Language features
Cyclone attempts to avoid some of the common pitfalls of C (programming language), C, while still maintaining its look and performance. To this end, Cyclone places the following limits on programs: *Null pointer, NULL checks are inserted to prevent segmentation faults
* Pointer arithmetic is limited
* Pointers must be initialized before use (this is enforced by definite assignment analysis)
* Dangling pointers are prevented through region analysis and limits on Malloc, free()
* Only "safe" casts and unions are allowed
* Control flow, goto into scopes is disallowed
* Control flow, switch labels in different scopes are disallowed
* Pointer-returning functions must execute return
* Setjmp/longjmp, setjmp and Setjmp/longjmp, longjmp are not supported
To maintain the tool set that C programmers are used to, Cyclone provides the following extensions:
* Never-NULL pointers do not require NULL checks
* "Fat" pointers support pointer arithmetic with run-time bounds checking
* Growable regions support a form of safe manual memory management
* Garbage collection (computer science), Garbage collection for heap-allocated values
* Tagged unions support type-varying arguments
* Injections help automate the use of tagged unions for programmers
* Polymorphism (computer science), Polymorphism replaces some uses of void pointer, void *
* varargs are implemented as fat pointers
* Exception handling, Exceptions replace some uses of setjmp and longjmp
For a better high-level introduction to Cyclone, the reasoning behind Cyclone and the source of these lists, sePointer types
Cyclone implements three kinds of pointer (computer science), pointer: ** (the normal type)
* @ (the never-NULL pointer), and
* ? (the only type with pointer arithmetic allowed, fat pointer, "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:
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 Unix signal, signal 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:
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 limit, 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:
'\0'). However, what would happen if char buf[6] = ; 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:
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 checking, bounds check, and casting from ? to @ invokes both a NULL check and a bounds check. Casting from * to ? results in no checks whatsoever; the resulting ? pointer has a size of 1.)
Dangling pointers and region analysis
Consider the following code, in C:itoa allocates an array of chars buf on the stack and returns a pointer to the start of buf. However the memory used on the stack for buf is deallocated when the function returns, so the returned value cannot be used safely outside of the function. While GNU Compiler Collection, gcc and other compilers will warn about such code, the following will typically compile without warnings:
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 Cyclone compiler would see that z is a pointer into the local stack, and would report an error.
See also
* C (programming language), C * ML (programming language), ML * Rust (programming language), RustReferences
External links