Tag Archives: breakpoints

Tip: gdb break if first function argument has a value

I wanted to find out who was closing stdout in my program (ie. who is calling close(1);).

This seems to work in gdb:

(gdb) break close if $rdi == 1

The explanation is that on x86-64, register %rdi is used to pass the first argument to a function. In gdb conditionals, register names are prefixed by $. Hence the condition checks if the first argument to the function is 1.

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