"failed external call" on bc and --posix-runtime

[jordr]

Running Chopper on the bc benchmark without any skip instructions yields:

$ kleegacy -libc=uclibc -simplify-sym-indices -search=nurs:covnew  -split-search -output-module bc.bc
...
KLEE: WARNING ONCE: calling external: __syscall_rt_sigaction(2, 94287618746464, 94287622407168, 8)
KLEE: ERROR: /home/ubuntu/code/klee-uclibc/libc/signal/sigaction.c:58: failed external call: __syscall_rt_sigaction
KLEE: NOTE: now ignoring this error at this location

When I add --posix-runtime, the error disappears (--allow-external-sym-calls does not seem to help). What KLEE says about --posix-runtime is:

  -posix-runtime                          - Link with POSIX runtime.  Options that can be passed as arguments to the programs are: --sym-arg <max-len>  --sym-args <min-argvs> <max-argvs> <max-len> + file model options

What happens?