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20. GLib

GLib is a lower-level library that provides many useful definitions and functions available for use when creating GDK and GTK applications. These include definitions for basic types and their limits, standard macros, type conversions, byte order, memory allocation, warnings and assertions, message logging, timers, string utilities, hook functions, a lexical scanner, dynamic loading of modules, and automatic string completion. A number of data structures (and their related operations) are also defined, including memory chunks, doubly-linked lists, singly-linked lists, hash tables, strings (which can grow dynamically), string chunks (groups of strings), arrays (which can grow in size as elements are added), balanced binary trees, N-ary trees, quarks (a two-way association of a string and a unique integer identifier), keyed data lists (lists of data elements accessible by a string or integer id), relations and tuples (tables of data which can be indexed on any number of fields), and caches.

A summary of some of GLib's capabilities follows; not every function, data structure, or operation is covered here. For more complete information about the GLib routines, see the GLib documentation. One source of GLib documentation is

If you are using a language other than C, you should consult your language's binding documentation. In some cases your language may have equivalent functionality built-in, while in other cases it may not.

20.1 Definitions

Definitions for the extremes of many of the standard types are:


Also, the following typedefs. The ones left unspecified are dynamically set depending on the architecture. Remember to avoid counting on the size of a pointer if you want to be portable! E.g., a pointer on an Alpha is 8 bytes, but 4 on Intel 80x86 family CPUs.

char   gchar;
short  gshort;
long   glong;
int    gint;
char   gboolean;

unsigned char   guchar;
unsigned short  gushort;
unsigned long   gulong;
unsigned int    guint;

float   gfloat;
double  gdouble;
long double gldouble;

void* gpointer;


20.2 Doubly Linked Lists

The following functions are used to create, manage, and destroy standard doubly linked lists. Each element in the list contains a piece of data, together with pointers which link to the previous and next elements in the list. This enables easy movement in either direction through the list. The data item is of type "gpointer", which means the data can be a pointer to your real data or (through casting) a numeric value (but do not assume that int and gpointer have the same size!). These routines internally allocate list elements in blocks, which is more efficient than allocating elements individually.

There is no function to specifically create a list. Instead, simply create a variable of type GList* and set its value to NULL; NULL is considered to be the empty list.

To add elements to a list, use the g_list_append(), g_list_prepend(), g_list_insert(), or g_list_insert_sorted() routines. In all cases they accept a pointer to the beginning of the list, and return the (possibly changed) pointer to the beginning of the list. Thus, for all of the operations that add or remove elements, be sure to save the returned value!

GList *g_list_append( GList    *list,
                      gpointer  data );

This adds a new element (with value data) onto the end of the list.

GList *g_list_prepend( GList    *list,
                       gpointer  data );

This adds a new element (with value data) to the beginning of the list.

GList *g_list_insert( GList    *list,
                      gpointer  data,
                      gint      position );

This inserts a new element (with value data) into the list at the given position. If position is 0, this is just like g_list_prepend(); if position is less than 0, this is just like g_list_append().

GList *g_list_remove( GList    *list,
                      gpointer  data );

This removes the element in the list with the value data; if the element isn't there, the list is unchanged.

void g_list_free( GList *list );

This frees all of the memory used by a GList. If the list elements refer to dynamically-allocated memory, then they should be freed first.

There are many other GLib functions that support doubly linked lists; see the glib documentation for more information. Here are a few of the more useful functions' signatures:

GList *g_list_remove_link( GList *list,
                           GList *link );

GList *g_list_reverse( GList *list );

GList *g_list_nth( GList *list,
                   gint   n );
GList *g_list_find( GList    *list,
                    gpointer  data );

GList *g_list_last( GList *list );

GList *g_list_first( GList *list );

gint g_list_length( GList *list );

void g_list_foreach( GList    *list,
                     GFunc     func,
                     gpointer  user_data );

20.3 Singly Linked Lists

Many of the above functions for singly linked lists are identical to the above. Here is a list of some of their operations:

GSList *g_slist_append( GSList   *list,
                        gpointer  data );
GSList *g_slist_prepend( GSList   *list,
                         gpointer  data );
GSList *g_slist_insert( GSList   *list,
                        gpointer  data,
                        gint      position );
GSList *g_slist_remove( GSList   *list,
                        gpointer  data );
GSList *g_slist_remove_link( GSList *list,
                             GSList *link );
GSList *g_slist_reverse( GSList *list );

