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Overview of KLEE’s main command-line options


KLEE usage

$ klee [klee-options] <program.bc> [program-options]

The general form of a KLEE command-line is first the arguments for KLEE itself ([klee-options]), then the LLVM bitcode file to execute (program.bc), and then any arguments to pass to the application ([program-options]). In particular, the KLEE option -posix-runtime enables the use of the symbolic environment options as part of the program’s options.

To get a complete list of KLEE’s command-line options run: klee --help. The remainder of this page illustrate KLEE’s main command-line options.

Symbolic Environment

KLEE provides several options as part of its symbolic environment:

  1. -sym-arg <N> - Replace by a symbolic argument with length N.
  2. -sym-args <MIN> <MAX> <N> - Replace by at least MIN arguments and at most MAX arguments, each with maximum length N.
  3. -sym-files <NUM> <N> - Make NUM symbolic files (‘A’, ‘B’, ‘C’, etc.), each with size N (excluding stdin)
  4. -sym-stdin <N> - Make stdin symbolic with size N.
  5. -sym-stdout - Make stdout symbolic.
  6. -save-all-writes - Allow write operations to execute as expected even if they exceed the file size (default=on). When off, all writes exceeding the initial file size are discarded. Note: file offset is always incremented.
  7. -max-fail <N> - Allow up to N injected failures.
  8. -fd-fail - Shortcut for ‘-max-fail 1’.

Some usage examples:

$ klee -posix-runtime <program.bc> -sym-arg 16
$ klee -posix-runtime <program.bc> -sym-files 2 30 -sym-stdin

Search Heuristics

Main search heuristics

KLEE provides four main search heuristics:

  1. Depth-First Search (DFS): Traverses states in depth-first order.
  2. Random State Search:Randomly selects a state to explore.
  3. Random Path Selection: Described in our KLEE OSDI’08 paper.
  4. Non Uniform Random Search (NURS): Selects a state randomly according to a given distribution. The distribution can be based on the minimum distance to an uncovered instruction (MD2U), the query cost, etc.

To select a search heuristic, use the --search option provided by KLEE. For example:

$ klee --search=dfs demo.o

runs demo.o using DFS, while

$ klee --search=random-path demo.o

runs it using the random path selection strategy. The full list of options is shown in KLEE’s help message:

$ klee --help
-search - Specify the search heuristic (default=random-path interleaved with nurs:covnew)
  =dfs - use Depth First Search (DFS)
  =random-state - randomly select a state to explore
  =random-path - use Random Path Selection (see OSDI'08 paper)
  =nurs:covnew - use Non Uniform Random Search (NURS) with Coverage-New heuristic
  =nurs:md2u - use NURS with Min-Dist-to-Uncovered heuristic
  =nurs:depth - use NURS with 2^depth heuristic
  =nurs:icnt - use NURS with Instr-Count heuristic
  =nurs:cpicnt - use NURS with CallPath-Instr-Count heuristic
  =nurs:qc - use NURS with Query-Cost heuristic

Interleaving search heuristics

Search heuristics in KLEE can be interleaved in a round-robin fashion. To interleave several search heuristics to be interleaved, use the --search multiple times. For example:

$ klee --search=random-state --search=nurs:md2u demo.o

interleaves the Random State and the NURS:MD2U heuristics in a round robin fashion.

Default search heuristics

The default heuristics used by KLEE are random-path interleaved with nurs:covnew.

Query Logging

To log the queries issued by KLEE during symbolic execution, you can use the following options:

  1. --use-query-log=TYPE:FORMAT, where:
    • TYPE is either all to log all the queries KLEE made during execution before any optimisation (e.g. caching, constraint independence) is performed, or solver to log only the queries passed to KLEE’s underlying solver. Note that it is possible that some of the unoptimized queries are never executed or are modified before being executed by KLEE’s underlying solver.
    • FORMAT is the format in which queries are logged and can be either kquery for the KQuery format, or smt2 for the SMT-LIBv2 format.
  2. --min-query-time-to-log=TIME (in ms) is used to log only queries that exceed a certain time limit. TIME can be:
    • 0 (default): to log all queries
    • <0: a negative value specifies that only queries that timed out should be logged. The timeout value is specified via the --max-solver-time option.
    • >0: only queries that took more that TIME milliseconds should be logged.
  3. --log-partial-queries-early=true is used to dump the query to the log file before the next part of the solver chain is called. Normally, KLEE prints the query and its solution after it has been solved. But if KLEE crashes inside the solver chain, the suspicious query will not be logged. Enable this option to debug such cases. This option comes with a performance penalty as the log buffer gets always flushed.
  4. --compress-query-log is used to compress query log files (default=off)

Entry Point

To change the entry point you can use the option -entry-point=FUNCTION_NAME, where FUNCTION_NAME is the name of the function to use as the entry point for execution.

Calls to klee-assume

By default, KLEE will report an error if the assumed condition is infeasible. The option -silent-klee-assume can be used to sliently terminate the current path exploration in such cases.


By default, KLEE generates two files containing statistics concerning the code exploration:

There are several options to modify how KLEE outputs statistics:

KLEE debug

KLEE provides several debugging options:

Memory Management

KLEE explicitly intercepts calls for memory management (like malloc() and free()) and forwards to an existing memory allocators. To change this behaviour, following options are provided:

Making KLEE Exit on Events

KLEE does not exit if a bug is found in the analyzed application by default. On the other hand, KLEE implicitly exits on some failures. This behaviour can be changed by the following options:


klee -exit-on-error input.bc
klee -exit-on-error-type=Assert -exit-on-error-type=Ptr input.bc

Linking External Libraries

Definitions of undefined functions are taken from files given using the option -link-llvm-lib.

If some functions in the input file are defined in an external LLVM IR file, an archive (.a) of LLVM IR files, or a shared object with LLVM IR code, these external files can be linked in using the option -link-llvm-lib=LIB_FILENAME.

For example, the following command runs KLEE on the program test.bc, linking a helper library:

$ klee -link-llvm-lib=libhelper.so.bc test.bc

The option can be provided multiple times. For instance, linking two libraries, helper and helper2, can be done with the following command:

$ klee -link-llvm-lib=libhelper.so.bc -link-llvm-lib=libhelper2.so.bc test.bc