In C if I simply declare a variable without initializing it, will compiler reserve space for it?

Question

In C if I simply declare a variable without initializing it, will compiler reserve space for it ?

Answer 1

Not in the object or in the executable file, but at runtime the space required for the unitialized global & static data will be allocated in the application's memory.

EDIT: You can see these in the memory-map file of the executable, generated by the compiler (set the necessary options if a mapfile is not generated by default).

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EDIT2: Well, because some people here were quick to downvote my answer, as if it had no merit at all, and wondering if I was actually wrong, I created a small test-project in Visual Studio 2017. I made three test-cases.

Case 1: Empty main() function

#include <stdio.h>

int main()
{
    return(0);
}

Case 2: main() function and uninitialized data

#include <stdio.h>

char g_data[16 * 1024];

extern char Ref_g_data();

int main()
{
    // Needed to actually include the data in the .exe, otherwise the linker will wipe them
    Ref_g_data();
    return(0);
}

This required writing a function in another source file, accessing the data:

extern char g_data[];

char Ref_g_data()
{
    return(g_data[0]);
}

Case 3: main() function and initialized data

Almost as above, just changed the g_data[] definition to initialized:

char g_data[16 * 1024] = "Hello";

And here are the results (filesizes):

Test    SrcMain.obj     TestCS.exe
Case 1  73,217 bytes    8,704 bytes
Case 2  73,307 bytes    8,704 bytes
Case 3  73,327 bytes    25,088 bytes

That is, the 16K data are indeed included in the executable if they are intialized.

Comparison of mapfiles:

Case 1:

DATA
 0002:000000f0 00000120H .rdata
DATA
 0003:00000000 00000018H .data

Case 2:

DATA
 0002:000000f0 00000120H .rdata
DATA
 0003:00000000 00000020H .data

Case 3:

DATA
 0002:000000f0 00004120H .rdata ---> 16K bigger
DATA
 0003:00000000 00000018H .data

I think the above prove that my answer was actually correct.