Debugging within the FreeType sources
=====================================

I. Configuration macros
-----------------------

There are  several ways to enable  debugging features in a  FreeType 2
builds.  This is  controlled through the definition  of special macros
located in the file `ftoption.h'.  The macros are:


  FT_DEBUG_LEVEL_ERROR

    #define this  macro if  you want to  compile the  `FT_ERROR' macro
    calls to print error messages during program execution.  This does
    not stop  the program.  Very  useful to spot invalid  fonts during
    development and to code workarounds for them.

  FT_DEBUG_LEVEL_TRACE

    #define this macro  if you want to compile  both macros `FT_ERROR'
    and  `FT_TRACE'.  This  also  includes  the variants  `FT_TRACE0',
    `FT_TRACE1', `FT_TRACE2', ..., `FT_TRACE7'.

    The  trace macros  are used  to  send debugging  messages when  an
    appropriate  `debug level'  is configured  at runtime  through the
    `FT2_DEBUG' environment variable (more on this later).

  FT_LOGGING

    As    mentioned   above    if       `FT_DEBUG_LEVEL_ERROR'     and 
    `FT_DEBUG_LEVEL_TRACE'  macros are defined, FreeType will send the
    debug and trace logs outputted from `FT_ERROR' and `FT_TRACE' 
    macros to   `stderr'.
    
    `FT_LOGGING' is a  new  macro  which is  introduced to enhance the
    capabilities of `FT_TRACE' and `FT_ERROR' macros.
    `FT_LOGGING'  macro    depends on   `FT_DEBUG_LEVEL_TRACE'     and
    `FT_DEBUG_LEVEL_ERROR', therefore these macros will  automatically
    be   #defined   once   `FT_LOGGING'   macro   is    #defined   and
    the behaviour of   `FT_TRACE'  and  `FT_ERROR' macros will change.
    Now,  `FT_TRACE'  and  `FT_ERROR'  macros  will start  sending the
    tracing  and  debugging   messages  to  the file system instead of
    `stderr'.
    On  the  file system  the  log  file will be generated on the path 
    specified  using  `FT_LOGGING_FILE'  environment  variable(more on
    this later).

    There is an additional support  of  logging  Time and the  name of
    the `FT_COMPONENT' from which the current  log is  generated along
    with the actual  tracing and  debugging message, and for this user
    will  have  to  define  additional  tags  under  the  `FT2_DEBUG' 
    environment  variable(More details under section `FT2_DEBUG' under
    Environment Variables heading).
    
  
  FT_DEBUG_MEMORY

    If this  macro is #defined, the  FreeType engine is linked  with a
    small  but  effective debugging  memory  manager  that tracks  all
    allocations and frees that are performed within the font engine.

    When  the `FT2_DEBUG_MEMORY'  environment variable  is defined  at
    runtime,  a call  to `FT_Done_FreeType'  dumps memory  statistics,
    including the list of leaked memory blocks and optionally with the
    source locations where these were  allocated.  It is always a very
    good idea to  define this in development builds.   This works with
    _any_  program linked  to FreeType,  but  requires a  big deal  of
    memory (the debugging memory manager never frees the blocks to the
    heap in order to detect double frees).

    When `FT2_DEBUG_MEMORY'  isn't defined  at runtime,  the debugging
    memory manager is ignored, and performance is unaffected.


II. Debugging macros
--------------------

Several  macros  can be  used  within  the  FreeType sources  to  help
debugging its code:


  1. FT_ERROR(( ... ))

    This macro is used to send debug messages that indicate relatively
    serious  errors  (like broken  font  files)  without stopping  the
    execution of the running program.   Its code is compiled only when
    either   `FT_DEBUG_LEVEL_ERROR'   or  `FT_DEBUG_LEVEL_TRACE'   are
    defined in `ftoption.h'.

    Note that you have to use a printf-like signature, but with double
    parentheses, like in

      FT_ERROR(( "your %s is not %s\n", "foo", "bar" ));


  2. FT_ASSERT( condition )

    This macro is used to check  strong assertions at runtime.  If its
    condition isn't  TRUE, the  program aborts  with a  panic message.
    Its  code  is  compiled   when  either  `FT_DEBUG_LEVEL_ERROR'  or
    `FT_DEBUG_LEVEL_TRACE'  are   defined.   You  don't   need  double
    parentheses here.  Example:

      FT_ASSERT( ptr != NULL );


  3. FT_TRACE( level, (message...) )

    The  `FT_TRACE' macro  is used  to send  general-purpose debugging
    messages during program execution.   This macro uses an *implicit*
    macro  named  `FT_COMPONENT',  which names  the  current  FreeType
    component being run.

