Most real time systems require a certain amount of reentrant code,
yet too many programmers have no idea what this entails.
Published in Embedded Systems Programming, February 1993
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For many reasons, debugging an interrupt-based system is much
harder than one built of simple looping code. One of the most
insidious sorts of bugs, that is tough to find and sometimes tougher
to understand, is a reentrancy problem.
Reentrant functions, AKA "pure code", are often falsely
thought to be any code that does not modify itself. Too many programmers
feel if they simply avoid self-modifying code, then their routines
are guaranteed to be reentrant, and thus interrupt-safe. Nothing
could be further from the truth.
A function is reentrant if, while it is being executed, it can
be re-invoked by itself, or by any other routine, by interrupting
the present execution for a while.
Reentrancy was originally invented for mainframes, in the days
when memory was a valuable commodity. System operators noticed
that a dozen or hundreds of identical copies of a few big programs
would be in the computer's memory array at any time. At the University
of Maryland, my old hacking grounds, the monster Univac 1108 had
one of the early reentrant FORTRAN compilers. It burned up a (for
those days) breathtaking 32kw of system memory, but being reentrant,
it required only 32k even if 50 users were running it. Each user
executed the same code, from the same set of addresses.
A routine must satisfy the following conditions to be reentrant:
- It never modifies itself. That is, the instructions of the
program are never changed. Period. Under any circumstances. Far
too many embedded systems still violate this cardinal rule.
- Any variables changed by the routine must be allocated to
a particular "instance" of the function's invocation.
Thus, if reentrant function FOO is called by three different functions,
then FOO's data must be stored in three different areas of RAM.
Item (2) deserves a bit more discussion. One of the better trends
in this industry is the use of professional software engineers
in firmware development projects. In the old days the hardware
designer, who had perhaps little formal software engineering education
other than that gained in the school of hard knocks, wrote the
code as an afterthought to building the hardware. Real software
types understand and use more sophisticated program structures,
like recursion, leading to cleaner code (usually), but with new
sorts of perils.
A recursive function calls itself. The classic example is computing
n! (n factorial), which is most elegantly done with a few lines
of recursive code. Any recursive function must be reentrant, because
each instance of its execution must have its own set of local
variables to avoid corrupting any other instance.
For example, consider the following very simple recursive function,
taken from an example in the Borland C++ Programmer's Guide
double power(double x, int exp)
if (exp<=0) return(1);
This function will behave correctly in its obvious recursive environment
as well as in an interrupting real-time system, where "power"
may be called from both a main-line routine and from an interrupt
service routine. The function is indeed pure - all the variables
used are created for each instance of its execution.
Suppose we modify the function as follows:
double power(double x)
where exp is now defined as a public variable, accessible to many
other functions. The function still works correctly. However,
if this function was called by, say, main(), and then an interrupt
which calls the same function came along while it is executing,
it will return an incorrect result. The variable exp is fixed
in memory; it is not unique to each call, and is therefore shared
between callers, a disaster in an interrupting environment.
So, pure code must never modify itself, and must never, ever share
data with any other instance of itself, whether invoked by recursion,
an interrupt service routine, or some other process.
In the example of the previously mentioned FORTRAN compiler, while
the compiler itself was loaded but once into system memory, each
user allocated a chunk of his or her memory area to the compiler
for its own internal use. Every variable or data array the compiler
used was referenced via a base register that set the beginning
of the compiler's data space for that user.
C is elegantly oriented towards reentrancy, as automatic local
variables are generated as stack frame entries each time a function
is invoked. Still, as we've seen, it is possible to write problematic
code in C. In assembly, of course, chaos rules.
Any real time embedded system must address reentrancy issues,
as all but the simplest systems will most likely require reentrant
code. Before seeing where reentrancy is critical, perhaps we should
look at typical areas where code is "impure".
Some processors have limited I/O addressing capabilities. For
example, the 8085 can only send data to specific, hard coded addresses.
There is no mechanism to generate an indirect I/O port (like "output
to the port contained in register C"). The traditional work
around was to generate output and return instructions in RAM,
dynamically patch in the desired port, and then call the RAM-based
code. This violates every rule of reentrancy. A better approach
is to generate a table of output instructions in ROM space, and
indirectly call the proper one. This does, however, consume vast
amounts of memory.
