Build options and Environment Variables
Makefiles are provided for a variety of simulators in
The common Makefile
src/cocotb/share/makefiles/Makefile.sim includes the appropriate simulator Makefile based on the contents of the
Makefiles defines the targets
sim, the default target is
Both rules create a results file with the name taken from
COCOTB_RESULTS_FILE, defaulting to
This file is a xUnit-compatible output file suitable for use with e.g. Jenkins.
sim targets unconditionally re-runs the simulator whereas the
regression target only re-builds if any dependencies have changed.
In addition, the target
clean can be used to remove build and simulation artifacts.
help lists these available targets and the variables described below.
Typically the makefiles provided with cocotb for various simulators use a separate
This allows for a rapid re-running of a simulator if none of the RTL source files have changed and therefore the simulator does not need to recompile the RTL.
The following sections document environment variables and makefile variables according to their owner/consumer.
Of the environment variables, only
MODULE is mandatory to be set
(typically done in a makefile or run script), all others are optional.
Use this to indicate the instance in the hierarchy to use as the DUT. If this isn’t defined then the first root instance is used. Leading and trailing whitespace are automatically discarded.
Changed in version 1.6.0: Strip leading and trailing whitespace
Seed the Python random module to recreate a previous test stimulus. At the beginning of every test a message is displayed with the seed used for that execution:
INFO cocotb.gpi __init__.py:89 in _initialise_testbench Seeding Python random module with 1377424946
To recreate the same stimuli use the following:
Use this to override the default behavior of annotating cocotb output with ANSI color codes if the output is a terminal (
forces output to be ANSI-colored regardless of the type of
stdoutor the presence of
suppresses the ANSI color output in the log messages
All command-line software which outputs text with ANSI color added should check for the presence of a
NO_COLORenvironment variable that, when present (regardless of its value), prevents the addition of ANSI color.
Defaults internally to
1. If the value is
1, log lines displayed in the terminal will be shorter. It will print only time, message type (
ERROR, …) and the log message itself. Set to
0if you wish to have the full format message.
In order to give yourself time to attach a debugger to the simulator process before it starts to run, you can set the environment variable
COCOTB_ATTACHto a pause time value in seconds. If set, cocotb will print the process ID (PID) to attach to and wait the specified time before actually letting the simulator run.
Enable performance analysis of the Python portion of cocotb. When set, a file
test_profile.pstatwill be written which contains statistics about the cumulative time spent in the functions.
From this, a callgraph diagram can be generated with gprof2dot and
The default logging level to use. This is set to
INFOunless overridden. Valid values are
TRACEis used for internal low-level logging and produces very verbose logs.
Defines how to resolve bits with a value of
Wwhen being converted to integer. Valid settings are:
randomly resolve to a
The absolute path to the Python library associated with the current Python installation; i.e.
python.dllon Windows. This is determined with
cocotb-config --libpythonin cocotb’s makefiles.
The name of the Python module(s) to search for test functions - if your tests are in a file called
MODULEwould be set to
test_mydesign. Multiple modules can be specified using a comma-separated list. All tests will be run from each specified module in order of the module’s appearance in this list.
The is the only environment variable that is required for cocotb, all others are optional.
Multiple test function regexes can be specified using a comma-separated list.
The file name where xUnit XML tests results are stored. If not provided, the default is
New in version 1.3.
Enable to report Python coverage data. For some simulators, this will also report HDL coverage.
This needs the
coveragePython module to be installed.
Enable additional log output of the coroutine scheduler.
A comma-separated list of extra libraries that are dynamically loaded at runtime. A function from each of these libraries will be called as an entry point prior to elaboration, allowing these libraries to register system functions and callbacks. Note that HDL objects cannot be accessed at this time. An entry point function must be named following a
:separator, which follows an existing simulator convention.
GPI_EXTRA=libnameA.so:entryA,libnameB.so:entryBwill first load
libnameA.sowith entry point
entryA, then load
libnameB.sowith entry point
Changed in version 1.4.0: Support for the custom entry point via
:was added. Previously
:was used as a separator between libraries instead of
Changed in version 1.5.0: Library name must be fully specified. This allows using relative or absolute paths in library names, and loading from libraries that aren’t prefixed with “lib”. Paths should not contain commas.
PyGPI is an experimental feature and subject to change.
The Python module and callable that starts up the Python cosimulation environment. This defaults to
cocotb:_initialise_testbench, which is the cocotb standard entry point. User overloads can be used to enter alternative Python frameworks or to hook existing cocotb functionality. The variable is formatted as
path.to.entry.module:entry_point.function. The string before the colon is the Python module to import and the string following the colon is the object to call as the entry function.
The entry function must be a callable matching this form:
entry_function(argv: List[str]) -> None
The entry module must have the following additional functions defined. These additional requirements on the entry module may be relaxed over time.