    The developer should always  define `FT_COMPONENT' as appropriate,
    for example as in

      #undef  FT_COMPONENT
      #define FT_COMPONENT  io

    The  value of  the `FT_COMPONENT'  macro is  one of  the component
    names defined  in the internal file  `internal/fttrace.h'.  If you
    modify the  FreeType source code  and insert a  new `FT_COMPONENT'
    macro,  you must  register it  in `fttrace.h'.   If you  insert or
    remove many  trace macros,  you can test  for undefined  or unused
    trace macros with the script `src/tools/chktrcmp.py'.

    Each  such component  is assigned  a `debug  level', ranging  from
    value  0 to  7, through  the  use of  the `FT2_DEBUG'  environment
    variable  (described below)  when a  program linked  with FreeType
    starts.

    When `FT_TRACE' is called, its level is compared to the one of the
    corresponding component.  Messages with trace levels *higher* than
    the  corresponding  component level  are  filtered  out and  never
    printed.  This means  that trace messages with level  0 are always
    printed, those  with level 2  are only printed when  the component
    level is *at least* 2, etc.

    The second  parameter to  `FT_TRACE' must contain  parentheses and
    corresponds to a printf-like call, as in

      FT_TRACE( 2, ( "your %s is not %s\n", "foo", "bar" ) )

    The  shortcut macros  `FT_TRACE0', `FT_TRACE1',  `FT_TRACE2', ...,
    `FT_TRACE7' can be used with  constant level indices, and are much
    cleaner to use, as in

      FT_TRACE2(( "your %s is not %s\n", "foo", "bar" ));


III. Environment variables
--------------------------

The  following  environment  variables control  debugging  output  and
behaviour of FreeType at runtime.


  FT2_DEBUG

    This  variable   is  only  used   when  FreeType  is   built  with
    `FT_DEBUG_LEVEL_TRACE' defined.   It contains a list  of component
    level definitions, following this format:

      component1:level1 component2:level2 component3:level3 ...

    where `componentX' is the name  of a tracing component, as defined
    in `fttrace.h'.  `levelX' is  the corresponding level  to  use  at
    runtime.

    `any' is a special component  name that is interpreted as `any/all
    components'.  For example, the following definitions

      set FT2_DEBUG=any:2 memory:5 io:4        (on Windows)
      export FT2_DEBUG="any:2 memory:5 io:4"   (on Linux with bash)

    both stipulate that all components should have level 2, except for
    the memory and io components, which  are set to the trace levels 5
    and 4, respectively.

    `FT_LOGGING' related tags:

      As mentioned above in the  `FT_LOGGING'  section  below are  the
      additional tags which can be  added in  `FT2_DEBUG'  environment
      variable to log the time and the value of  `FT_COMPONENT' in the
      actual log message: 
        * -v: For printing the name of FreeType's component from which
        the current log is produced.
        * -t: For printing the time(in milliseconds) along with actual
        log message.

        Example:
        
          Case 1: FT2_DEBUG="any:7 memory:5 -vt"

            [20:32:02:44969 ttload]    table directory loaded

          Case 2: FT2_DEBUG="any:7 memory:5 -t"

            [20:32:02:44969 ttload]    table directory loaded

          Case 3: FT2_DEBUG="any:7 memory:5 -v"

            [ttload]    table directory loaded

  FT_LOGGING_FILE

    This  variable   is  only  used   when  FreeType  is   built  with
    `FT_LOGGING' macro defined. It contains the path to the file where
    the user wants to put his log file.

      On Linux with bash:
      export FT_LOGGING_FILE="/tmp/freetype2.log"      

      On Windows:
      set FT_LOGGING_FILE=C:\Users\AppData\Local\Temp\freetype2.log

  FT2_DEBUG_MEMORY

    This environment variable,  when defined, tells FreeType  to use a
    debugging memory manager that tracks leaking memory blocks as well
    as other common  errors like double frees.  It is  also capable of
    reporting  _where_  the  leaking   blocks  were  allocated,  which
    considerably  saves  time  when  debugging new  additions  to  the
    library.