The 1802 had no stack... at all. There were no call or return
instructions (no kidding!). Reentrancy was all but impossible.
Even modern real time operating systems sometimes have short machine-specific
segments of code that are not reentrant. Since there is probably
no other more dangerous spot for impure code, all the vendors
do a good job of protecting the impure sections by disabling interrupts
for a brief time.
Reentrancy is crucial in any section of code that may be invoked
by another process. In a real time OS, each task is independent
and subject to reentrancy concerns. Any subroutine shared between
tasks can be a real source of concern, since the RTOS can context
switch on a timer tick during the execution of this critical routine,
and then schedule another task that invokes the same function.
There is a much more insidious problem. Suppose your main line
routine and the ISRs are all coded in C. The compiler will certainly
invoke runtime functions to support floating point math, I/O,
string manipulations, etc. If the runtime package is only partially
reentrant, than your ISRs may very well corrupt the execution
of the main line code. This problem is common, but is virtually
impossible to troubleshoot since symptoms result only occasionally
and erratically. Can you imagine the difficulty of isolating
a bug which manifests itself only occasionally, and with totally
different characteristics each time?
One moral is to be sure your compiler has a pure runtime package.
Even assembly programmers suffer from this malaise, as any common
low-level routine, shared between an ISR and other code, must
be pure. If you purchase a floating point library, communications
library, etc., be sure the vendor guarantees reentrancy in all
Since so many embedded systems execute from ROM, some level of
reentrancy is assured. No matter how hard one tries, it is impossible
to write self modifying code, at least in the ROM space!
Unfortunately, this breeds a certain complacency. My company sells
emulators. Our number one source of customer support calls comes
from programmers whose code works from ROM, but crashes when executing
from the emulator's internal memory. The problem is always the
same - the user's code inadvertently writes over itself. In ROM,
the problem goes unnoticed. Surely, though, this indicates a more
serious problem. That write should probably have occurred to some
data area in RAM, which is not getting updated. Or, if the write
is indeed spurious, then perhaps some other random setting of
the index registers will cause the write to trash the stack or
some other critical data structure.
Given a halfway decent CPU it's not too hard to write reentrant
code from scratch (decent stack resources are the biggest requirement),
but I've found that it is almost impossible to make a big impure
program reentrant. Generally these are monsters that relay on
heaps of global variables.
If reentrancy is needed in only small areas it is sometimes possible
to protect the worst code by disabling interrupts around it. Then,
no ISR can synchronously muck with the critical resources. This
may not work well in a multitasking system.
Unfortunately, disabling interrupts brings its own problems. Interrupt
latency increases, and in some cases the code could miss interrupts
Many years ago I had to convert an assembly language floating
point package to pure code. I tried to shoehorn fixes in, but
in the end found that only a total rewrite cured the problem.
How do you know if your code is truly reentrant? No matter how
carefully you design a new project, it is far too easy to inadvertently
slip in the occasional impure data reference. In an upgrade to
an old project the problem is more severe, as you may not be totally
comfortable with the code's structure or the original programmer's
I doubt if there is a complete test exist for reentrancy. Some
partial tests are easy and well worth the effort.
It's pretty easy to find code that accidentally (or otherwise)
writes over itself. Any decent emulator lets you catch writes
to program space. When using a ROM monitor, simulator, or the
like, regularly run checksums on the code. If the checksum ever
changes (for a particular version of the code), then something
I do global searches for STATIC declarations in C. STATICS can
indicate a potential reentrancy problem.
In assembly language routines, any direct reference to a RAM address
is cause to examine reentrancy issues. There is nothing implicitly
wrong with a MOV [COUNT],AX; most of the time this sort of effective
address generation will be fine even in a reentrant routine. Still,
it might be a flag that the code is impure. A sure-fire solution
is to store local variables in the current stack frame.
How can you tell if a purchased runtime package is reentrant without
reading all the code? One crude approach is to link the program
without the package (stubbing out the called routines), and again
with it. Compare the linker's reported use of data space. A really
clean reentrant package will require no additional RAM beyond
the stack and/or heap.