_sim_event(message: str) -> None
_log_from_c(logger_name: str, level: int, filename: str, lineno: int, msg: str, function_name: str)) -> None
_filter_from_c(logger_name: str, level: int) -> bool
Changed in version 1.8:
_sim_eventis no longer passed, it is assumed to be SIM_FAIL (2).
Makefile-based Test Scripts
The following variables are makefile variables, not environment variables.
Set this to 1 to enable the GUI mode in the simulator (if supported).
Selects which simulator Makefile to use. Attempts to include a simulator specific makefile from
Set this to 1 to enable wave traces dump for the Aldec Riviera-PRO, Mentor Graphics Questa, and Icarus Verilog simulators. To get wave traces in Verilator see Waveforms.
Used to inform the makefile scripts which HDL language the top-level design element is written in. Currently it supports the values
verilogfor Verilog or SystemVerilog tops, and
vhdlfor VHDL tops. This is used by simulators that support more than one interface (VPI, VHPI, or FLI) to select the appropriate interface to start cocotb.
The variable is also made available to cocotb tests conveniently as
Explicitly sets the simulator interface to use when
vhdl. This includes the initial GPI interface loaded, and
GPI_EXTRAlibrary loaded in mixed language simulations. Valid values are
fli. Not all simulators support all values; refer to the Simulator Support section for details.
Setting this variable is rarely needed as cocotb chooses a suitable default value depending on the simulator used.
A list of the Verilog source files to include. Paths can be absolute or relative; if relative, they are interpreted as relative to the location where
A list of the Verilog directories to search for include files. Paths can be absolute or relative; if relative, they are interpreted as relative to the location where
A list of the VHDL source files to include. Paths can be absolute or relative; if relative, they are interpreted as relative to the location where
A list of the VHDL source files to include in the VHDL library lib (currently for GHDL/ModelSim/Questa/Xcelium/Incisive only).
A space-separated list defining the order in which VHDL libraries should be compiled (needed for ModelSim/Questa/Xcelium/Incisive, GHDL determines the order automatically).
Any arguments or flags to pass to the compile (analysis) stage of the simulation.
Any arguments or flags to pass to the execution of the compiled simulation.
Any argument to be passed to the “first” invocation of a simulator that runs via a TCL script. One motivating usage is to pass -noautoldlibpath to Questa to prevent it from loading the out-of-date libraries it ships with. Used by Aldec Riviera-PRO and Mentor Graphics Questa simulator.
Passed to both the compile and execute phases of simulators with two rules, or passed to the single compile and run command for simulators which don’t have a distinct compilation stage.
“Plusargs” are options that are starting with a plus (
+) sign. They are passed to the simulator and are also available within cocotb as
cocotb.plusargs. In the simulator, they can be read by the Verilog/SystemVerilog system functions
The special plusargs
+seed, if present, are evaluated to set the random seed value if
RANDOM_SEEDis not set. If both
Prefix for simulation command invocations.
This can be used to add environment variables or other commands before the invocations of simulation commands. For example,
SIM_CMD_PREFIX := LD_PRELOAD="foo.so bar.so"can be used to force a particular library to load. Or,
SIM_CMD_PREFIX := gdb --argsto run the simulation with the GDB debugger.
New in version 1.6.0.
The default time unit that should be assumed for simulation when not specified by modules in the design. If this isn’t specified then it is assumed to be
1ns. Allowed values are 1, 10, and 100. Allowed units are
New in version 1.3.
The default time precision that should be assumed for simulation when not specified by modules in the design. If this isn’t specified then it is assumed to be
1ps. Allowed values are 1, 10, and 100. Allowed units are
New in version 1.3.
Use to add additional dependencies to the compilation target; useful for defining additional rules to run pre-compilation or if the compilation phase depends on files other than the RTL sources listed in
Use to add additional dependencies to the simulation target.
Use to define a scratch directory for use by the simulator. The path is relative to the location where
makewas invoked. If not provided, the default scratch directory is
Used to specify how to redirect simulator output. Setting to the empty string will prevent redirection. Defaults to sending both stdout and stderr to
New in version 2.0.
The name of a simulator script that is run as part of the simulation, e.g. for setting up wave traces. You can usually write out such a file from the simulator’s GUI. This is currently supported for the Mentor Questa, Mentor ModelSim and Aldec Riviera simulators.
The name of the library that contains the
TOPLEVELmodule/entity. Defaults to the
RTL_LIBRARY. Only supported by the Siemens EDA Questa simulator.
Library Build Process
You can pass additional options to the library build process
(which is usually happening as part of the installation with
pip) using the
for C and C++ compilation and linking:
The following variables are used for cocotb internals. They may change at any time, and users should not rely on them.
Path to the directory containing the cocotb Python package in the
Path to the directory containing the cocotb Makefiles and simulator libraries in the subdirectories
Enable code coverage collection for cocotb internals. When set, a file
.coverage.cocotbwill be written which contains statistics about the code coverage. This is mainly useful for cocotb’s own Continuous Integration setup.