libcfg

Table of Contents

• Introduction
• Compilation and linking
• Getting started
  • Initialisation
  • Parameter registration
  • Function registration
  • Parsing command line options
  • Parsing configuration file
  • Result validation
  • Releasing memory
  • Error handling
  • Examples

Introduction

This is a simple library written in C, for parsing command line options and plain configuration files. It can be used to retrieve variables and arrays into runtime memory, or call functions indicated by command line flags.

This library is compliant with the ISO C99 standard, and relies only on the C standard library. It is written by Cheng Zhao (赵成), and is distributed under the MIT license.

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Compilation and linking

Since this library is tiny and portable, it is recommended to compile the only two source files — libcfg.h and libcfg.c — along with your own program. To this end, one only needs to include the header libcfg.h in the source file for parsing configurations:

#include "libcfg.h"

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Getting started

Initialisation

A cfg_t type structure has to be initialised in the first place, for storing all the configuration information. This can be done using the function cfg_init, e.g

cfg_t *cfg = cfg_init();

This function returns the NULL pointer on error.

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Parameter registration

To retrieve variables and arrays from command line options or configuration files, they need to be registered as configuration parameters, which are represented by the cfg_param_t type structure:

typedef struct {
  int opt;                      /* short command line option            */
  char *lopt;                   /* long command line option             */
  char *name;                   /* name of the parameter                */
  cfg_dtype_t dtype;            /* data type of the parameter           */
  void *var;                    /* variable for the retrieved value     */
} cfg_param_t;

The format of the attributes are:

• opt: a single case-sensitive letter or 0;
• lopt: a string composed of characters with graphical representations (ensured by isgraph), or a NULL pointer;
• name: a string composed of case-sensitive letters, digits, and the underscore character, and starting with either a letter or an underscore;
• dtype: a pre-defined data type indicator;
• var: pointer to the address of the variable/array for holding the retrieved value, and no memory allocation is needed.

In particular, if opt is set to 0, or lopt is set to NULL, the value will not be retrieved from short or long command line options, respectively. For safety consideration, the length of lopt and name must be smaller than the pre-defined CFG_MAX_LOPT_LEN and CFG_MAX_NAME_LEN values respectively.

The supported data types and their indicators are listed below:

Data type	Indicator	Native C type
Boolean variable	CFG_DTYPE_BOOL	bool
Character variable	CFG_DTYPE_CHAR	char
Integer variable	CFG_DTYPE_INT	int
Long integer variable.	CFG_DTYPE_LONG	long
Single-precision floating-point number	CFG_DTYPE_FLT	float
Double-precision floating-point number	CFG_DTYPE_DBL	double
String variable	CFG_DTYPE_STR	char *
Boolean array	CFG_ARRAY_BOOL	bool *
Character array	CFG_ARRAY_CHAR	char *
Integer array	CFG_ARRAY_INT	int *
Long integer array	CFG_ARRAY_LONG	long *
Single-precision floating-point array	CFG_ARRAY_FLT	float *
Double-precision floating-point array	CFG_ARRAY_DBL	double *
String array	CFG_ARRAY_STR	char **

Once the configuration parameters are set, they can be registered using the function

int cfg_set_params(cfg_t *cfg, const cfg_param_t *params, const int npar);

Here, cfg indicates the structure for storing all configuration information, or the entry for the registration. params denotes the address of the configuration parameter structure, and npar indicates the number of parameters to be registered at once. This function returns 0 on success, and a non-zero integer on error.

Note that the cfg_param_t type structure for parameter registration cannot be deconstructed until the command line options and configuration files containing this parameter are parsed (see Parsing command line options and Parsing configuration file),

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Function registration

Functions that can be called with command line flags must be declared with the prototype

void func(void *args);

Similar to configuration parameters, functions can also be registered in the form of a structure:

typedef struct {
  int opt;                      /* short command line option            */
  char *lopt;                   /* long command line option             */
  void (*func) (void *);        /* pointer to the function              */
  void *args;                   /* pointer to the arguments             */
} cfg_func_t;

The opt and lopt variables are the short and long command line option for calling this function, respectively. And at least one of them has to be set, i.e., a case-sensitive letter for opt, or a string composed of graphical characters for lopt. Again, the length of lopt must be smaller than the pre-defined CFG_MAX_LOPT_LEN limit. The pointers func and args are the address of the function to be called, and the corresponding arguments, respectively.

The functions can then be registered using

int cfg_set_funcs(cfg_t *cfg, const cfg_func_t *funcs, const int nfunc);

Here, cfg indicates the entry for the registration, funcs denotes the address to the structure holding the registration information of functions, and nfunc indicates the number of functions to be registered at once. This function returns 0 on success, and a non-zero integer on error.

Note that the cfg_func_t type structure for function registration cannot be deconstructed until the command line options are parsed (see Parsing command line options).

As an example, a typical demand for calling functions via command line is to print the usage of a program, when there is the -h or --help flag. In this case, the help function and the corresponding structure can be defined as

void help(void *arg) {
  printf("Display the help messages.\n");
  exit(0);
}

const cfg_func_t help_func = {'h', "help", help, NULL};

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Parsing command line options

Command line options are passed to the main function at program startup, as the argc and argv parameters. These two parameters can be used by the function cfg_read_opts for parsing the options, and retrieving parameter values or calling functions. This function is defined as

int cfg_read_opts(cfg_t *cfg, const int argc, char *const *argv, const int priority, int *optidx);

It returns 0 on success, and a non-zero integer on error. And the arguments are:

• cfg: the structure for all configuration information;
• argc: the number of command line arguments, obtained from main;
• argv: the command line argument list, obtained from main;
• priority: the priority of values retrieved from command line, must be positive;
• optidx: the index of the argument list at which the parser is terminated.

