I still use dtrace just about every day in my job and found that I had to print an argument to a function which was an array of strings.
The array was variable length up to about 10 items. I'm not sure if
the is
the right way to do it, but it seems to work and is not too painful if
the array size is small.Here's an example. Suppose in your application, you have
the following function, where n is number of item in
the array s.void arraytest(int n, char **s){ /* Loop thru s[0] to s[n-1] */}How do you use DTrace to print out
the values of s[i] or of s[0] to s[n-1]? DTrace does not have if-then blocks or for loops, so you can't do something like: for i=0; i<arg0; i++ trace arg1[i]; It turns out that you can use probe ordering as a kind of iterator. Probes with
the same name will fire in
the order that they appear in
the script, so I can
save the value of "n" in
the first probe and then use it as part of
the predicate of
the next probe to determine if
the other probe should fire or not. So
the first probe for tracing
the arraytest function is:pid$target::arraytest:entry{ self->n = arg0;}Then, if I want to print out
the first few items of
the array, I first check
the value of n. If it's greater than
the index that I want to print out, then I can print that index. For example, if I want to print out
the 3rd element of
the array, I would do something like:pid$target::arraytest:entry/self->n > 2/{ printf("%s",stringof(arg1 + 2 * sizeof(pointer)));}Actually, that doesn't quite work because arg1 is a pointer to an array of pointers and needs to be copied twice from
the user process space to
the kernel space (which is where dtrace is). Also,
the sizeof(char *) is 8, but for some reason, I have to use 4 which is
the sizeof(uint32_t). (I still don't know how that works.) So,
the script that prints
the 3rd element of
the array should look like:pid$target::arraytest:entry{ /* first,
save the size of
the array so that we don't get invalid address errors when indexing arg1+n. */ self->n = arg0;}pid$target::arraytest:entry/self->n > 2/{ /* print
the 3rd element (index = 2) of
the second arg. */ i = 2; size = 4; self->a_t = copyin(arg1+size*i,size); printf("%s: a[%d]=%s",probefunc,i,copyinstr(*(uint32_t *)self->a_t));}If your array is large, then it's quite painful since you have to write one probe for every array index. For example, here's
the full script for printing
the first 5 elements of
the array:#!/usr/sbin/dtrace -spid$target::arraytest:entry{ /* first,
save the size of
the array so that we don't get invalid address errors when indexing arg1+n. */ self->n = arg0;}pid$target::arraytest:entry/self->n > 0/{ i = 0; size = sizeof(uint32_t); self->a_t = copyin(arg1+size*i,size); printf("%s: a[%d]=%s",probefunc,i,copyinstr(*(uint32_t *)self->a_t));}pid$target::arraytest:entry/self->n > 1/{ i = 1; size = sizeof(uint32_t); self->a_t = copyin(arg1+size*i,size); printf("%s: a[%d]=%s",probefunc,i,copyinstr(*(uint32_t *)self->a_t));}pid$target::arraytest:entry/self->n > 2/{ i = 2; size = sizeof(uint32_t); self->a_t = copyin(arg1+size*i,size); printf("%s: a[%d]=%s",probefunc,i,copyinstr(*(uint32_t *)self->a_t));}pid$target::arraytest:entry/self->n > 3/{ i = 3; size = sizeof(uint32_t); self->a_t = copyin(arg1+size*i,size); printf("%s: a[%d]=%s",probefunc,i,copyinstr(*(uint32_t *)self->a_t));}pid$target::arraytest:entry/self->n > 4/{ i = 4; size = sizeof(uint32_t); self->a_t = copyin(arg1+size*i,size); printf("%s: a[%d]=%s",probefunc,i,copyinstr(*(uint32_t *)self->a_t));}
If
the array is large, then your script will also have to be very long to print out all values of
the array.
