The Cyclone programming language is intended to be a safe dialect of the C programming language. Cyclone is designed to avoid buffer overflows and other vulnerabilities that are endemic in C programs, without losing the power and convenience of C as a tool for systems programming.
Cyclone development was started as a joint project of AT&T Labs Research and Greg Morrisett's group at Cornell in 2001. Version 1.0 was released on May 8, 2006.
Language features
Cyclone attempts to avoid some of the common pitfalls of the C programming language, while still maintaining the look and performance of C. To this end, Cyclone places the following restrictions upon programs:
NULLchecks are inserted to prevent segmentation faults
- Pointer arithmetic is restricted
- Pointers must be initialized before use (this is enforced by definite assignment analysis)
- Dangling pointers are prevented through region analysis and limitations on
free()
- Only "safe" casts and unions are allowed
gotointo scopes is disallowed
switchlabels in different scopes are disallowed
- Pointer-returning functions must execute
return
setjmpand
longjmpare not supported
In order to maintain the tool set that C programmers are used to, Cyclone provides the following extensions:
- Never-
NULLpointers do not require
NULLchecks
- "Fat" pointers support pointer arithmetic with run-time bounds checking
- Growable regions support a form of safe manual memory management
- Garbage collection for heap-allocated values
- Tagged unions support type-varying arguments
- Injections help automate the use of tagged unions for programmers
- Polymorphism replaces some uses of
void *
- varargs are implemented as fat pointers
- Exceptions replace some uses of
setjmpand
longjmp
For a better high-level introduction to Cyclone, the reasoning behind Cyclone and the source of these lists, please see [1].
Although Cyclone looks, in general, much like C, it should be thought of as a C-like language.
Pointer/reference types
Cyclone implements three kinds of reference (following C terminology these are called pointers):
*(the normal type)
@(the never-
NULLpointer), and
?(the only type with pointer arithmetic allowed, "fat" pointers).
The purpose of introducing these new pointer types is to avoid common problems when using pointers. Take for instance a function, called
foo that takes a pointer to an int:
int foo(int *);
Although the person who wrote the function
foo could have inserted
NULL checks, let us assume that for performance reasons they did not. Calling
foo(NULL); will result in undefined behavior (typically, although not necessarily, a SIGSEGV being sent to the application). To avoid such problems, Cyclone introduces the
@ pointer type, which can never be
NULL. Thus, the "safe" version of
foo would be:
int foo(int @);
This tells the Cyclone compiler that the argument to
foo should never be
NULL, avoiding the aforementioned undefined behavior. The simple change of
* to
@ saves the programmer from having to write
NULL checks and the operating system from having to trap
NULL pointer dereferences. This extra restriction, however, can be a rather large stumbling block for most C programmers, who are used to being able to manipulate their pointers directly with arithmetic. Although this is desirable, it can lead to buffer overflows and other "off-by-one"-style mistakes. To avoid this, the
? pointer type is delimited by a known bound, the size of the array. Although this adds overhead due to the extra information stored about the pointer, it improves safety and security. Take for instance a simple (and naïve)
strlen function, written in C:
int strlen(const char *s) { int iter = 0; if (s == NULL) return 0; while (s[iter] != '\0') { iter++; } return iter; }
This function assumes that the string being passed in is terminated by NUL (
'\0'). However, what would happen if
char buf[] = {'h','e','l','l','o','!'}; were passed to this string? This is perfectly legal in C, yet would cause
strlen to iterate through memory not necessarily associated with the string
s. There are functions, such as
strnlen which can be used to avoid such problems, but these functions are not standard with every implementation of ANSI C. The Cyclone version of
strlen is not so different from the C version:
int strlen(const char ? s) { int iter, n = s.size; if (s == NULL) return 0; for (iter = 0; iter < n; iter++, s++) { if (*s == '\0') return iter; } return n; }
Here,
strlen bounds itself by the length of the array passed to it, thus not going over the actual length. Each of the kinds of pointer type can be safely cast to each of the others, and arrays and strings are automatically cast to
? by the compiler. (Casting from
? to
* invokes a bounds check, and casting from
? to
@ invokes both a
NULL check and a bounds check. Casting from
* or
@ results in no checks whatsoever; the resulting
? pointer has a size of 1.)
Dangling pointers and region analysis
Consider the following code, in C:
char *itoa(int i) { char buf[20]; sprintf(buf,"%d",i); return buf; }
This returns an object that is allocated on the stack of the function
itoa, which is not available after the function returns. While gcc and other compilers will warn about such code, the following will typically compile without warnings:
char *itoa(int i) { char buf[20], *z; sprintf(buf,"%d",i); z = buf; return z; }
Cyclone does regional analysis of each segment of code, preventing dangling pointers, such as the one returned from this version of
itoa. All of the local variables in a given scope are considered to be part of the same region, separate from the heap or any other local region. Thus, when analyzing
itoa, the compiler would see that
z is a pointer into the local stack, and would report an error.
Manual memory management
Examples
The best example to start with is the classic Hello world program:
#include <stdio.h> #include <core.h> using Core; int main(int argc, string_t ? args) { if (argc <= 1) { printf("Usage: hello-cyclone <name>\n"); return 1; } printf("Hello from Cyclone, %s\n", args[1]); return 0; }
References
- Cyclone User Manual
- Cyclone: a Type-safe Dialect of C by Dan Grossman, Michael Hicks, Trevor Jim, and Greg Morrisett - published January 2005
See also
External links
- Cyclone Homepage
- Cyclone 1.0 source code RPM
- Cyclone - Source code repositories
- Cyclone - FAQ
- Cyclone for C programmers
Presentations:
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