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(libc.info)Other Input Conversions

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Other Input Conversions
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   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' (Note:
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 Note: 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.