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3.1.1.2 Gperf Declarations

The declaration section can contain gperf declarations. They influence the way gperf works, like command line options do. In fact, every such declaration is equivalent to a command line option. There are three forms of declarations:

  1. Declarations without argument, like ‘%compare-lengths’.
  2. Declarations with an argument, like ‘%switch=count’.
  3. Declarations of names of entities in the output file, like ‘%define lookup-function-name name’.

When a declaration is given both in the input file and as a command line option, the command-line option's value prevails.

The following gperf declarations are available.

%delimiters=delimiter-list

Allows you to provide a string containing delimiters used to separate keywords from their attributes. The default is ",". This option is essential if you want to use keywords that have embedded commas or newlines.

%struct-type

Allows you to include a struct type declaration for generated code; see above for an example.

%ignore-case

Consider upper and lower case ASCII characters as equivalent. The string comparison will use a case insignificant character comparison. Note that locale dependent case mappings are ignored.

%language=language-name

Instructs gperf to generate code in the language specified by the option's argument. Languages handled are currently:

KR-C

Old-style K&R C. This language is understood by old-style C compilers and ANSI C compilers, but ANSI C compilers may flag warnings (or even errors) because of lacking ‘const’.

C

Common C. This language is understood by ANSI C compilers, and also by old-style C compilers, provided that you #define const to empty for compilers which don't know about this keyword.

ANSI-C

ANSI C. This language is understood by ANSI C (C89, ISO C90) compilers, ISO C99 compilers, and C++ compilers.

C++

C++. This language is understood by C++ compilers.

The default is C.

%define slot-name name

This declaration is only useful when option ‘-t’ (or, equivalently, the ‘%struct-type’ declaration) has been given. By default, the program assumes the structure component identifier for the keyword is ‘name’. This option allows an arbitrary choice of identifier for this component, although it still must occur as the first field in your supplied struct.

%define initializer-suffix initializers

This declaration is only useful when option ‘-t’ (or, equivalently, the ‘%struct-type’ declaration) has been given. It permits to specify initializers for the structure members following slot-name in empty hash table entries. The list of initializers should start with a comma. By default, the emitted code will zero-initialize structure members following slot-name.

%define hash-function-name name

Allows you to specify the name for the generated hash function. Default name is ‘hash’. This option permits the use of two hash tables in the same file.

%define lookup-function-name name

Allows you to specify the name for the generated lookup function. Default name is ‘in_word_set’. This option permits multiple generated hash functions to be used in the same application.

%define class-name name

This option is only useful when option ‘-L C++’ (or, equivalently, the ‘%language=C++’ declaration) has been given. It allows you to specify the name of generated C++ class. Default name is Perfect_Hash.

%7bit

This option specifies that all strings that will be passed as arguments to the generated hash function and the generated lookup function will solely consist of 7-bit ASCII characters (bytes in the range 0..127). (Note that the ANSI C functions isalnum and isgraph do not guarantee that a byte is in this range. Only an explicit test like ‘c >= 'A' && c <= 'Z'’ guarantees this.)

%compare-lengths

Compare keyword lengths before trying a string comparison. This option is mandatory for binary comparisons (see section Use of NUL bytes). It also might cut down on the number of string comparisons made during the lookup, since keywords with different lengths are never compared via strcmp. However, using ‘%compare-lengths’ might greatly increase the size of the generated C code if the lookup table range is large (which implies that the switch option ‘-S’ or ‘%switch’ is not enabled), since the length table contains as many elements as there are entries in the lookup table.

%compare-strncmp

Generates C code that uses the strncmp function to perform string comparisons. The default action is to use strcmp.

%readonly-tables

Makes the contents of all generated lookup tables constant, i.e., “readonly”. Many compilers can generate more efficient code for this by putting the tables in readonly memory.

%enum

Define constant values using an enum local to the lookup function rather than with #defines. This also means that different lookup functions can reside in the same file. Thanks to James Clark <jjc@ai.mit.edu>.

%includes

Include the necessary system include file, <string.h>, at the beginning of the code. By default, this is not done; the user must include this header file himself to allow compilation of the code.

%global-table

Generate the static table of keywords as a static global variable, rather than hiding it inside of the lookup function (which is the default behavior).

%pic

Optimize the generated table for inclusion in shared libraries. This reduces the startup time of programs using a shared library containing the generated code. If the ‘%struct-type’ declaration (or, equivalently, the option ‘-t’) is also given, the first field of the user-defined struct must be of type ‘int’, not ‘char *’, because it will contain offsets into the string pool instead of actual strings. To convert such an offset to a string, you can use the expression ‘stringpool + o’, where o is the offset. The string pool name can be changed through the ‘%define string-pool-name’ declaration.

%define string-pool-name name

Allows you to specify the name of the generated string pool created by the declaration ‘%pic’ (or, equivalently, the option ‘-P’). The default name is ‘stringpool’. This declaration permits the use of two hash tables in the same file, with ‘%pic’ and even when the ‘%global-table’ declaration (or, equivalently, the option ‘-G’) is given.

%null-strings

Use NULL strings instead of empty strings for empty keyword table entries. This reduces the startup time of programs using a shared library containing the generated code (but not as much as the declaration ‘%pic’), at the expense of one more test-and-branch instruction at run time.

%define word-array-name name

Allows you to specify the name for the generated array containing the hash table. Default name is ‘wordlist’. This option permits the use of two hash tables in the same file, even when the option ‘-G’ (or, equivalently, the ‘%global-table’ declaration) is given.

%define length-table-name name

Allows you to specify the name for the generated array containing the length table. Default name is ‘lengthtable’. This option permits the use of two length tables in the same file, even when the option ‘-G’ (or, equivalently, the ‘%global-table’ declaration) is given.

%switch=count

Causes the generated C code to use a switch statement scheme, rather than an array lookup table. This can lead to a reduction in both time and space requirements for some input files. The argument to this option determines how many switch statements are generated. A value of 1 generates 1 switch containing all the elements, a value of 2 generates 2 tables with 1/2 the elements in each switch, etc. This is useful since many C compilers cannot correctly generate code for large switch statements. This option was inspired in part by Keith Bostic's original C program.

%omit-struct-type

Prevents the transfer of the type declaration to the output file. Use this option if the type is already defined elsewhere.


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