<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
symbolic	B-General_Concept
execution	I-General_Concept
(	O
also	O
symbolic	B-General_Concept
evaluation	I-General_Concept
or	O
symbex	O
)	O
is	O
a	O
means	O
of	O
analyzing	O
a	O
program	O
to	O
determine	O
what	O
inputs	B-General_Concept
cause	O
each	O
part	O
of	O
a	O
program	O
to	O
execute	B-General_Concept
.	O
</s>
<s>
An	O
interpreter	B-Application
follows	O
the	O
program	O
,	O
assuming	O
symbolic	O
values	O
for	O
inputs	B-General_Concept
rather	O
than	O
obtaining	O
actual	O
inputs	B-General_Concept
as	O
normal	O
execution	O
of	O
the	O
program	O
would	O
.	O
</s>
<s>
Finally	O
,	O
the	O
possible	O
inputs	B-General_Concept
that	O
trigger	O
a	O
branch	O
can	O
be	O
determined	O
by	O
solving	O
the	O
constraints	O
.	O
</s>
<s>
The	O
field	O
of	O
symbolic	B-Application
simulation	I-Application
applies	O
the	O
same	O
concept	O
to	O
hardware	O
.	O
</s>
<s>
Symbolic	B-Algorithm
computation	I-Algorithm
applies	O
the	O
concept	O
to	O
the	O
analysis	O
of	O
mathematical	O
expressions	O
.	O
</s>
<s>
During	O
symbolic	B-General_Concept
execution	I-General_Concept
,	O
the	O
program	O
reads	O
a	O
symbolic	O
value	O
(	O
e.g.	O
,	O
λ	O
)	O
and	O
assigns	O
it	O
to	O
y	O
.	O
</s>
<s>
At	O
this	O
point	O
of	O
the	O
program	O
,	O
λ	O
could	O
take	O
any	O
value	O
,	O
and	O
symbolic	B-General_Concept
execution	I-General_Concept
can	O
therefore	O
proceed	O
along	O
both	O
branches	O
,	O
by	O
"	O
forking	O
"	O
two	O
paths	O
.	O
</s>
<s>
When	O
paths	O
terminate	O
(	O
e.g.	O
,	O
as	O
a	O
result	O
of	O
executing	O
fail( )	O
or	O
simply	O
exiting	O
)	O
,	O
symbolic	B-General_Concept
execution	I-General_Concept
computes	O
a	O
concrete	O
value	O
for	O
λ	O
by	O
solving	O
the	O
accumulated	O
path	O
constraints	O
on	O
each	O
path	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
constraint	B-Application
solver	I-Application
would	O
determine	O
that	O
in	O
order	O
to	O
reach	O
the	O
fail( )	O
statement	O
,	O
λ	O
would	O
need	O
to	O
equal	O
6	O
.	O
</s>
<s>
One	O
example	O
of	O
merging	O
is	O
veritesting	O
,	O
which	O
"	O
employs	O
static	O
symbolic	B-General_Concept
execution	I-General_Concept
to	O
amplify	O
the	O
effect	O
of	O
dynamic	O
symbolic	B-General_Concept
execution	I-General_Concept
"	O
.	O
</s>
<s>
Symbolic	B-General_Concept
execution	I-General_Concept
is	O
used	O
to	O
reason	O
about	O
a	O
program	O
path-by-path	O
which	O
is	O
an	O
advantage	O
over	O
reasoning	O
about	O
a	O
program	O
input-by-input	O
as	O
other	O
testing	O
paradigms	O
use	O
(	O
e.g.	O
</s>
<s>
However	O
,	O
if	O
few	O
inputs	B-General_Concept
take	O
the	O
same	O
path	O
through	O
the	O
program	O
,	O
there	O
is	O
little	O
savings	O
over	O
testing	O
each	O
of	O
the	O
inputs	B-General_Concept
separately	O
.	O
</s>
<s>
Symbolic	B-General_Concept
execution	I-General_Concept
is	O
harder	O
when	O
the	O
same	O
memory	O
location	O
can	O
be	O
accessed	O
through	O
different	O
names	O
(	O
aliasing	O
)	O
.	O
</s>
<s>
Aliasing	O
cannot	O
always	O
be	O
recognized	O
statically	O
,	O
so	O
the	O
symbolic	B-General_Concept
execution	I-General_Concept
engine	O
ca	O
n't	O
recognize	O
that	O
a	O
change	O
to	O
the	O
value	O
of	O
one	O
variable	O
also	O
changes	O
the	O
other	O
.	O
</s>
<s>
Programs	O
interact	O
with	O
their	O
environment	O
by	O
performing	O
system	B-Operating_System
calls	I-Operating_System
,	O
receiving	O
signals	O
,	O
etc	O
.	O
</s>
<s>
Consistency	O
problems	O
may	O
arise	O
when	O
execution	O
reaches	O
components	O
that	O
are	O
not	O
under	O
control	O
of	O
the	O
symbolic	B-General_Concept
execution	I-General_Concept
tool	O
(	O
e.g.	O
,	O
kernel	O
or	O
libraries	O
)	O
.	O
</s>
<s>
In	O
theory	O
,	O
symbolic	B-General_Concept
execution	I-General_Concept
would	O
fork	O
two	O
paths	O
at	O
line	O
5	O
and	O
each	O
path	O
from	O
there	O
on	O
would	O
have	O
its	O
own	O
copy	O
of	O
the	O
file	O
.	O
</s>
<s>
In	O
practice	O
,	O
file	O
operations	O
are	O
implemented	O
as	O
system	B-Operating_System
calls	I-Operating_System
in	O
the	O
kernel	O
,	O
and	O
are	O
outside	O
the	O
control	O
of	O
the	O
symbolic	B-General_Concept
execution	I-General_Concept
tool	O
.	O
</s>
<s>
The	O
disadvantage	O
is	O
that	O
the	O
side	O
effects	O
of	O
such	O
calls	O
will	O
clobber	O
all	O
states	O
managed	O
by	O
the	O
symbolic	B-General_Concept
execution	I-General_Concept
engine	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
engine	O
instruments	O
the	O
system	B-Operating_System
calls	I-Operating_System
with	O
a	O
model	O
that	O
simulates	O
their	O
effects	O
and	O
that	O
keeps	O
all	O
the	O
side	O
effects	O
in	O
per-state	O
storage	O
.	O
</s>
<s>
The	O
disadvantage	O
is	O
that	O
one	O
needs	O
to	O
implement	O
and	O
maintain	O
many	O
potentially	O
complex	O
models	O
of	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
Tools	O
such	O
as	O
KLEE	O
,	O
Cloud9	O
,	O
and	O
Otter	O
take	O
this	O
approach	O
by	O
implementing	O
models	O
for	O
file	O
system	O
operations	O
,	O
sockets	O
,	O
IPC	B-Operating_System
,	O
etc	O
.	O
</s>
<s>
Symbolic	B-General_Concept
execution	I-General_Concept
tools	O
based	O
on	O
virtual	O
machines	O
solve	O
the	O
environment	O
problem	O
by	O
forking	O
the	O
entire	O
VM	O
state	O
.	O
</s>
<s>
The	O
concept	O
of	O
symbolic	B-General_Concept
execution	I-General_Concept
was	O
introduced	O
academically	O
in	O
the	O
1970s	O
with	O
descriptions	O
of	O
:	O
the	O
Select	O
system	O
,	O
</s>
