Pointer Safety

If you are a C or C++ programmer, you may be a bit frightened after reading the previous section because you know that misuse of pointers in your programs can cause your programs to crash. DTrace is a robust, safe environment for executing your D programs where these mistakes cannot cause program crashes. You may indeed write a buggy D program, but invalid D pointer accesses will not cause DTrace or the operating system kernel to fail or crash in any way. Instead, the DTrace software will detect any invalid pointer accesses, disable your instrumentation, and report the problem back to you for debugging.

If you have programmed in the Java programming language, you probably know that the Java language does not support pointers for precisely the same reasons of safety. Pointers are needed in D because they are an intrinsic part of the operating system's implementation in C, but DTrace implements the same kind of safety mechanisms found in the Java programming language that prevent buggy programs from damaging themselves or each other. DTrace's error reporting is similar to the run-time environment for the Java programming language that detects a programming error and reports an exception back to you.

To see DTrace's error handling and reporting, write a deliberately bad D program using pointers. In an editor, type the following D program and save it in a file named badptr.d:

Example 5-1 badptr.d: Demonstration of DTrace Error Handling

BEGIN
{
    x = (int *)NULL;
    y = *x;
    trace(y);
}

The badptr.d program creates a D pointer named x that is a pointer to int. The program assigns this pointer the special invalid pointer value NULL, which is a built-in alias for address 0. By convention, address 0 is always defined to be invalid so that NULL can be used as a sentinel value in C and D programs. The program uses a cast expression to convert NULL to be a pointer to an integer. The program then dereferences the pointer using the expression *x, and assigns the result to another variable y, and then attempts to trace y. When the D program is executed, DTrace detects an invalid pointer access when the statement y = *x is executed and reports the error:

# dtrace -s badptr.d
dtrace: script '/dev/stdin' matched 1 probe
CPU     ID                    FUNCTION:NAME
dtrace: error on enabled probe ID 1 (ID 1: dtrace:::BEGIN): invalid address
(0x0) in action #2 at DIF offset 4
dtrace: 1 error on CPU 0
^C
#

The other problem that can arise from programs that use invalid pointers is an alignment error. By architectural convention, fundamental data objects such as integers are aligned in memory according to their size. For example, 2-byte integers are aligned on addresses that are multiples of 2, 4-byte integers on multiples of 4, and so on. If you dereference a pointer to a 4-byte integer and your pointer address is an invalid value that is not a multiple of 4, your access will fail with an alignment error. Alignment errors in D almost always indicate that your pointer has an invalid or corrupt value due to a bug in your D program. You can create an example alignment error by changing the source code of badptr.d to use the address (int *)2 instead of NULL. Because int is 4 bytes and 2 is not a multiple of 4, the expression *x results in a DTrace alignment error.

For details about the DTrace error mechanism, see ERROR Probe.