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cdecl, c++decl - Compose C and C++ type declarations


cdecl [-a | -+ | -p | -r] [-ciqdDV]
[[ files ...] | explain ... | declare ... | cast ... | set ... | help | ? ]
c++decl [-a | -+ | -p | -r] [-ciqdDV]
[[ files ...] | explain ... | declare ... | cast ... | set ... | help | ? ]
explain ...
declare ...
cast ...


Cdecl (and c++decl) is a program for encoding and decoding C (or C++) type declarations.
The C language is based on the (draft proposed) X3J11 ANSI Standard; optionally, the C
language may be based on the pre-ANSI definition defined by Kernighan & Ritchie's The C
Programming Language book, or the C language defined by the Ritchie PDP-11 C compiler.
The C++ language is based on Bjarne Stroustrup's The C++ Programming Language, plus the
version 2.0 additions to the language.


-a Use the ANSI C dialect of the C language.

-p Use the pre-ANSI dialect defined by Kernighan & Ritchie's book.

-r Use the dialect defined by the Ritchie PDP-11 C compiler.

-+ Use the C++ language, rather than C.

-i Run in interactive mode (the default when reading from a terminal). This also
turns on prompting, line editing, and line history.

-q Quiet the prompt. Turns off the prompt in interactive mode.

-c Create compilable C or C++ code as output. Cdecl will add a semicolon to the end
of a declaration and a pair of curly braces to the end of a function definition.

-d Turn on debugging information (if compiled in).

-D Turn on YACC debugging information (if compiled in).

-V Display version information and exit.


Cdecl may be invoked under a number of different names (by either renaming the executable,
or creating a symlink or hard link to it). If it is invoked as cdecl then ANSI C is the
default language. If it is invoked as c++decl then C++ is the default. If it is invoked
as either explain, cast, or declare then it will interpret the rest of the command line
options as parameters to that command, execute the command, and exit. It will also do
this if the first non-switch argument on the command line is one of those three commands.
Input may also come from a file.

Cdecl reads the named files for statements in the language described below. A
transformation is made from that language to C (C++) or pseudo-English. The results of
this transformation are written on standard output. If no files are named, or a filename
of ``-'' is encountered, standard input will be read. If standard input is coming from a
terminal, (or the -i option is used), a prompt will be written to the terminal before each
line. The prompt can be turned off by the -q option (or the set noprompt command). If
cdecl is invoked as explain, declare or cast, or the first argument is one of the commands
discussed below, the argument list will be interpreted according to the grammar shown
below instead of as file names.

When it is run interactively, cdecl uses the GNU readline library to provide keyword
completion and command line history, very much like bash(1) (q.v.). Pressing TAB will
complete the partial keyword before the cursor, unless there is more than one possible
completion, in which case a second TAB will show the list of possible completions and
redisplay the command line. The left and right arrow keys and backspace can be used for
editing in a natural way, and the up and down arrow keys retrieve previous command lines
from the history. Most other familiar keys, such as Ctrl-U to delete all text from the
cursor back to the beginning of the line, work as expected. There is an ambiguity between
the int and into keywords, but cdecl will guess which one you meant, and it always guesses

You can use cdecl as you create a C program with an editor like vi(1) or emacs(1). You
simply type in the pseudo-English version of the declaration and apply cdecl as a filter
to the line. (In vi(1), type ``!!cdecl<cr>''.)

If the create program option -c is used, the output will include semi-colons after
variable declarations and curly brace pairs after function declarations.

The -V option will print out the version numbers of the files used to create the process.
If the source is compiled with debugging information turned on, the -d option will enable
it to be output. If the source is compiled with YACC debugging information turned on, the
-D option will enable it to be output.


There are six statements in the language. The declare statement composes a C type
declaration from a verbose description. The cast statement composes a C type cast as
might appear in an expression. The explain statement decodes a C type declaration or
cast, producing a verbose description. The help (or ?) statement provides a help
message. The quit (or exit) statement (or the end of file) exits the program. The set
statement allows the command line options to be set interactively. Each statement is
separated by a semi-colon or a newline.


