalent to `-ip0'.

ASCII C allows white space to be placed on preprocessor command lines between the character `#' and the command name. By default, indent removes this space, but specifying the `-lps' option directs indent to leave this space unmodified.

Copyright (c) 1989, 1992 Free Software Foundation
Copyright (c) 1985 Sun Microsystems, Inc.
Copyright (c) 1980 The Regents of the University of California.
Copyright (c) 1976 Board of Trustees of the University of Illinois.
All rights reserved.

Redistribution and use in source and binary forms are permitted
provided that the above copyright notice and this paragraph are
duplicated in all such forms and that any documentation,
advertising materials, and other materials related to such
distribution and use acknowledge that the software was developed
by the University of California, Berkeley, the University of Illinois,
Urbana, and Sun Microsystems, Inc.  The name of either University
or Sun Microsystems may not be used to endorse or promote products
derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR
PURPOSE.