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A Guide To The Erlang Source

Revision as of 21:25, 19 January 2011 by Vijay Nishad (Talk | contribs)

A guide to the erlang source.



The source can be downloaded from:


Most of the interesting code is in: erts/emulator/beam


The atoms used in erlang are listed in erts/emulator/beam/atom.names and are referenced in the code as am_foo. So the 'EXIT' atom is am_EXIT in the code.

Basic types

See sys.h for the basic datatypes:

** Data types:
** Eterm: A tagged erlang term (possibly 64 bits)
** BeamInstr: A beam code instruction unit, possibly larger than Eterm, not smaller.
** UInt:  An unsigned integer exactly as large as an Eterm.
** SInt:  A signed integer exactly as large as an eterm and therefor large
**        enough to hold the return value of the signed_val() macro.
** UWord: An unsigned integer at least as large as a void * and also as large
**          or larger than an Eterm
** SWord: A signed integer at least as large as a void * and also as large
**          or larger than an Eterm
** Uint32: An unsigned integer of 32 bits exactly
** Sint32: A signed integer of 32 bits exactly
** Uint16: An unsigned integer of 16 bits exactly
** Sint16: A signed integer of 16 bits exactly.

See erts/emulator/beam/big.c for the conversion between types. For example:

 uint_to_big(Uint x, Eterm *y).
 term_to_Uint(Eterm term, Uint *up).

An Eterm can contain any erlang term like atoms, integers, etc. Some util methods can be found in erl_term.h to get the value out:


More util methods in


The bifs are summed up in the file. For example: bif 'erl.lang':exit/1 ebif_exit_1 bif erlang:exit/2

This means the exit bif is mapped to the exit_1 method in the bif.c file. The bif.c file holds the bif implementations like:

BIF_RETTYPE spawn_3(BIF_ALIST_3) {...

BIF_ALIST_3 means you have BIF_P, BIF_ARG1 to 3 and The arguments BIF_ARG1 are Eterms, so you have to check them with

is_number(BIF_ARG_1), is_atom, is_tuple, is_list etc

As defined in bif.h:

#define BIF_RETTYPE Eterm

#define BIF_P A__p

#define BIF_ALIST_0 Process* A__p
#define BIF_ALIST_1 Process* A__p, Eterm A_1
#define BIF_ALIST_2 Process* A__p, Eterm A_1, Eterm A_2
#define BIF_ALIST_3 Process* A__p, Eterm A_1, Eterm A_2, Eterm A_3

#define BIF_ARG_1  A_1
#define BIF_ARG_2  A_2
#define BIF_ARG_3  A_3

To bif to set process flags:

BIF_RETTYPE process_flag_2(BIF_ALIST_2)
   Eterm old_value;

   else if (BIF_ARG_1 == am_priority) {
       erts_smp_proc_lock(BIF_P, ERTS_PROC_LOCK_STATUS);
       old_value = erts_set_process_priority(BIF_P, BIF_ARG_2);
       erts_smp_proc_unlock(BIF_P, ERTS_PROC_LOCK_STATUS);
       if (old_value == THE_NON_VALUE)
	   goto error;

am_priority is one of the predefined atoms (like am_EXIT). BIF_ARG_1 is an Eterm that can contain the erlang atom 'priority'. BIF_RETTYPE is an Eterm: we return the old priority as an Eterm.

More bifs in bif.c (these are called in erlang like erlang:register(,...))

  • process_flag/2
  • process_flag/3, ...
  • register/2
  • whereis/1
  • exit/2

More types


  • ErlSpawnOpts
  • Process (see erl_process.h )
  • ErtsProcLocks
  • ErtsMonitor
  • ErtsRunQueue

SMP You will see this a lot in the code: #ifdef ERTS_SMP Things get more complicated, with regards to locking etc. if SMP is enabled.




The process struct is defined in erts/emulator/beam/process.h. The important function is Process *schedule(Process *p, int calls) in process.c . The next process is picked in "pick_next_process" of that method.


The main struct is Process as defined in erl_process.h, Each process has its own heap. Some variables:

  • int prio
  • Uint reds
  • ErlMessageQueue msg

Process statuses are:

  1. define P_FREE 0
  2. define P_RUNABLE 1
  3. define P_WAITING 2
  4. define P_RUNNING 3
  5. define P_EXITING 4
  6. define P_GARBING 5
  7. define P_SUSPENDED 6

Monitors and links

See erl_monitor.c and .h. The main structs are ErtsLinks and ErtsMonitor. Each process has a *ErtLinks and *ErtsMonitor (herein all its links/monitors are kept as an AVL tree). Actions are done with erts_sweep_links(ErtsLink *root, ...) and erts_sweep_monitors(ErtsMonitor *root, ...).