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(libc.info)String Input Conversions

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String Input Conversions
------------------------

   This section describes the `scanf' input conversions for reading
string and character values: `%s', `%S', `%[', `%c', and `%C'.

   You have two options for how to receive the input from these
conversions:

   * Provide a buffer to store it in.  This is the default.  You should
     provide an argument of type `char *' or `wchar_t *' (the latter of
     the `l' modifier is present).

     *Warning:* To make a robust program, you must make sure that the
     input (plus its terminating null) cannot possibly exceed the size
     of the buffer you provide.  In general, the only way to do this is
     to specify a maximum field width one less than the buffer size.
     *If you provide the buffer, always specify a maximum field width
     to prevent overflow.*

   * Ask `scanf' to allocate a big enough buffer, by specifying the `a'
     flag character.  This is a GNU extension.  You should provide an
     argument of type `char **' for the buffer address to be stored in.
     Note: Dynamic String Input.

   The `%c' conversion is the simplest: it matches a fixed number of
characters, always.  The maximum field width says how many characters to
read; if you don't specify the maximum, the default is 1.  This
conversion doesn't append a null character to the end of the text it
reads.  It also does not skip over initial whitespace characters.  It
reads precisely the next N characters, and fails if it cannot get that
many.  Since there is always a maximum field width with `%c' (whether
specified, or 1 by default), you can always prevent overflow by making
the buffer long enough.

   If the format is `%lc' or `%C' the function stores wide characters
which are converted using the conversion determined at the time the
stream was opened from the external byte stream.  The number of bytes
read from the medium is limited by `MB_CUR_LEN * N' but at most N wide
character get stored in the output string.

   The `%s' conversion matches a string of non-whitespace characters.
It skips and discards initial whitespace, but stops when it encounters
more whitespace after having read something.  It stores a null character
at the end of the text that it reads.

   For example, reading the input:

      hello, world

with the conversion `%10c' produces `" hello, wo"', but reading the
same input with the conversion `%10s' produces `"hello,"'.

   *Warning:* If you do not specify a field width for `%s', then the
number of characters read is limited only by where the next whitespace
character appears.  This almost certainly means that invalid input can
make your program crash--which is a bug.

   The `%ls' and `%S' format are handled just like `%s' except that the
external byte sequence is converted using the conversion associated
with the stream to wide characters with their own encoding.  A width or
precision specified with the format do not directly determine how many
bytes are read from the stream since they measure wide characters.  But
an upper limit can be computed by multiplying the value of the width or
precision by `MB_CUR_MAX'.

   To read in characters that belong to an arbitrary set of your choice,
use the `%[' conversion.  You specify the set between the `[' character
and a following `]' character, using the same syntax used in regular
expressions.  As special cases:

   * A literal `]' character can be specified as the first character of
     the set.

   * An embedded `-' character (that is, one that is not the first or
     last character of the set) is used to specify a range of
     characters.

   * If a caret character `^' immediately follows the initial `[', then
     the set of allowed input characters is the everything _except_ the
     characters listed.

   The `%[' conversion does not skip over initial whitespace characters.

   Here are some examples of `%[' conversions and what they mean:

`%25[1234567890]'
     Matches a string of up to 25 digits.

`%25[][]'
     Matches a string of up to 25 square brackets.

`%25[^ \f\n\r\t\v]'
     Matches a string up to 25 characters long that doesn't contain any
     of the standard whitespace characters.  This is slightly different
     from `%s', because if the input begins with a whitespace character,
     `%[' reports a matching failure while `%s' simply discards the
     initial whitespace.

`%25[a-z]'
     Matches up to 25 lowercase characters.

   As for `%c' and `%s' the `%[' format is also modified to produce
wide characters if the `l' modifier is present.  All what is said about
`%ls' above is true for `%l['.

   One more reminder: the `%s' and `%[' conversions are *dangerous* if
you don't specify a maximum width or use the `a' flag, because input
too long would overflow whatever buffer you have provided for it.  No
matter how long your buffer is, a user could supply input that is
longer.  A well-written program reports invalid input with a
comprehensible error message, not with a crash.