(libc.info.gz) Input Conversion Syntax

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 12.14.2 Input Conversion Syntax
 -------------------------------
 
 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;
  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.)   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.   Declaring Attributes
 of Functions (gcc.info)Function Attributes, for more information.
 

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(libc.info.gz) Table of Input Conversions