The most basic use of signals is to implement simple event notification: for example, if we have a MamanFile object, and if this object has a write method, we might wish to be notified whenever someone uses this method. The code below shows how the user can connect a callback to the write signal. Full code for this simple example is located in sample/signal/maman-file.{h|c} and in sample/signal/test.c

file = g_object_new (MAMAN_FILE_TYPE, NULL);

g_signal_connect (G_OBJECT (file), "write",
                  (GCallback)write_event,
                  NULL);

maman_file_write (file, buffer, 50);

The MamanFile signal is registered in the class_init function:

klass->write_signal_id = 
  g_signal_newv ("write",
                 G_TYPE_FROM_CLASS (g_class),
                 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
                 NULL /* class closure */,
                 NULL /* accumulator */,
                 NULL /* accu_data */,
                 g_cclosure_marshal_VOID__VOID,
                 G_TYPE_NONE /* return_type */,
                 0     /* n_params */,
                 NULL  /* param_types */);

and the signal is emitted in maman_file_write:

void maman_file_write (MamanFile *self, guint8 *buffer, guint32 size)
{
  /* First write data. */
  /* Then, notify user of data written. */
  g_signal_emit (self, MAMAN_FILE_GET_CLASS (self)->write_signal_id,
                 0 /* details */, 
                 NULL);
}

As shown above, you can safely set the details parameter to zero if you do not know what it can be used for. For a discussion of what you could used it for, see the section called “The detail argument”

The signature of the signal handler in the above example is defined as g_cclosure_marshal_VOID__VOID. Its name follows a simple convention which encodes the function parameter and return value types in the function name. Specifically, the value in front of the double underscore is the type of the return value, while the value(s) after the double underscore denote the parameter types. The header gobject/gmarshal.h defines a set of commonly needed closures that one can use.