Chapter 11 The runtime system (ocamlrun)
The ocamlrun command executes bytecode files produced by the
linking phase of the ocamlc command.
The ocamlrun command comprises three main parts: the bytecode
interpreter, that actually executes bytecode files; the memory
allocator and garbage collector; and a set of C functions that
implement primitive operations such as input/output.
The usage for ocamlrun is:
ocamlrun options bytecode-executable arg1 ... argn
The first non-option argument is taken to be the name of the file
containing the executable bytecode. (That file is searched in the
executable path as well as in the current directory.) The remaining
arguments are passed to the OCaml program, in the string array
Sys.argv. Element 0 of this array is the name of the
bytecode executable file; elements 1 to n are the remaining
arguments arg1 to argn.
As mentioned in chapter 9, the bytecode executable files
produced by the ocamlc command are self-executable, and manage to
launch the ocamlrun command on themselves automatically. That is,
assuming a.out is a bytecode executable file,
a.out arg1 ... argn
works exactly as
ocamlrun a.out arg1 ... argn
Notice that it is not possible to pass options to ocamlrun when
invoking a.out directly.
Under several versions of Windows, bytecode executable files are
self-executable only if their name ends in .exe. It is recommended
to always give .exe names to bytecode executables, e.g. compile
with ocamlc -o myprog.exe ... rather than ocamlc -o myprog ....
The following command-line options are recognized by ocamlrun.
When the program aborts due to an uncaught exception, print a detailed
“back trace” of the execution, showing where the exception was
raised and which function calls were outstanding at this point. The
back trace is printed only if the bytecode executable contains
debugging information, i.e. was compiled and linked with the -g
option to ocamlc set. This is equivalent to setting the b flag
in the OCAMLRUNPARAM environment variable (see below).
- -I dir
Search the directory dir for dynamically-loaded libraries,
in addition to the standard search path (see
Print the magic number of the bytecode executable given as argument
Print the magic number expected by this version of the runtime and exit.
Print the names of the primitives known to this version of
ocamlrun and exit.
Direct the memory manager to print some progress messages on
standard error. This is equivalent to setting v=63 in the
OCAMLRUNPARAM environment variable (see below).
Print version string and exit.
Print short version number and exit.
The following environment variables are also consulted:
- Additional directories to search for
dynamically-loaded libraries (see section 11.3).
- The directory containing the OCaml standard
library. (If OCAMLLIB is not set, CAMLLIB will be used instead.)
Used to locate the ld.conf configuration file for
dynamic loading (see section 11.3). If not set,
default to the library directory specified when compiling OCaml.
- Set the runtime system options
and garbage collection parameters.
(If OCAMLRUNPARAM is not set, CAMLRUNPARAM will be used instead.)
This variable must be a sequence of parameter specifications separated
A parameter specification is an option letter followed by an =
sign, a decimal number (or an hexadecimal number prefixed by 0x),
and an optional multiplier. The options are documented below;
the last six correspond to the fields of the
control record documented in
The multiplier is k, M, or G, for multiplication by 210,
220, and 230 respectively.
- (backtrace) Trigger the printing of a stack backtrace
when an uncaught exception aborts the program.
This option takes no argument.
- (parser trace) Turn on debugging support for
ocamlyacc-generated parsers. When this option is on,
the pushdown automaton that executes the parsers prints a
trace of its actions. This option takes no argument.
- (randomize) Turn on randomization of all hash tables by default
This option takes no argument.
- The initial size of the major heap (in words).
- (allocation_policy) The policy used for allocating in the
OCaml heap. Possible values are 0 for the next-fit policy, and 1
for the first-fit policy. Next-fit is usually faster, but first-fit
is better for avoiding fragmentation and the associated heap
- (minor_heap_size) Size of the minor heap. (in words)
- (major_heap_increment) Default size increment for the
major heap. (in words)
- (space_overhead) The major GC speed setting.
- (max_overhead) The heap compaction trigger setting.
- (stack_limit) The limit (in words) of the stack size.
- (verbose) What GC messages to print to stderr. This
is a sum of values selected from the following:
1 (= 0x001)
- Start of major GC cycle.
- 2 (= 0x002)
- Minor collection and major GC slice.
- 4 (= 0x004)
- Growing and shrinking of the heap.
- 8 (= 0x008)
- Resizing of stacks and memory manager tables.
- 16 (= 0x010)
- Heap compaction.
- 32 (= 0x020)
- Change of GC parameters.
- 64 (= 0x040)
- Computation of major GC slice size.
- 128 (= 0x080)
- Calling of finalization functions
- 256 (= 0x100)
- Startup messages (loading the bytecode
executable file, resolving shared libraries).
- 512 (= 0x200)
- Computation of compaction-triggering condition.
- 1024 (= 0x400)
- Output GC statistics at program exit.
- (cleanup_on_exit) Shut the runtime down gracefully on exit (see
caml_shutdown in section 20.7.5). The option also enables
pooling (as in caml_startup_pooled). This mode can be used to detect
leaks with a third-party memory debugger.