    This  code  is only  compiled  when  FreeType  is built  with  the
    `FT_DEBUG_MEMORY'  macro #defined  in `ftoption.h'  though, it  is
    ignored in other builds.


  FT2_ALLOC_TOTAL_MAX

    This variable is ignored if `FT2_DEBUG_MEMORY' is not defined.  It
    allows  you  to  specify  a  maximum  heap  size  for  all  memory
    allocations performed  by FreeType.  This  is very useful  to test
    the robustness  of the  font engine  and programs  that use  it in
    tight memory conditions.

    If it is  undefined, or if its value is  not strictly positive, no
    allocation bounds are checked at runtime.


  FT2_ALLOC_COUNT_MAX

    This variable is ignored if `FT2_DEBUG_MEMORY' is not defined.  It
    allows  you to  specify  a maximum  number  of memory  allocations
    performed    by    FreeType    before    returning    the    error
    `FT_Err_Out_Of_Memory'.  This is useful  for debugging and testing
    the engine's robustness.

    If it is  undefined, or if its value is  not strictly positive, no
    allocation bounds are checked at runtime.


  FT2_KEEP_ALIVE

    This  variable is  ignored if  `FT2_DEBUG_MEMORY' is  not defined.
    `Keep alive' means that freed  blocks aren't released to the heap.
    This is  useful to detect  double-frees or weird  heap corruption,
    reporting the source code location  of the original allocation and
    deallocation  in case  of a  problem.   It uses  large amounts  of
    memory, however.

    If it  is undefined,  or if  its value  is not  strictly positive,
    freed blocks are released at runtime.


IV. Additional Capabilities with `FT_LOGGING'
--------------------------------------------

With the support of  `FT_LOGGING', four  APIs have been introduced for
providing  additional  debugging  support  to the  applications  using
FreeType.

  FT_Trace_Set_Level( const char* )
    
    If FreeType is configured with  `FT_LOGGING' the levels of tracing
    component set using `FT2_DEBUG' environment variable is considered
    as a default value.
    And  this  function can be used change/set the value of the levels
    of tracing components at run-time.

    NOTE: If the  environment  variable  `FT2_DEBUG is not defined the
    default value of  the levels of  tracing  components will be NULL.

  FT_Trace_Set_Default_Level():

    If previously, `FT_Trace_Set_Level'  functions is used  to set new
    values of the tracing components of  FreeType, this function could
    be  used to reset the level of  tracing  components to the default
    value.
    Hence,   the   functions   `FT_Trace_Set_Default_Level'  and
    `FT_Trace_Set_Level'  could  be  combined  and  used  together  to
    change the  trace levels of FreeType's components for a particular 
    block of code.
  
  FT_Set_Log_Handler( ft_custom_log_handler ):

    This API has been provided under the `FT_LOGGING' macro to support
    a feature  wherein   a custom log handler can be provided based on
    user’s choice. Doing so,  FreeType  will start  calling the custom
    handler  for  new  log  messages  and  will  no longer write these
    messages   in   predefined   log file of FreeType. It will totally
    depend on user how the log messages will be handled. 
    User   just   need to call this   API  with an  argument  of  type 
    ft_custom_log_handler   which   should  match  following  function
    prototype:

     void  (*ft_custom_log_handler)( const char* ft_component,       \
                                     const char* fmt, va_list args ); 


  FT_Set_Default_Log_Handler():

    If the user has chosen to set  a custom  log  handler  using   the
    `FT_Set_Log_Handler'  API   above,  the current API can be used to
    reset back the log handler to the Freetype's  inbuilt log handler. 
    Hence, the    functions     `FT_Set_Default_Log_Handler'       and
    `FT_Set_Log_Handler'  in  combination   can be used  to separately
    handle  the  trace or  error logs  generated by a particular block
    of code.


Applications need to  #include  the following header file to avail the
above APIs:

    #include<freetype/ftlogging.h>


------------------------------------------------------------------------

Copyright (C) 2002-2020 by
David Turner, Robert Wilhelm, and Werner Lemberg.

This  file is  part  of the  FreeType  project, and  may  only be  used,
modified,  and  distributed under  the  terms  of  the FreeType  project
license, LICENSE.TXT.  By continuing  to use, modify, or distribute this
file  you indicate that  you have  read the  license and  understand and
accept it fully.


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