FreeType 2.0 I/O Frames

© 2000 David Turner (david@freetype.org)
© 2000 The FreeType Development Team (www.freetype.org)

Introduction

This document explains the concept of I/O frames as used in the FreeType 2 source code. It also enumerates the various functions and macros that can be used to read them.

It is targeted to FreeType hackers, or more simply to developers who would like a better understanding of the library's source code.

I. What frames are

Simply speaking, a frame is an array of bytes in a font file that is `preloaded' into memory in order to be rapidly parsed. Frames are useful to ensure that every `load' is checked against end-of-file overruns, and provides nice functions to extract data in a variety of distinct formats.

But an example is certainly more meaningful than anything else. The following code

    error = read_short( stream, &str.value1 );
    if ( error ) goto ...

    error = read_ulong( stream, &str.value2 );
    if ( error ) goto ...

    error = read_ulong( stream, &str.value3 );
    if ( error ) goto ...

can easily be replaced with

    error = FT_Access_Frame( stream, 2 + 4 + 4 );
    if ( error ) goto ...

    str.value1 = FT_Get_Short( stream );
    str.value2 = FT_Get_ULong( stream );
    str.value3 = FT_Get_ULong( stream );

    FT_Forget_Frame( stream );

Here, the call to FT_Access_Frame() will

Ensure that there are at least 2+4+4=10 bytes left in the stream.
`Preload' (for disk-based streams) 10 bytes from the current stream position.
Set the frame `cursor' to the first byte in the frame.

Each FT_Get_Short() or FT_Get_ULong() call will read a big-endian integer from the stream (2 bytes for FT_Get_Short(), 4 bytes for FT_Get_ULong) and advance the frame cursor accordingly.

FT_Forget_Frame() `releases' the frame from memory.

There are several advantages to using frames:

Single-check when loading tables.
Making code clearer by providing simple parsing functions while keeping code safe from file over-runs and invalid offsets.

II. Accessing and reading a frame with macros

By convention in the FreeType source code, macros are able to use two implicit variables named error and stream. This is useful because these two variables are extremely often used in the library, and doing this only reduces our typing requirements and make the source code much clearer.

Note that error must be a local variable of type FT_Error, while stream must be a local variable or argument of type FT_Stream.

The macro used to access a frame is ACCESS_Frame(_size_), it will translate to

    ( error = FT_Access_Frame( stream, _size_ ) )
        != FT_Err_Ok

Similarly, the macro FORGET_Frame() translates to

    FT_Forget_Frame( stream )

Extracting integers can be performed with the GET_xxx() macros, like

Macro name	Translation	Description
`GET_Byte()`	`FT_Get_Byte(stream)`	Reads an 8-bit unsigned byte.
`GET_Char()`	`(FT_Char) FT_Get_Byte(stream)`	Reads an 8-bit signed byte.
`GET_Short()`	`FT_Get_Short(stream)`	Reads a 16-bit signed big-endian integer.
`GET_UShort()`	`(FT_UShort) FT_Get_Short(stream)`	Reads a 16-bit unsigned big-endian integer.
`GET_Offset()`	`FT_Get_Offset(stream)`	Reads a 24-bit signed big-endian integer.
`GET_UOffset()`	`(FT_UOffset) FT_Get_Offset(stream)`	Reads a 24-bit unsigned big-endian integer.
`GET_Long()`	`FT_Get_Long(stream)`	Reads a 32-bit signed big-endian integer.
`GET_ULong()`	`(FT_ULong) FT_Get_Long(stream)`	Reads a 32-bit unsigned big-endian integer.

(Note that an Offset is an integer stored with 3 bytes on the file.)

All this means that the following code

    error = FT_Access_Frame( stream, 2 + 4 + 4 );
    if ( error ) goto ...

    str.value1 = FT_Get_Short( stream );
    str.value2 = FT_Get_ULong( stream );
    str.value3 = FT_Get_ULong( stream );

    FT_Forget_Frame( stream );

can be simplified with macros:

    if ( ACCESS_Frame( 2 +4 + 4 ) ) goto ...

    str.value1 = GET_Short();
    str.value2 = GET_ULong();
    str.value3 = GET_ULong();

    FORGET_Frame();

Which is clearer. Notice that error and stream must be defined locally though for this code to work!

III. Alternatives

It is sometimes useful to read small integers from a font file without using a frame. Some functions have been introduced in FreeType 2 to do just that, and they are of the form FT_Read_xxxx.

For example, FT_Read_Short(stream, &error) reads and returns a 2-byte big-endian integer from a stream, and places an error code in the error variable.

Thus, reading a single big-endian integer is shorter than using a frame for it.

Note that there are also macros READ_xxx() which translate to

    FT_Read_xxx( stream, &error ), error != FT_Err_Ok

and can be used as in

    if ( READ_UShort( variable1 ) ||
         READ_ULong ( variable2 ) )
      goto Fail;