How to get the length of a function in bytes?

Question

I want to know the length of C function (written by me) at runtime. Any method to get it? It seems sizeof doesn't work here.

Answer 1

There is a way to determine the size of a function. The command is:

 nm -S <object_file_name> 

This will return the sizes of each function inside the object file. Consult the manual pages in the GNU using 'man nm' to gather more information on this.

Answer 2

You can get this information from the linker if you are using a custom linker script. Add a linker section just for the given function, with linker symbols on either side:

mysec_start = .; *(.mysection) mysec_end = .; 

Then you can specifically assign the function to that section. The difference between the symbols is the length of the function:

#include <stdio.h>  int i;   __attribute__((noinline, section(".mysection"))) void test_func (void) {     i++; }  int main (void) {     extern unsigned char mysec_start[];     extern unsigned char mysec_end[];      printf ("Func len: %lu\n", mysec_end - mysec_start);     test_func ();      return 0; } 

This example is for GCC, but any C toolchain should have a way to specify which section to assign a function to. I would check the results against the assembly listing to verify that it's working the way you want it to.