- (custom_major_ratio) Target ratio of floating garbage to
major heap size for out-of-heap memory held by custom values
(e.g. bigarrays) located in the major heap. The GC speed is adjusted
to try to use this much memory for dead values that are not yet
collected. Expressed as a percentage of major heap size. Default:
44. Note: this only applies to values allocated with
- (custom_minor_ratio) Bound on floating garbage for
held by custom values in the minor heap. A minor GC is triggered
when this much memory is held by custom values located in the minor
heap. Expressed as a percentage of minor heap size. Default:
100. Note: this only applies to values allocated with
- (custom_minor_max_size) Maximum amount of out-of-heap
memory for each custom value allocated in the minor heap. When a custom
value is allocated on the minor heap and holds more than this many
bytes, only this value is counted against custom_minor_ratio and
the rest is directly counted against custom_major_ratio.
Default: 8192 bytes. Note:
this only applies to values allocated with caml_alloc_custom_mem.
If the option letter is not recognized, the whole parameter is ignored;
if the equal sign or the number is missing, the value is taken as 1;
if the multiplier is not recognized, it is ignored.
For example, on a 32-bit machine, under bash the command
tells a subsequent ocamlrun to print backtraces for uncaught exceptions,
set its initial minor heap size to 1 megabyte and
print a message at the start of each major GC cycle, when the heap
size changes, and when compaction is triggered.
- If OCAMLRUNPARAM is not found in the
environment, then CAMLRUNPARAM will be used instead. If
CAMLRUNPARAM is also not found, then the default values will be used.
- List of directories searched to find the bytecode
11.3 Dynamic loading of shared libraries
On platforms that support dynamic loading, ocamlrun can link
dynamically with C shared libraries (DLLs) providing additional C primitives
beyond those provided by the standard runtime system. The names for
these libraries are provided at link time as described in
section 20.1.4), and recorded in the bytecode executable
file; ocamlrun, then, locates these libraries and resolves references
to their primitives when the bytecode executable program starts.
The ocamlrun command searches shared libraries in the following
directories, in the order indicated:
Directories specified on the ocamlrun command line with the
- Directories specified in the CAML_LD_LIBRARY_PATH environment
- Directories specified at link-time via the -dllpath option to
ocamlc. (These directories are recorded in the bytecode executable
- Directories specified in the file ld.conf. This file resides
in the OCaml standard library directory, and lists directory
names (one per line) to be searched. Typically, it contains only one
line naming the stublibs subdirectory of the OCaml standard
library directory. Users can add there the names of other directories
containing frequently-used shared libraries; however, for consistency
of installation, we recommend that shared libraries are installed
directly in the system stublibs directory, rather than adding lines
to the ld.conf file.
- Default directories searched by the system dynamic loader.
Under Unix, these generally include /lib and /usr/lib, plus the
directories listed in the file /etc/ld.so.conf and the environment
variable LD_LIBRARY_PATH. Under Windows, these include the Windows
system directories, plus the directories listed in the PATH
11.4 Common errors
This section describes and explains the most frequently encountered
- filename: no such file or directory
If filename is the name of a self-executable bytecode file, this
means that either that file does not exist, or that it failed to run
the ocamlrun bytecode interpreter on itself. The second possibility
indicates that OCaml has not been properly installed on your
- Cannot exec ocamlrun
(When launching a self-executable bytecode file.) The ocamlrun
could not be found in the executable path. Check that OCaml
has been properly installed on your system.
- Cannot find the bytecode file
The file that ocamlrun is trying to execute (e.g. the file given as
first non-option argument to ocamlrun) either does not exist, or is
not a valid executable bytecode file.
- Truncated bytecode file
The file that ocamlrun is trying to execute is not a valid executable
bytecode file. Probably it has been truncated or mangled since
created. Erase and rebuild it.
- Uncaught exception
The program being executed contains a “stray” exception. That is,
it raises an exception at some point, and this exception is never
caught. This causes immediate termination of the program. The name of
the exception is printed, along with its string, byte sequence, and
(arguments of more complex types are not correctly printed).
To locate the context of the uncaught exception, compile the program
with the -g option and either run it again under the ocamldebug
debugger (see chapter 17), or run it with ocamlrun -b
or with the OCAMLRUNPARAM environment variable set to b=1.
- Out of memory
The program being executed requires more memory than available. Either
the program builds excessively large data structures; or the program
contains too many nested function calls, and the stack overflows. In
some cases, your program is perfectly correct, it just requires more
memory than your machine provides. In other cases, the “out of
memory” message reveals an error in your program: non-terminating
recursive function, allocation of an excessively large array,
string or byte sequence, attempts to build an infinite list or other
data structure, …
To help you diagnose this error, run your program with the -v option
to ocamlrun, or with the OCAMLRUNPARAM environment variable set to
v=63. If it displays lots of “Growing stack…”
messages, this is probably a looping recursive function. If it
displays lots of “Growing heap…” messages, with the heap size
growing slowly, this is probably an attempt to construct a data
structure with too many (infinitely many?) cells. If it displays few
“Growing heap…” messages, but with a huge increment in the
heap size, this is probably an attempt to build an excessively large
array, string or byte sequence.