12.13.2 Conversion Specifier Options

If you define a meaning for ‘%A’, what if the template contains ‘%+23A’ or ‘%-#A’? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template.

Both the handler-function and arginfo-function accept an argument that points to a struct printf_info, which contains information about the options appearing in an instance of the conversion specifier. This data type is declared in the header file printf.h.

Type: struct printf_info

This structure is used to pass information about the options appearing in an instance of a conversion specifier in a printf template string to the handler and arginfo functions for that specifier. It contains the following members:

int prec

This is the precision specified. The value is -1 if no precision was specified. If the precision was given as ‘*’, the printf_info structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.

int width

This is the minimum field width specified. The value is 0 if no width was specified. If the field width was given as ‘*’, the printf_info structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.

wchar_t spec

This is the conversion specifier character specified. It’s stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called.

unsigned int is_long_double

This is a boolean that is true if the ‘L’, ‘ll’, or ‘q’ type modifier was specified. For integer conversions, this indicates long long int, as opposed to long double for floating point conversions.

unsigned int is_char

This is a boolean that is true if the ‘hh’ type modifier was specified.

unsigned int is_short

This is a boolean that is true if the ‘h’ type modifier was specified.

unsigned int is_long

This is a boolean that is true if the ‘l’ type modifier was specified.

unsigned int alt

This is a boolean that is true if the ‘#’ flag was specified.

unsigned int space

This is a boolean that is true if the ‘ ’ flag was specified.

unsigned int left

This is a boolean that is true if the ‘-’ flag was specified.

unsigned int showsign

This is a boolean that is true if the ‘+’ flag was specified.

unsigned int group

This is a boolean that is true if the ‘'’ flag was specified.

unsigned int extra

This flag has a special meaning depending on the context. It could be used freely by the user-defined handlers but when called from the printf function this variable always contains the value 0.

unsigned int wide

This flag is set if the stream is wide oriented.

wchar_t pad

This is the character to use for padding the output to the minimum field width. The value is '0' if the ‘0’ flag was specified, and ' ' otherwise.