What is the easiest way to find the sizeof a type without compiling and executing code?

Question

I wrote a bash script to determine the size of gcc\'s datatypes (e.g. ./sizeof int double outputs the respective sizes of int and

Answer 1

Using nm with no code

Just make your thing a global variable. nm can report its size.

// getsize.c

struct foo {
    char str[3];
    short s;     // expect padding galore...
    int i;
} my_struct;

Compile but don't link, then use nm:

$ gcc -c getsize.c
$ nm getsize.o --format=posix
my_struct C 000000000000000c 000000000000000c

Note that the last column is the size (in hex), here is how we can get it:

$ nm test.o -P | cut -d ' ' -f 4
000000000000000c

# or in decimal
$ printf %d 0x`nm test.o -P | cut -d ' ' -f 4`
12

 

Using objdump with no code

If nm doesn't work for some reason, you can store the size itself in a global variable.

Start with this C file:

// getsize.c
struct foo { char str[3]; short s; int i; };

unsigned long my_sizeof = sizeof(struct foo);

Now we have to find the value of this variable from the object file.

$ gcc -c sizeof.c
$ objdump -Sj .data sizeof.o

test.o:     file format elf64-x86-64


Disassembly of section .data:

0000000000000000 :
   0:   0c 00 00 00 00 00 00 00                             ........

Darn, little endian! You could write a script to parse this, but the following solution (assuming GCC extensions) will force it to always be big endian:

// getsize.c
struct foo { char str[3]; short s; int i; };

struct __attribute__ ((scalar_storage_order("big-endian"))) {
    unsigned long v;
} my_sizeof = { sizeof(struct foo) };

This yields:

0000000000000000 :
   0:   00 00 00 00 00 00 00 0c                             ........

Watch out! You can't just strip out all non-hex characters because sometimes the "...." stuff on the right will be valid ASCII. But the first one should always be a .. The following command keeps things between the : and the first ..

$ gcc -c sizeof.c
$ objdump -Sj .data sizeof.o |

        sed '$!d                     # keep last line only
             s/\s//g                 # remove tabs and spaces
             s/.*:\([^.]*\)\..*/\1/' # only keep between : and .'

000000000000000c