How can I print the result of sizeof() at compile time in C?

Question

I was mucking around looking for similar functionality when I stumbled on this:

Is it possible to print out the size of a C++ class at compile-time?

Which gave me the idea for this:

char (*__kaboom)[sizeof( YourTypeHere )] = 1;

Which results in the following warning in VS2015:

warning C4047: 'initializing': 'DWORD (*)[88]' differs in levels of indirection from 'int'

where 88 in this case would be the size you're looking for.

Super hacky, but it does the trick. Probably a couple years too late, but hopefully this will be useful to someone.

I haven't had a chance to try with gcc or clang yet, but I'll try to confirm whether or not it works if someone doesn't get to it before me.

Edit: Works out of the box for clang 3.6

The only trick I could get to work for GCC was abusing -Wformat and having the macro define a function like the following:

void kaboom_print( void )
{
    printf( "%d", __kaboom );
}

Which will give you a warning like:

...blah blah blah... argument 2 has type 'char (*)[88]'

Slightly more gross than the original suggestion, but maybe someone who knows gcc a bit better can think of a better warning to abuse.

Duplicate case constant is a trick that is guaranteed to work IN ALL C COMPILERS regardless of how each of them reports error. For Visual C++, it is simple:

struct X {
    int a,b;
    int c[10];
};
int _tmain(int argc, _TCHAR* argv[])
{
    int dummy;

    switch (dummy) {
    case sizeof(X):
    case sizeof(X):
        break;
    }
    return 0;
}

Compilation result:

 ------ Build started: Project: cpptest, Configuration: Debug Win32 ------
 cpptest.cpp c:\work\cpptest\cpptest\cpptest.cpp(29): error C2196: case value '48' already used
 ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

So sizeof the struct X is 48

EDITED (3jun2020): For gcc or any other compilers that only print "duplicate case value", I use this trick to narrow down the value:

1) add a case value 1==2 (to represent false)

2) by trial and error, narrow down the value e.g. I try to guess the sizeof(X) is >16:

#include <stdio.h>
typedef struct _X {
    int a;
    char b[10];
} X;
int main()
{
    printf("Hello World");

    int dummy=0   ;
    switch (dummy) {
    case  1==2:
    case sizeof( X)>16:
    //case 16:
    break;
    }
    return 0;
}

result:

main.c: In function ‘main’:
main.c:14:5: error: duplicate case value
     case sizeof( X)>16:
     ^~~~
main.c:13:5: error: previously used here
     case  1==2:

so it is false, i.e. sizeof(X)<=16.

3) repeat with some other sensible values. e.g. try to guess that it is 16 i.e. sizeof(X)==16. If it doesn't complain about duplicate case value. Then the expression is true.

4) optionally add a case 16 to verify it e.g.

#include <stdio.h>
typedef struct _X {
    int a;
    char b[10];
} X;
int main()
{
    printf("Hello World");

    int dummy=0   ;
    switch (dummy) {
   // case  1==2:
    case sizeof( X):
    case 16:
    break;
    }
    return 0;
}

result

main.c: In function ‘main’:
main.c:15:5: error: duplicate case value
     case 16:
     ^~~~
main.c:14:5: error: previously used here
     case sizeof( X):

confirming that sizeof(X) is 16.

Alternatively, it is observed that gcc can report multiple duplicates, so this trick is possible for making multiple guesses on a single pass:

#include <stdio.h>
typedef struct _X {
    int a;
    char b[10];
} X;
int main()
{
    printf("Hello World");

    int dummy=0   ;
    switch (dummy) {
    case  1==2: //represents false
    case 1==1: //represents true
    case sizeof( X)>10:
    case sizeof( X)>12:
    case sizeof( X)>14:
    case sizeof( X)>16:
    case  sizeof( X)==16:
    //case 16:
    break;
    }
    return 0;
}

result

main.c: In function ‘main’:
main.c:14:5: error: duplicate case value
     case sizeof( X)>10:
     ^~~~
main.c:13:5: error: previously used here
     case 1==1:
     ^~~~
main.c:15:5: error: duplicate case value
     case sizeof( X)>12:
     ^~~~
main.c:13:5: error: previously used here
     case 1==1:
     ^~~~
main.c:16:5: error: duplicate case value
     case sizeof( X)>14:
     ^~~~
main.c:13:5: error: previously used here
     case 1==1:
     ^~~~
main.c:17:5: error: duplicate case value
     case sizeof( X)>16:
     ^~~~
main.c:12:5: error: previously used here
     case  1==2:
     ^~~~
main.c:18:5: error: duplicate case value
     case  sizeof( X)==16:
     ^~~~
main.c:13:5: error: previously used here
     case 1==1:
     ^~~~

suggesting the sizeof(X) is >10, >12, >14 but is not >16. The ==16 is added as a final guess.

The following way, which works in GCC, Clang, MSVC and more, even in older versions, is based on failed conversion of a function parameter from pointer to array to a scalar type. The compilers do print size of the array, so you can get the value from the output. Works both in C and C++ mode.

Example code to find out sizeof(long) (play with it online):

char checker(int);
char checkSizeOfInt[sizeof(long)]={checker(&checkSizeOfInt)};

Examples of relevant output:

• GCC 4.4.7

<source>:1: note: expected 'int' but argument is of type 'char (*)[8]'

• clang 3.0.0

<source>:1:6: note: candidate function not viable: no known conversion from 'char (*)[8]' to 'int' for 1st argument;

• MSVC 19.14

<source>(2): warning C4047: 'function': 'int' differs in levels of indirection from 'char (*)[4]'

One more way (that actually works):

char __foo[sizeof(MyStruct) + 1] = {[sizeof(MyStruct)] = ""};

Works with old'ish gcc 5.x. Yields an error like this:

a.c:8:54: error: initializer element is not computable at load time
a.c:8:54: note: (near initialization for 'a[8]')

p.s. obviously, this one is (very) gcc specific. All other methods weren't working for me.

Quick and simple solution that worked for me (GCC):

(char[sizeof(long long)])"bla";

This results in an error message that reveals the size of long long:

ISO C++ forbids casting to an array type 'char [8]'