Get texture target by texture id

As far as I know, there isn't.

A possible workaround could be querying the current binding for every texture target used by your application and compare the current texture against the id you have.

GLuint currentTex;

glGetIntegerv(GL_TEXTURE_BINDING_1D, &currentTex);
if (currentTex == testTex)
{
    target = GL_TEXTURE_1D;
    return;
}

glGetIntegerv(GL_TEXTURE_BINDING_2D, &currentTex);
if (currentTex == testTex)
{
    target = GL_TEXTURE_2D;
    return
}

// and so on ...

Of course that you must have a texture bound for this to work. If binding with glBindTexture then you need the target anyway.

But this solution is so clumsy and non-scalable that it is generally much easier to just keep an extra int together with the id for the texture target.

Since OpenGL 4.5 this can be done by:

GLenum target;
glGetTextureParameteriv(textureId, GL_TEXTURE_TARGET, (GLint*)&target);

It's also true that since the introduction of the direct-state-access API (DSA) in OpenGL 4.5, knowing the target of the texture became not as useful.

There really isn't a pretty way to do this that I could find, even after looking at the state tables in the specs. Two possibilities that are both far from attractive:

1. Try binding it to various targets, and see if you get a GL_INVALID_OPERATION error:

   glBindTexture(GL_TEXTURE_1D, texId);
   if (glGetError() != GL_INVALID_OPERATION) {
       return GL_TEXTURE_1D;
   }
   glBindTexture(GL_TEXTURE_2D, texId);
   if (glGetError() != GL_INVALID_OPERATION) {
       return GL_TEXTURE_2D;
   }
   ...
   

2. This is similar to what @glamplert suggested. Bind the texture to a given texture unit with glBindTextures(), and then query the textures bound to the various targets for that unit:

   glBindTextures(texUnit, 1, &texId);
   glActiveTexture(GL_TEXTURE0 + texUnit);
   GLuint boundId = 0;
   glGetIntegerv(GL_TEXTURE_BINDING_1D, &boundId);
   if (boundId == texId) {
       return GL_TEXTURE_1D;
   }
   glGetIntegerv(GL_TEXTURE_BINDING_2D, &boundId);
   if (boundId == texId) {
       return GL_TEXTURE_2D;
   }
   

But I think you would be much happier if you simply store away which target is used for each texture when you first create it.