C - access specifiers

In C, the term "access specifiers" (like in C++: public, private, protected) doesn’t actually exist for normal variables and functions — C does not have object-oriented access control.

But C does have ways to control scope and visibility of variables and functions, which serve a similar purpose.

1. Levels of Access Control in C

C controls visibility using:

1. Scope — Where in the code the variable/function can be used.

2. Linkage — Whether it can be accessed from other files.

2. Storage Class Specifiers for Access Control

These are the main keywords that affect "access" in C:

            - For global vars/functions → internal linkage (file-level access only)  
            - For local vars → value persists between calls |

| extern | Makes a variable/function accessible from other files (external linkage) |
| register | Suggests storing in CPU registers (scope stays local) — mostly obsolete now |

2.1 File-Level Access

// file1.c
static int hiddenVar = 5;   // only accessible in this file

int publicVar = 10;         // accessible from other files via extern

static void hiddenFunction() { // only in this file
    printf("Hidden\n");
}

void publicFunction() {     // accessible from other files
    printf("Public\n");
}

// file2.c
#include <stdio.h>

extern int publicVar; // declare to use from file1.c

int main() {
    printf("%d\n", publicVar); // OK
    // printf("%d", hiddenVar); // ERROR: not visible here
}

2.2 Block Scope

void func() {
    int local = 10;  // only inside func()
    static int persist = 0; // inside func, but keeps value between calls
}

3. Summary Table

4. Key Takeaway

• C doesn’t have public / private / protected like C++.

• Instead, it uses scope rules (static, extern) to restrict or expose variables/functions.

• static at file level is the closest thing to "private".

• extern is the closest thing to "public".