GSList *g_slist_nth( GSList *list,
                     gint    n );
GSList *g_slist_find( GSList   *list,
                      gpointer  data );
GSList *g_slist_last( GSList *list );

gint g_slist_length( GSList *list );

void g_slist_foreach( GSList   *list,
                      GFunc     func,
                      gpointer  user_data );

20.4 Memory Management

gpointer g_malloc( gulong size );

This is a replacement for malloc(). You do not need to check the return value as it is done for you in this function. If the memory allocation fails for whatever reasons, your applications will be terminated.

gpointer g_malloc0( gulong size );

Same as above, but zeroes the memory before returning a pointer to it.

gpointer g_realloc( gpointer mem,
                    gulong   size );

Relocates "size" bytes of memory starting at "mem". Obviously, the memory should have been previously allocated.

void g_free( gpointer mem );

Frees memory. Easy one. If mem is NULL it simply returns.

void g_mem_profile( void );

Dumps a profile of used memory, but requires that you add #define MEM_PROFILE to the top of glib/gmem.c and re-make and make install.

void g_mem_check( gpointer mem );

Checks that a memory location is valid. Requires you add #define MEM_CHECK to the top of gmem.c and re-make and make install.

20.5 Timers

Timer functions can be used to time operations (e.g., to see how much time has elapsed). First, you create a new timer with g_timer_new(). You can then use g_timer_start() to start timing an operation, g_timer_stop() to stop timing an operation, and g_timer_elapsed() to determine the elapsed time.

GTimer *g_timer_new( void );

void g_timer_destroy( GTimer *timer );

void g_timer_start( GTimer  *timer );

void g_timer_stop( GTimer  *timer );

void g_timer_reset( GTimer  *timer );

gdouble g_timer_elapsed( GTimer *timer,
                         gulong *microseconds );

20.6 String Handling

GLib defines a new type called a GString, which is similar to a standard C string but one that grows automatically. Its string data is null-terminated. What this gives you is protection from buffer overflow programming errors within your program. This is a very important feature, and hence I recommend that you make use of GStrings. GString itself has a simple public definition:

struct GString 
  gchar *str; /* Points to the string's current \0-terminated value. */
  gint len; /* Current length */

As you might expect, there are a number of operations you can do with a GString.

GString *g_string_new( gchar *init );

This constructs a GString, copying the string value of init into the GString and returning a pointer to it. NULL may be given as the argument for an initially empty GString.

void g_string_free( GString *string,
                    gint     free_segment );

This frees the memory for the given GString. If free_segment is TRUE, then this also frees its character data.

GString *g_string_assign( GString     *lval,
                          const gchar *rval );

This copies the characters from rval into lval, destroying the previous contents of lval. Note that lval will be lengthened as necessary to hold the string's contents, unlike the standard strcpy() function.

The rest of these functions should be relatively obvious (the _c versions accept a character instead of a string):

GString *g_string_truncate( GString *string,
                            gint     len );
GString *g_string_append( GString *string,
                          gchar   *val );
GString *g_string_append_c( GString *string,
                            gchar    c );
GString *g_string_prepend( GString *string,
                           gchar   *val );
GString *g_string_prepend_c( GString *string,
                             gchar    c );
void g_string_sprintf( GString *string,
                       gchar   *fmt,
void g_string_sprintfa ( GString *string,
                         gchar   *fmt,
                         ... );

20.7 Utility and Error Functions

gchar *g_strdup( const gchar *str );

Replacement strdup function. Copies the original strings contents to newly allocated memory, and returns a pointer to it.

gchar *g_strerror( gint errnum );

I recommend using this for all error messages. It's much nicer, and more portable than perror() or others. The output is usually of the form:

program name:function that failed:file or further description:strerror

Here's an example of one such call used in our hello_world program:

g_print("hello_world:open:%s:%s\n", filename, g_strerror(errno));

void g_error( gchar *format, ... );

Prints an error message. The format is just like printf, but it prepends "** ERROR **: " to your message, and exits the program. Use only for fatal errors.

void g_warning( gchar *format, ... );

Same as above, but prepends "** WARNING **: ", and does not exit the program.

void g_message( gchar *format, ... );

Prints "message: " prepended to the string you pass in.

void g_print( gchar *format, ... );

Replacement for printf().

And our last function:

gchar *g_strsignal( gint signum );

Prints out the name of the Unix system signal given the signal number. Useful in generic signal handling functions.

All of the above are more or less just stolen from glib.h. If anyone cares to document any function, just send me an email!

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