C const Pointers

How it works

const can qualify the pointed-to type, the pointer object, or both. const int *view permits moving view to another valid int but prevents writing through *view. int *const owner prevents changing the stored pointer while permitting modification of its int. Neither spelling expresses ownership or allocation length. Read a declaration around the identifier and make the intended access rule part of the helper signature so the compiler can reject accidental writes through a read-only view.

Run it locally

Use a hosted C17 compiler in a separate lab folder. The command assumes a driver named cc; substitute gcc, clang or zig cc as appropriate. C does not currently run in Bhylu’s browser playground. Apache, PHP and MySQL are not needed for these C programs.

cc -std=c17 -Wall -Wextra -Wpedantic -Werror const-pointers.c -o lesson

On Windows, use -o lesson.exe and run .\lesson.exe in PowerShell. On Unix-like systems run ./lesson. Math-library linking differs by toolchain; GCC on many Unix systems needs -lm after the source.

Download the complete C source. File-writing labs must run in an empty private directory, away from existing reports.

Example

#include <stdio.h>

int main(void) {
    int depth = 20;
    int replacement = 25;
    const int *view = &depth;
    int *const selected = &depth;
    *selected = 23;
    printf("Viewed: %d\n", *view);
    view = &replacement;
    printf("Other: %d\n", *view);
    return 0;
}

Expected result

Viewed: 23
Other: 25

The read-only view observes a change made through a different permitted alias. const is an access rule through a type, not a promise that an object never changes by any means. The const pointer stays connected to depth throughout its lifetime. Casting away const does not make an originally const object writable and can conceal a design error.