Type casting gcc error with const modifier

I’m playing around with OpenGL and decided to implement a function that would load shader source code from file to string.

OpenGL is expecting to get const char*.

So here is the function I got:

const char* string_from_file(const char* filename) {
    FILE* f = fopen(filename, "r");
    if (f == NULL) {
       fprintf(stderr, "Error while trying to open %s file\n", filename);
       exit(1);
    }
    char* buf = (char*) malloc(MAX_SHADER_FILE_SIZE);
    char c;
    size_t i = 0;
    while((c = fgetc(f)) != EOF) {
        buf[i] = c;
        i++;
    }
    return buf; 
}

Error I’m getting from gcc:

#include <GLFW/glfw3.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_SHADER_FILE_SIZE 200

void framebuffer_size_callback(GLFWwindow *window, int width, int height);
void processInput(GLFWwindow *window);
void _print_shader_info_log(GLuint shader_index); 

const char* string_from_file(const char *filename);

const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;

const char *vertexShaderSource = string_from_file("src/shaders/main_vert.glsl");
const char *fragmentShaderSource = string_from_file("src/shaders/main_frag.glsl");

If compiled with g++ there is no error, it builds and runs.

So error says: initializer element is not compile-time constant. Why does it have to be compile-time constant? Isn’t const type* supposed to mean: the thing that this is pointed to should not be altered? Why is this error not there if compiled with g++?

>Solution :

This has nothing to do with your variable being const. It’s because you’re trying to call a function from the global scope, outside of any function, which you absolutely can’t do in C. It will probably work better if you do it like:

const char *vertexShaderSource;
...
int main(void)
{
    vertexShaderSource = string_from_file("src/shaders/main_vert.glsl");
...
}