Now let's look at how to define the handler and arginfo functions
which are passed as arguments to register_printf_function.
Compatibility Note: The interface changed in GNU libc
version 2.0. Previously the third argument was of type
You should define your handler functions with a prototype like:
int function (FILE *stream, const struct printf_info *info,
const void *const *args)
The stream argument passed to the handler function is the stream to
which it should write output.
The info argument is a pointer to a structure that contains
information about the various options that were included with the
conversion in the template string. You should not modify this structure
inside your handler function. See Conversion Specifier Options, for
a description of this data structure.
The args is a vector of pointers to the arguments data.
The number of arguments was determined by calling the argument
information function provided by the user.
Your handler function should return a value just like printf
does: it should return the number of characters it has written, or a
negative value to indicate an error.
— Data Type: printf_function
This is the data type that a handler function should have.
If you are going to use parse_printf_format in your
application, you must also define a function to pass as the
arginfo-function argument for each new conversion you install with
You have to define these functions with a prototype like:
int function (const struct printf_info *info,
size_t n, int *argtypes)
The return value from the function should be the number of arguments the
conversion expects. The function should also fill in no more than
n elements of the argtypes array with information about the
types of each of these arguments. This information is encoded using the
various `PA_' macros. (You will notice that this is the same
calling convention parse_printf_format itself uses.)
— Data Type: printf_arginfo_function
This type is used to describe functions that return information about
the number and type of arguments used by a conversion specifier.
Published under the terms of the GNU General Public License