GLSL 150 GL 3.2 black textures - c++

Question

I can't get my textures to work, all the screen is black.
Here is my code for loading the images, I use lodepng:
std::vector<unsigned char> image;
unsigned int error = lodepng::decode(image, w, h, filename);
GLuint texture_id;
glGenTextures(1, &texture_id);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture_id);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, 4, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, &image[0]);
glBindTexture(GL_TEXTURE_2D, 0);
For the rendering I do this:
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture_id_from_above); //texture_id were checked, seemed fine
glUniform1i(shader_sampler_loc, GL_TEXTURE0);
and my frag shader(trimmed version) is basically doing this:
uniform sampler2D sampler;
void main(void) {
gl_FragColor = texture2D(sampler, uv_coord);
}
The UV-coordinates are fine, the vector from lodepng containes many elements and there is no error returned. To further pin the problem I tried this:
gl_FragColor = texture2D(sampler, uv_coord)*0.5 + vec4(1, 1, 1, 1)*0.5f
To see if the whole assignment is somehow skipped or the texture in fact black. As a result I still only get a black window. But by removing
glActiveTexture(GL_TEXTURE0); //x2, and
glUniform1i(sampler_loc, GL_TEXTURE0);
all my objects appear gray. I have no clue what is wrong.
BTW: it was working before moving to OpenGL 3.2 (had 2.1 before), and all images are ^2. I use CORE_PROFILE && FORWARD_COMPAT.
Vertex shader:
#version 150
//VBO vertex attributes
attribute vec3 pos;
attribute vec2 tex;
attribute vec3 normal;
varying vec2 uv_coord;
uniform mat4 mvp_mat;
void main(void) {
gl_Position = mvp_mat * vec4(pos, 1);
uv_coord = tex;
}

Answer 1

glUniform1i(shader_sampler_loc, GL_TEXTURE0);
. . . should be
glUniform1i(shader_sampler_loc, 0);
etc.

Answer 2

So I kind of solved it, by using OPENGL_COMPAT_PROFILE it works. Though I would really want to go full 3.2, and find which parts are deprecated...
EDIT:
In my case, I finally found the dumb error, I was using
glTexImage2D(GL_TEXTURE_2D, 0, 4, ... //instead of
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, ...
So I guess the with the old GL I was lucky, and with 3.2 the enum were changed?

Answer 3

As comments suggest try changing the following:
Vertex shader:
out instead of varying
Add layout(location = #) to your attributes and change attribute for in.
Make sure, number of locations match the code
Fragment shader: (Assuming since its not complete)
in instead of varying
Add layout(location = #) to your sampler uniform
For the code:
Change
glUniform1i(sampler_loc, GL_TEXTURE0)
to
glUniform1i(sampler_loc, 0)