From Erlang Community

Current revision

A guide to the erlang source.

[edit] Download

The source can be downloaded from: https://github.com/erlang/otp

[edit] Tree

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

[edit] Atoms

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.

[edit] 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:

is_atom
is_fun
...

More util methods in

[edit] Trivia

• rp = receiving process / registered process
• cp = calling process
• rq = runqueue

[edit] BIFs

The bifs are summed up in the bif.tab 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;
       BIF_RET(old_value);
   }

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, ...
• process_info/1 and /2
• register/2
• whereis/1
• exit/2

[edit] More types

structs

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

[edit] SMP

http://en.wikipedia.org/wiki/Symmetric_multiprocessing 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.

[edit] Locking

...

[edit] Scheduler

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.

[edit] Process

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
• Eterm id (the pid)
• Uint flags

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

[edit] 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, ...).