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

Question

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

For now I am using a static assert (home brewed based on other web resources) to compare the sizeof() re

Answer 1

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 
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 
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 
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.