expected 'const GLchar * const' {aka 'const char const*'} but argument is of type 'char **'

Question

I tried to convert my old openGl code (written in c++) to c, I always encounter this weird warning about an incompatible pointer type when I try to use glCompileShader()

char *buffer = 0;
long length;
FILE *f = fopen(vertexPath, "rb");

if(f) {
    fseek(f, 0, SEEK_END);
    length = ftell(f);
    fseek(f, 0, SEEK_SET);
    buffer = malloc(length);
    if(buffer) {
        fread(buffer, 1, length, f);
    }
    fclose(f);
}

if(buffer) {
    unsigned int vertex;
    int success;
    
    vertex = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertex, 1, &buffer, NULL);
    glCompileShader(vertex);
    glGetShaderiv(vertex, GL_COMPILE_STATUS, &success);
    
    if(!success) {
        char *infoLog;
        infoLog = malloc(512);
        glGetShaderInfoLog(vertex, 512, NULL, infoLog);
        printf("%s%s", "Error Fragment\n", infoLog);
        free(infoLog);
    }
}

Here is the warning message that I got:

..\src.\shader.c: In function 'compileShader':
..\src.\shader.c:29:35: warning: passing argument 3 of 'glad_glShaderSource' from 
incompatible pointer type [-Wincompatible-pointer-types]
   29 |         glShaderSource(vertex, 1, &buffer, NULL);
      |                                   ^~~~~~~
      |                                   |
      |                                   char **
..\src.\shader.c:29:35: note: expected 'const GLchar * const*' {aka 'const char * 
const*'} but argument is of type 'char **'

I don't know what this warning means, if it is important, or if I can ignore it. Thx 4 helping.

score 1 · Accepted Answer · answered Mar 20 '22 at 16:23

I don't know what this warning means, if it is important, or if I can ignore it.

It means the &buffer is a char ** (pointer to char pointer) but the function expects a char const * const * (pointer to const pointer to const char).

Basically it's saying that glShaderSource can't modify the pointer or the buffer it points to.

You can resolve it by casting to match the signature:

(char const * const *)&buffer

expected 'const GLchar * const*' {aka 'const char * const*'} but argument is of type 'char **'

1 Answers1

expected 'const GLchar * const' {aka 'const char const*'} but argument is of type 'char **'