Some synonyms are permitted during a declaration:

character is a synonym for char
constant is a synonym for const
enumeration is a synonym for enum
func is a synonym for function
integer is a synonym for int
ptr is a synonym for pointer
ref is a synonym for reference
ret is a synonym for returning
structure is a synonym for struct
vector is a synonym for array

The TAB completion feature only knows about the keywords in the right column of the
structure, not the ones in the left column. TAB completion is a lot less useful when the
leading characters of different keywords are the same (the keywords confict with one
another), and putting both columns in would cause quite a few conflicts.


The following grammar describes the language. In the grammar, words in "<>" are non-
terminals, bare lower-case words are terminals that stand for themselves. Bare upper-case
words are other lexical tokens: NOTHING means the empty string; NAME means a C identifier;
NUMBER means a string of decimal digits; and NL means the new-line or semi-colon

<program> ::= NOTHING
| <program> <stmt> NL
<stmt> ::= NOTHING
| declare NAME as <adecl>
| declare <adecl>
| cast NAME into <adecl>
| cast <adecl>
| explain <optstorage> <ptrmodlist> <type> <cdecl>
| explain <storage> <ptrmodlist> <cdecl>
| explain ( <ptrmodlist> <type> <cast> ) optional-NAME
| set <options>
| help | ?
| quit
| exit
<adecl> ::= array of <adecl>
| array NUMBER of <adecl>
| function returning <adecl>
| function ( <adecl-list> ) returning <adecl>
| <ptrmodlist> pointer to <adecl>
| <ptrmodlist> pointer to member of class NAME <adecl>
| <ptrmodlist> reference to <adecl>
| <ptrmodlist> <type>
<cdecl> ::= <cdecl1>
| * <ptrmodlist> <cdecl>
| NAME :: * <cdecl>
| & <ptrmodlist> <cdecl>
<cdecl1> ::= <cdecl1> ( )
| <cdecl1> ( <castlist> )
| <cdecl1> [ ]
| <cdecl1> [ NUMBER ]
| ( <cdecl> )
<cast> ::= NOTHING
| ( )
| ( <cast> ) ( )
| ( <cast> ) ( <castlist> )
| ( <cast> )
| NAME :: * <cast>
| * <cast>
| & <cast>
| <cast> [ ]
| <cast> [ NUMBER ]
<type> ::= <typename> | <modlist>
| <modlist> <typename>
| struct NAME | union NAME | enum NAME | class NAME
<castlist> ::= <castlist> , <castlist>
| <ptrmodlist> <type> <cast>
| <name>
<adecllist> ::= <adecllist> , <adecllist>
| <name>
| <adecl>
| <name> as <adecl>
<typename> ::= int | char | double | float | void
<modlist> ::= <modifier> | <modlist> <modifier>
<modifier> ::= short | long | unsigned | signed | <ptrmod>
<ptrmodlist> ::= <ptrmod> <ptrmodlist> | NOTHING
<ptrmod> ::= const | volatile | noalias
<storage> ::= auto | extern | register | static
<optstorage> ::= NOTHING | <storage>
<options> ::= NOTHING | <options>
| create | nocreate
| prompt | noprompt
| ritchie | preansi | ansi | cplusplus
| debug | nodebug | yydebug | noyydebug


The set command takes several options. You can type set or set options to see the
currently selected options and a summary of the options which are available. The first
four correspond to the -a, -p, -r, and -+ command line options, respectively.

ansi Use the ANSI C dialect of the C language.

Use the pre-ANSI dialect defined by Kernighan & Ritchie's book.

Use the dialect defined by the Ritchie PDP-11 C compiler.

Use the C++ language, rather than C.

Turn on or off the prompt in interactive mode.