In particular, priority decides which value to take if the parameter is defined in multiple sources, say, both in a configuration file and command line options. For instance, if a variable has already been set in a configuration file with a lower priority value than the one passed to cfg_read_opts, then its value will be overwritten by the one obtained from command line.

The supported formats of command line options are listed below:

Description	Format	Example	Note
Short option	-OPT VALUE or -OPT=VALUE	-n=10	OPT must be a letter; VALUE is optional.
Long option	--LOPT VALUE or --LOPT=VALUE	--help	LOPT is a string with graphical characters, with length smaller than CFG_MAX_LOPT_LEN; VALUE is optional.
Option terminator	--		It terminates option scanning.

Note that the - and = symbols in the formats are customisable. They are actually defined as CFG_CMD_FLAG and CFG_CMD_ASSIGN in libcfg.h, respectively.

All command line arguments satisfying the above formats are interpreted as options, otherwise they are treated as values. And values can only be omitted for boolean type variables — which implies true — or function calls. If the value contains space or special characters that are reserved by the environment, then it should be enclosed by pairs of single or double quotation marks. Besides, values that may be confused with options (such as -x) are recommended to be passed with the assignment symbol =.

Furthermore, if the -- option is found, then the option scanning will be terminated, and the current index of the argument list is reported as optidx. Therefore, when calling the program, non-option parameters should always be passed after all the options. And when optidx is equal to argc, it means that all command line arguments are parsed.

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Parsing configuration file

Plain text files can be parsed using the function

int cfg_read_file(cfg_t *cfg, const char *filename, const int priority);

The argument cfg indicates the structure for storing all configuration information, filename denotes the name of the input file, and priority defines the priority of values read from this file. This function returns 0 on success, and a non-zero integer on error.

By default the format of a valid configuration file has to be

# This is a comment.
name_variable = value                # inline comment
name_array = [ element1, element2 ]  # entry for an array

Here, name_variable, name_array, and name_long_array indicate the registered name of configuration parameters (see Parameter registration), and value, element1, and element2 are the values to be loaded into memory.

In particular, scalar type definitions can be parsed as arrays with a single element. And by default array type definitions with multiple elements have to be enclosed by a pair of brackets []. In addition, multiple-line definitions are only allowed for arrays, and the line break symbol \ can only be placed after the array element separator ,. These symbols, including [, ], \, ,, as well as the comment indicator #, are customisable in libcfg.h. And if a value or an element of an array contains special characters, the full value or element has to be enclosed by a pair of single or double quotation marks.

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Result validation

The functions cfg_read_opts and cfg_read_file extract the parameter value from command line options and configuration files respectively. The value is then converted to the given data type, and passed to the address of the variable specified at registration.

To verify whether a variable or an array is set correctly, one can use the function

bool cfg_is_set(const cfg_t *cfg, const void *var);

It returns true if the variable or array values are set by the functions cfg_read_opts or cfg_read_file, and false if they are untouched. Here, var has to be the variable/array address specified at registration.

Moreover, the number of array elements read by the parser can be reported by the function

int cfg_get_size(const cfg_t *cfg, const void *var);

It returns 0 if the array is not set. So it may not be necessary to verify arrays using cfg_is_set. Note that the array is allocated with precisely the number of elements reported by this function, so the indices for accessing array elements must be smaller than this number.

Once the variable or array is verified successfully, it can then be used directly in the rest parts of the program.

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Releasing memory

Once all the variables and arrays are retrieved and verified, the cfg_t type structure for storing all the configuration information can be deconstructed by the function

void cfg_destroy(cfg_t *cfg);

After calling this function, the values of the variables and arrays are still accessible, but the size of arrays cannot be obtained using the cfg_get_size function anymore.

In addition, since the memory of arrays and string variables are allocated by this library, it is the user's responsibility to free them using the standard free function. In particular, since string arrays are represented two-dimensional character arrays, the pointers to both the string array and its first element have to be freed, e.g.

char **str;             /* declaration of the string array */

/* parameter registration and retriving */

free(*str);             /* free the first element of the array */
free(str);              /* free the array itself */

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Error handling

Errors can be caught by checking the return values of some of the functions, such as cfg_init, cfg_set_params, cfg_read_opts, etc. And once the cfg_init is executed successfully, error messages can be printed using the function

void cfg_perror(const cfg_t *cfg, FILE *stream, const char *msg);

It outputs the string indicated by msg, followed by a colon and a space, and then followed by the error message produced by this library, as well as a newline character \n. The results are written to stream, which is typically stderr.

Unexpected issues that are not critical enough to stop the program are treated as warnings. They cannot be handled by the return values of functions, but one can check warning messages using the function

void cfg_pwarn(cfg_t *cfg, FILE *stream, const char *msg);

This function is similar to cfg_perror. Note that there can be multiple warning messages, and once a warning message is printed, it is automatically removed from the message pool.

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Examples

An example for the usage of this library is provided in the example folder.

It registers variables and arrays for all the supported data types, as well as two functions to be called via command line. Command line options and the configuration file input.conf are then parsed. The variables and arrays are printed if they are set correctly.

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