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(libc.info)Input Conversion Syntax

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Input Conversion Syntax
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   A `scanf' template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with `%'.

   Any whitespace character (as defined by the `isspace' function;
Note: Classification of Characters) in the template causes any number
of whitespace characters in the input stream to be read and discarded.
The whitespace characters that are matched need not be exactly the same
whitespace characters that appear in the template string.  For example,
write ` , ' in the template to recognize a comma with optional
whitespace before and after.

   Other characters in the template string that are not part of
conversion specifications must match characters in the input stream
exactly; if this is not the case, a matching failure occurs.

   The conversion specifications in a `scanf' template string have the
general form:

     % FLAGS WIDTH TYPE CONVERSION

   In more detail, an input conversion specification consists of an
initial `%' character followed in sequence by:

   * An optional "flag character" `*', which says to ignore the text
     read for this specification.  When `scanf' finds a conversion
     specification that uses this flag, it reads input as directed by
     the rest of the conversion specification, but it discards this
     input, does not use a pointer argument, and does not increment the
     count of successful assignments.

   * An optional flag character `a' (valid with string conversions only)
     which requests allocation of a buffer long enough to store the
     string in.  (This is a GNU extension.)  Note: Dynamic String
     Input.

   * An optional decimal integer that specifies the "maximum field
     width".  Reading of characters from the input stream stops either
     when this maximum is reached or when a non-matching character is
     found, whichever happens first.  Most conversions discard initial
     whitespace characters (those that don't are explicitly
     documented), and these discarded characters don't count towards
     the maximum field width.  String input conversions store a null
     character to mark the end of the input; the maximum field width
     does not include this terminator.

   * An optional "type modifier character".  For example, you can
     specify a type modifier of `l' with integer conversions such as
     `%d' to specify that the argument is a pointer to a `long int'
     rather than a pointer to an `int'.

   * A character that specifies the conversion to be applied.

   The exact options that are permitted and how they are interpreted
vary between the different conversion specifiers.  See the descriptions
of the individual conversions for information about the particular
options that they allow.

   With the `-Wformat' option, the GNU C compiler checks calls to
`scanf' and related functions.  It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a `scanf'-style format string.  Note: Declaring Attributes
of Functions, for more information.