12.14.7 Other Input Conversions

This section describes the miscellaneous input conversions.

The ‘%p’ conversion is used to read a pointer value. It recognizes the same syntax used by the ‘%p’ output conversion for printf (see Other Output Conversions); that is, a hexadecimal number just as the ‘%x’ conversion accepts. The corresponding argument should be of type void **; that is, the address of a place to store a pointer.

The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.

The ‘%n’ conversion produces the number of characters read so far by this call. The corresponding argument should be of type int *. This conversion works in the same way as the ‘%n’ conversion for printf; see Other Output Conversions, for an example.

The ‘%n’ conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments. If the ‘%n’ follows the locus of a matching failure, then no value is stored for it since scanf returns before processing the ‘%n’. If you store -1 in that argument slot before calling scanf, the presence of -1 after scanf indicates an error occurred before the ‘%n’ was reached.

Finally, the ‘%%’ conversion matches a literal ‘%’ character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified.