1. First, solve the problem of error reporting as shown in the title;

Take a look at the following code:

#include <stdio.h>  
int a = 1;  
int b = 2;  
int c = a+b;  
  
int main() {  
    printf("c is %d\n", c);  
    return 0;  
}  

Error during compilation of GCC-O test test.c: Initializer Element is not Constant

Reason: the value of the global variable C cannot be determined at compile time; it must be determined at run time. Why is that?Because this is the standard:
C language standard: Initializers of external variables and static variables must be constant expressions [1].
So the solution:

#include <stdio.h>  
int a = 1;  
int b = 2;  
int c; 
  
int main() {  
    c = a + b; 
    printf("c is %d\n", c);  
    return 0;  
}

———-
Similarly, the following code does not compile, and the error message is the same as above: [4]

char * p1 = (char *) malloc(10);  
int main(void)
{
。。。   
}

2. Do GCC and g++ compile according to the suffix name of the file?

If you change the source file name *.c to *.cc, and then compile with g++, no error will be reported. Even if you just change the file name to *.cc, the compiler will also compile with GCC, which can also compile the past. What is the reason?

————–
Principle:
GCC started out as GNU C Compiler, which is, as you know, a C Compiler. But later, as the project integrated more compilers in different languages, GCC stood for the GNU Compiler Collection, thus representing a Collection of compilers.
So instead of calling GCC when you compile your code, it’s more like a driver that calls the c compiler or the c++ compiler (g++) based on the suffix of your code. For example, if your code suffix is *.c, it will call the C compiler and linker to link to C’s library. If your code suffix is CPP, it will call the g++ compiler, and of course the library call will also use the c++ version (however, the GCC command does not automatically link to the library used by c++ programs, you must follow the parameter gcc-lstdc ++)

—————–
Here’s a piece of code:

#include <iostream>

using namespace std;

int a = 1;
int b = a;

int main()
{
cout << b << endl;

}

The Initializer Element is not constant because GCC identifies the code as C, and C specifies that initializing global variables must be a constant expression.
If $GCC test.cc, undefined reference to ‘STD ::cout’… Wait… Error: ld returned 1 exit status. Although GCC recognized c++ code at this time, it would not automatically link c++ code base, so ld link error appeared. At this point only need to manually specify and C ++ library link C: $GCC test.cc-LSTDC ++, at this point you can smoothly through the compilation link.

———————
G++ is the c++ compiler for GCC. At compile time, g++ calls GCC, so for c++ code, the two are equivalent. But since the GCC command does not automatically associate with the library used by c++ programs, it is common to use g++ for linking, so for the sake of uniformity, compile/link all use g++, giving the illusion that c++ programs can only use g++.
So you can understand it this way: you can compile with GCC /g++, and you can link with either g++ or gcc-lstdc ++. Because GCC commands do not automatically join with libraries used by C++ programs, g++ is usually used to complete the join. But at compile time, g++ automatically calls GCC, which is equivalent.
For the above code, just $g++ test.c will compile because g++ identifies *.c files as c++ files [2].

The compiler is through the program suffix name to determine the language is C or C ++. Specifically, if the suffix is.c, GCC treats it like a c program, and g++ treats it like a c++ program; CPP suffix, both are considered as c++ programs, note that although c++ is a superset of c, but the syntax requirements of the two are different [2], for c language and c++ language suffix name problem, see [7]
(3) for 1 is the C language standard, not C++, for C++, global variables and static variables do not have to follow the initialization must be constant expression such requirements, so, if the compiler to identify the above code into C++ code, then it can be successfully compiled in the past. (And the program USES the same libraries as the C language, so it’s ok not to add -LSTDC ++ parameter.

Therefore, since the suffix name of the file is changed to *.cc, I guess that whether GCC or g++, this file is identified as c++ file, so it is clear that since it is identified as c++ file, of course not to report an error.

3, compiler strong and weak symbols

In C, global variables default to 0 if they are not initialized, that is, in the global space:
int x =0; With the int x; The effect looks the same. The difference is very big, but here it is strongly recommended that you all global variables
to initialization, the main difference between them is as follows:

the compiler when compiling for both conditions can produce two kind of symbols in the symbol table of the target file, for initialization, called
symbol, an uninitialized, called weak symbols.

if the connector encounters two duplicate symbols while connecting to the target file, it will have the following processing rule:
if there are strong symbols with multiple duplicate names, it will report an error.
if there is a strong symbol and more than one weak symbol, the strong symbol shall prevail.
if there is no strong symbol, but there are more than one weak symbol with the same name, then choose any weak symbol.

————-
So here’s a piece of code:

#include<stdio.h>
int i;
i =1;
int main()
{
printf("%d\n", i);
}

GCC test. C
Can be compiled, but with warning: Data Definition has no type or Storage class
What does that mean?I =1, the definition of an I variable has no type;
So the code looks like this:

#include<stdio.h>
#include<stdlib.h>
 
int i;
j=1;
 
int main()
{
printf( "%d\n", j);
printf( "%d\n", i);
system( "pause" );
return 0;
}

gcc test.c
This code also has warning; data definition has no type or storage class
So for I =1; The compiler takes it as the definition + initialization of a variable.
and the int I before; Because I is equal to 1; So int I is weak, it doesn’t work; [3]

However, the above two pieces of code, if using g++ compiler, that is, according to the rules of c++ compiler, will be directly error:
“J does not name a type” because c++ is a mandatory language, can not be so casual!
It is worth noting that the first paragraph of the above two code also reports an error in g++ : “I does not name a type”. It is understandable that j compilation should not pass, because j is not defined at all. Int I = 1; int I = 1; Int j = 2; int c = a+b; G++ is passable. What’s the problem?See section 4 of this article.

————–
Here’s another piece of code:

#include <stdio.h>
struct stu
{
   long number;
};

struct stu  student; 
student.number = 1;  

int main(int argc, char * argv[])
{
 printf("%d\n", student.number);
}

Why is that?
The assignment statement should be placed in the function body!
Initialization can be outside the body of the function… But the assignment must be inside the body of the function…
Moreover, not only are structures assigned, but most statements (other than declarations and definitions of functions and variables) do not allow…
So understand the difference between initialization and assignment;
(This applies to C and C ++)
**** It should also be noted that in C language, struct is required when defining structural variables, while in C ++, it is not required. Stu Student = {1}; You can, but with struct you can compile it, you can run it.

4. Why is it specified that the initial values of global and static variables must be constant expressions?
Now that we’ve solved the problem, and we’ve analyzed why we did it, we can go deeper. Why do global variables and static variables have to be initialized as constant expressions?
Global (static) storage: Divided into DATA segments and BSS segments. The DATA section (global initialization section) holds initialized global and static variables; The BSS section (global uninitialized area) holds uninitialized global and static variables as well as initialized to zero global and static variables (see here). Automatic release at the end of the program. The BBS section is automatically cleared by the system before the program execution, so uninitialized global variables and static variables are already 0 before the program execution. – The program is released by the system after completion. [5]
——-
A comparison, for the statement:

int i = 3
int main()
{
    int j = i;
    ...
}

Instead of determining the value of the local variable J at compile time, the value of I is read at run time to assign to J. The compiled connected executable does not contain the value of J, only the code for the corresponding assignment statement. In contrast, since I is a global variable and is stored in static storage, its value needs to be determined at compile time. Space will be allocated in the target file to store the value of I. There will be no assignment statement to assign the value of I at run time, and there is no corresponding code.

for the statement:

int i = 3;
int j = i;

Because J is a global variable and is stored in static storage, its value also needs to be determined at compile time. I is a variable, not a constant, and the value of I cannot be determined at compile time, which means that the value of J cannot be determined at compile time, so C will report an error.
Instead, C++ simply puts j as an uninitialized global variable in the.bss area at compile time, with the default value of 0, and then adds a statement to assign j at run time, ensuring that this statement is executed before the main function begins. So the initialization of j is actually done at run time. [6]

So, we can now answer the question raised in red in section 3:
1. For int I = 1; int j = 2; int c = a+b; This sentence, in g + + through because the statement is the c at compile time as a variable declaration, because the front of the variable c has keyword int, then the compiler will be additional add a statement c = a + b, and ensure that the statement before the main function, g + + this way to make this statement to the right by compiling.
2. For int I; I = 1; G++ does not compile. The reason is that an assignment statement like I = 1, which is different from an int c = a+b, requires a value at run time; I =1 cannot be regarded as a declaration of a variable (I variable has no type in front of it),,,,,,, etc

Tip: GCC compiler plus the -v parameter, displays compile-time details, compiler version, compilation process, etc.