Parsing a Template String - The GNU C Library

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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.