Turn on or off the appending of semicolon or curly braces to the declarations
output by cdecl. This corresponds to the -c command line option.

Turn on or off debugging information.

Turn on or off YACC debugging information.

Note: debugging information and YACC debugging information are only available if they have
been compiled into cdecl. The last two options correspond to the -d and -D command line
options, respectively. Debugging information is normally used in program development, and
is not generally compiled into distributed executables.


To declare an array of pointers to functions that are like malloc(3), do

declare fptab as array of pointer to function returning pointer to char

The result of this command is

char *(*fptab[])()

When you see this declaration in someone else's code, you can make sense out of it by

explain char *(*fptab[])()

The proper declaration for signal(2), ignoring function prototypes, is easily described in
cdecl's language:

declare signal as function returning pointer to function returning void

which produces

void (*signal())()

The function declaration that results has two sets of empty parentheses. The author of
such a function might wonder where to put the parameters:

declare signal as function (arg1,arg2) returning pointer to function returning void

provides the following solution (when run with the -c option):

void (*signal(arg1,arg2))() { }

If we want to add in the function prototypes, the function prototype for a function such
as _exit(2) would be declared with:

declare _exit as function (retvalue as int) returning void


void _exit(int retvalue) { }

As a more complex example using function prototypes, signal(2) could be fully defined as:

declare signal as function(x as int, y as pointer to function(int) returning void)
returning pointer to function(int) returning void

giving (with -c)

void (*signal(int x, void (*y)(int )))(int ) { }

Cdecl can help figure out the where to put the "const" and "volatile" modifiers in
declarations, thus

declare foo as pointer to const int


const int *foo


declare foo as const pointer to int


int * const foo

C++decl can help with declaring references, thus

declare x as reference to pointer to character


char *&x

C++decl can help with pointers to member of classes, thus declaring a pointer to an
integer member of a class X with

declare foo as pointer to member of class X int


int X::*foo


declare foo as pointer to member of class X function (arg1, arg2) returning pointer
to class Y


class Y *(X::*foo)(arg1, arg2)


The declare, cast and explain statements try to point out constructions that are not
supported in C. In some cases, a guess is made as to what was really intended. In these
cases, the C result is a toy declaration whose semantics will work only in Algol-68. The
list of unsupported C constructs is dependent on which version of the C language is being
used (see the ANSI, pre-ANSI, and Ritchie options). The set of supported C++ constructs
is a superset of the ANSI set, with the exception of the noalias keyword.


ANSI Standard X3.159-1989 (ANSI C)

ISO/IEC 9899:1990 (the ISO standard)

The comp.lang.c FAQ

Section 8.4 of the C Reference Manual within The C Programming Language by B. Kernighan &
D. Ritchie.

Section 8 of the C++ Reference Manual within The C++ Programming Language by B.


The pseudo-English syntax is excessively verbose.

There is a wealth of semantic checking that isn't being done.

Cdecl was written before the ANSI C standard was completed, and no attempt has been made
to bring it up-to-date. Nevertheless, it is very close to the standard, with the obvious
exception of noalias.

Cdecl's scope is intentionally small. It doesn't help you figure out initializations. It
expects storage classes to be at the beginning of a declaration, followed by the the
const, volatile and noalias modifiers, followed by the type of the variable. Cdecl
doesn't know anything about variable length argument lists. (This includes the ``,...''

Cdecl thinks all the declarations you utter are going to be used as external definitions.
Some declaration contexts in C allow more flexibility than this. An example of this is:

declare argv as array of array of char

where cdecl responds with

Warning: Unsupported in C -- 'Inner array of unspecified size'
(maybe you mean "array of pointer")
char argv[][]

Tentative support for the noalias keyword was put in because it was in the draft ANSI


Originally written by Graham Ross, improved and expanded by David Wolverton, Tony Hansen,
and Merlyn LeRoy.

GNU readline support and Linux port by David R. Conrad, <[email protected]>

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