bash unable to execute the executable file - c

Question

I'm trying to compile c program on my Linux using make utility
this what happened if i try to create the .o file
#make size_of.o
cc -c -o size_of.o size_of.c
compiling process run correctly,but when i execute the executable file i got this error
#./size_of.o
bash: ./size_of.o: cannot execute binary file
Then once again i run make without .o suffix
#make size_of
cc size_of.o -o size_of
The compiling and executing process run as i expected.
is there any problem with the program or can you tell me what's wrong?
How can i fix this problem and is there any different between executable file in C?
This the program:
#include <stdio.h>
int main (void){
printf("char %d bytes\n",sizeof(char));
printf("short %d bytes\n",sizeof(short));
printf("int %d bytes\n",sizeof(int));
printf("long %d bytes\n",sizeof(long));
printf("float %d bytes\n",sizeof(float));
printf("double %d bytes\n",sizeof(double));
printf("long double %d bytes\n",sizeof(long double));
return 0;
}
and this is the output:
char 1 bytes
short 2 bytes
int 4 bytes
long 4 bytes
float 4 bytes
double 8 bytes
long double 12 bytes

Answer 1

.o files are object files, not executables. You have specifically told the compiler to only create object files, because you used the -c flag. You don't run object files, they feed into a linker (along with other things) to create the executable file.
The general (simplified) process is:
Phase
-----
+---------+
| main.c | (source)
+---------+
|
Compile........|............................
|
V
+---------+ +-----------+
| main.o | (object) | libs etc. |
+---------+ +-----------+
| |
Link...........|.........................|....
| |
+-------------------------+
|
V
+---------+
| main | (executable)
+---------+
You fix that by either using turning the object file into an executable as you've done later in the process, though I would do it as:
cc -o size_of size_of.o
Or simply create the executable directly from the source file:
cc -o size_of size_of.c
And, if you're using make, make sure you have an actual Makefile. Otherwise, you get default rules which may not be what you want. It could be as simple as:
size_of: size_of.c Makefile
gcc -o size_of size_of.c

Answer 2

In the first make invocation you are setting make target to be an object file (target has an .o extension). Built in make rule for object files is just to compile and assemble them (no linking) and that is what you get an object file.
Second invocation is actually asking make to build an executable file.
Gnumake has a set of built in rules for different targets. Please see this link for details:
https://www.gnu.org/software/make/manual/html_node/Catalogue-of-Rules.html