DTrace's interaction with processes is a little different than most traditional debuggers or observability tools. Many such tools appear to execute within the scope of the process, letting users dereference pointers to program variables directly. Rather than appearing to execute within or as part of the process itself, DTrace probes execute in the Oracle Linux kernel. To access process data, a probe needs to use the copyin or copyinstr subroutines to copy user process data into the address space of the kernel.

For example, consider the following write() system call:

ssize_t write(int fd, const void *buf, size_t nbytes);

The following D program illustrates an incorrect attempt to print the contents of a string passed to the write() system call:

syscall::write:entry
{
  printf("%s", stringof(arg1)); /* incorrect use of arg1 */
}

If you try to run this script, DTrace produces error messages similar to the following example:

dtrace: error on enabled probe ID 1 (ID 37: syscall::write:entry): \
invalid address (0x10038a000) in action #1

The arg1 variable, containing the value of the buf parameter, is an address that refers to memory in the process executing the system call. To read the string at that address, use the copyinstr subroutine and record its result with the printf action:

syscall::write:entry
{
  printf("%s", copyinstr(arg1)); /* correct use of arg1 */
}

The output of this script shows all of the strings being passed to the write() system call. Occasionally, however, you might see irregular output similar to the following example:

 0         37                     write:entry mada&^%**&

The copyinstr subroutine acts on an input argument that is the user address of a null-terminated ASCII string. However, buffers passed to the write() system call might refer to binary data rather than ASCII strings or to ASCII strings which do not include a terminating null byte. To print only as much of the string as the caller intended, use the two parameter version of the copyinstr subroutine which includes the size of the targeted string buffer:

syscall::write:entry { printf("%s", copyinstr(arg1, arg2)); }

Alternatively, use the copyin subroutine which takes an address and size:

syscall::write:entry
{
  printf("%s", stringof(copyin(arg1, arg2)));
}

Notice that the stringof operator is necessary so that DTrace properly converts the user data retrieved using copyin to a string. The use of stringof is not necessary with copyinstr because it always returns type string.