(libc.info.gz) Parsing a Template String

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 12.12.10 Parsing a Template String
 ----------------------------------
 
 You can use the function `parse_printf_format' to obtain information
 about the number and types of arguments that are expected by a given
 template string.  This function permits interpreters that provide
 interfaces to `printf' to avoid passing along invalid arguments from
 the user's program, which could cause a crash.
 
    All the symbols described in this section are declared in the header
 file `printf.h'.
 
  -- Function: size_t parse_printf_format (const char *TEMPLATE, size_t
           N, int *ARGTYPES)
      This function returns information about the number and types of
      arguments expected by the `printf' template string TEMPLATE.  The
      information is stored in the array ARGTYPES; each element of this
      array describes one argument.  This information is encoded using
      the various `PA_' macros, listed below.
 
      The argument N specifies the number of elements in the array
      ARGTYPES.  This is the maximum number of elements that
      `parse_printf_format' will try to write.
 
      `parse_printf_format' returns the total number of arguments
      required by TEMPLATE.  If this number is greater than N, then the
      information returned describes only the first N arguments.  If you
      want information about additional arguments, allocate a bigger
      array and call `parse_printf_format' again.
 
    The argument types are encoded as a combination of a basic type and
 modifier flag bits.
 
  -- Macro: int PA_FLAG_MASK
      This macro is a bitmask for the type modifier flag bits.  You can
      write the expression `(argtypes[i] & PA_FLAG_MASK)' to extract
      just the flag bits for an argument, or `(argtypes[i] &
      ~PA_FLAG_MASK)' to extract just the basic type code.
 
    Here are symbolic constants that represent the basic types; they
 stand for integer values.
 
 `PA_INT'
      This specifies that the base type is `int'.
 
 `PA_CHAR'
      This specifies that the base type is `int', cast to `char'.
 
 `PA_STRING'
      This specifies that the base type is `char *', a null-terminated
      string.
 
 `PA_POINTER'
      This specifies that the base type is `void *', an arbitrary
      pointer.
 
 `PA_FLOAT'
      This specifies that the base type is `float'.
 
 `PA_DOUBLE'
      This specifies that the base type is `double'.
 
 `PA_LAST'
      You can define additional base types for your own programs as
      offsets from `PA_LAST'.  For example, if you have data types `foo'
      and `bar' with their own specialized `printf' conversions, you
      could define encodings for these types as:
 
           #define PA_FOO  PA_LAST
           #define PA_BAR  (PA_LAST + 1)
 
    Here are the flag bits that modify a basic type.  They are combined
 with the code for the basic type using inclusive-or.
 
 `PA_FLAG_PTR'
      If this bit is set, it indicates that the encoded type is a
      pointer to the base type, rather than an immediate value.  For
      example, `PA_INT|PA_FLAG_PTR' represents the type `int *'.
 
 `PA_FLAG_SHORT'
      If this bit is set, it indicates that the base type is modified
      with `short'.  (This corresponds to the `h' type modifier.)
 
 `PA_FLAG_LONG'
      If this bit is set, it indicates that the base type is modified
      with `long'.  (This corresponds to the `l' type modifier.)
 
 `PA_FLAG_LONG_LONG'
      If this bit is set, it indicates that the base type is modified
      with `long long'.  (This corresponds to the `L' type modifier.)
 
 `PA_FLAG_LONG_DOUBLE'
      This is a synonym for `PA_FLAG_LONG_LONG', used by convention with
      a base type of `PA_DOUBLE' to indicate a type of `long double'.
 
    For an example of using these facilities, see  Example of
 Parsing.
 

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