How to get the value of stack pointer? (MIPS GCC)

Question

I am trying to get the call stack, for some reason the following code returns a wrong stack pointer:

 unsigned int stack_pointer = 0; __asm("la $26, %[spAddr]\n\t" "or $27, $0, $sp\n\t" "sw $27, 0($26)\n\t" "nop"::[spAddr] "m" (stack_pointer)); return stack_pointer;

What am I missing here?

You aren't declaring any output parameters (declared after the 1st colon). Where are you expecting things to get returned? See the docs. — David Wohlferd
– David Wohlferd, Commented Aug 20, 2017 at 9:04

Jester · Accepted Answer · 2017-08-20 10:46:17Z

To get the stack pointer use the proper output constraint like so:

register unsigned sp asm("29"); asm("" : "=r" (sp));

Note that mips uses a register for the return address, but of course non-leaf functions might store it on the stack.

To implement a backtrace, you can however use the builtins __builtin_return_address and __builtin_extract_return_addr as described in the gcc manual.

Also, if glibc is available, it already has backtrace function, see man backtrace.

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