How to break in gdb based on array contents? - c

Question

I am trying to place a conditional breakpoint on a function that would check the contents of an array. My idea was to use memcmp() in the condition:
typedef struct {
uint8_t arr[4];
} arg_t;
Then in gdb (declaring an array inline):
b func() if memcmp(arg.arr, (uint8_t[]){1, 2, 3, 4}, sizeof(arg.arr)) == 0
However, this does not work:
(gdb) c
Continuing.
Error in testing breakpoint condition:
Too many array elements
I can do it with if arg.arr[0] == 1 && arg.arr[1] == 2 && ... but in my real case access to the array (containing an IPv6 address) is rather convoluted, so it becomes unwieldy rather quickly.
UPDATE: After Mark's comment, I tried the following test program:
#include <stdio.h>
#include <stdint.h>
#include <string.h>
int main(void) {
uint8_t a[] = { 1, 2, 3, 4 };
printf("memcmp returns %d\n", memcmp(a, (uint8_t[4]){1,2,3,4}, sizeof(a)));
return 0;
}
After compiling it, I placed a conditional break on the return using the explicit array size syntax, and it worked but it does not seem to stop as expected:
(gdb) br hello.c:8 if memcmp(a, (uint8_t[4]){1,2,3,4}, sizeof(a)) == 0
Breakpoint 1 at 0x75c: file hello.c, line 8.
(gdb) run
Starting program: /mnt/c/stuff/src/test/hello
memcmp returns 0
[Inferior 1 (process 153) exited normally]
I was trying to evaluate the return value of the function at the break site manually:
(gdb) br hello.c:8
Note: breakpoint 1 also set at pc 0x800075c.
Breakpoint 2 at 0x800075c: file hello.c, line 8.
(gdb) dele 1
(gdb) run
Starting program: /mnt/c/stuff/src/test/hello
memcmp returns 0
Breakpoint 2, main () at hello.c:8
8 return 0;
(gdb) p memcmp(a, (uint8_t[4]){1,2,3,4}, sizeof(a))
$1 = (int (*)(const void *, const void *, size_t)) 0x7fffff18aba0 <__memcmp_avx2_movbe>
I was surprised to see this, I suspect this may be due to memcmp() being a compiler intrinsic to the avx2 instruction, in which case I might need to cast it somehow?

Answer 1

This may not be what you want, but for complex tests, I get the program to help gdb by adding some check routines.
Method 1:
I create a void checkon (void) { if (expr_to_stop_on) badnews(); }
And then have: void badnews { stopme = 1; }
I then instrument the code with calls to checkon
I then tell gdb to do b badnews.
Method 2:
An alternative may be to create: int checkfail(void) { return expr_to_stop_on; }
Then, tell gdb to watch myarray.
This creates a watchpoint (using H/W assist). The watchpoint is like a breakpoint.
You can then do: cond 1 checkfail()
Method 3:
Similar to method 2, but instead of a watchpoint, use a tracepoint [with the same cond command (Actually, you may need to use actions instead).
This essentially single steps the program and allows a set of commands to be executed at every tracepoint.
This can be slow because every line has to do these things.
I think method 2 is probably the best bet.

Answer 2

(Posting my own answer to be able to close this, but credit goes to #mark-plotnick)
It works with the gdb builtin $_memeq instead of using memcmp - suspecting because of the latter not being a real function in my environment but YMMV:
(gdb) br arrbreak.c:8 if $_memeq(a, (uint8_t[4]){1,2,3,4}, sizeof(a))
Breakpoint 2 at 0x800075c: file arrbreak.c, line 8.
(gdb) run
Starting program: /mnt/c/stuff/src/test/arrbreak
memcmp returns 0
Breakpoint 2, main () at arrbreak.c:8
8 return 0;
(gdb) p $_memeq(a, (uint8_t[4]){1,2,3,4}, sizeof(a))
$1 = 1
(gdb) p $_memeq(a, (uint8_t[4]){1,2,3,5}, sizeof(a))
$2 = 0
(gdb)