Use uniform buffer of uvec3[] - opengl

Question

I have the following uniform buffer:
uniform EDGE_ID_TO_START_POS{
uint[12*3] pos;
} edgeIdToStartPos;
Writing to this buffer works perfectly fine:
#define EDGE_ID_TO_START_POS_SIZE (12*3)
const uint32_t edgeIdToStartPos_constBuffer[EDGE_ID_TO_START_POS_SIZE] = {
/* 0*/ 0, 0, 0, /* 1*/ 0, 1, 0, /* 2*/ 1, 0, 0, /* 3*/ 0, 0, 0,
/* 4*/ 0, 0, 1, /* 5*/ 0, 1, 1, /* 6*/ 1, 0, 1, /* 7*/ 0, 0, 1,
/* 8*/ 0, 0, 0, /* 9*/ 0, 1, 0, /*10*/ 1, 1, 0, /*11*/ 1, 0, 0
};
glBindBuffer(GL_UNIFORM_BUFFER, ubo);
glBufferData(GL_UNIFORM_BUFFER, EDGE_ID_TO_START_POS_SIZE * sizeof(uint32_t), edgeIdToStartPos_constBuffer, GL_STATIC_DRAW);
glBindBuffer(GL_UNIFORM_BUFFER, 0);
Now I want to change the uniform buffer to the following structure:
uniform EDGE_ID_TO_START_POS{
uvec3[12] pos;
} edgeIdToStartPos;
This structure makes more sense and is easier to use. And it should have the same memory layout than the previous version (?).
However, when reading from this struct, only the vectors 0..3 contain data. All uvecs with an index of 4 or higher contain zeros.
Why does this happen and how can I initialise such a uniform buffer correctly?
(C/C++, glew, glfw)

Answer 1

A uvec3 has an alignment of 16 bytes, so you must add padding. See OpenGL 4.5 spec, section 7.6.22:
If the member is a three-component vector with components consuming N basic machine units, the base alignment is 4N.
Here is how it would look:
static const int EDGE_ID_TO_START_POS_SIZE = 12 * 4;
const uint32_t edgeIdToStartPos_constBuffer[EDGE_ID_TO_START_POS_SIZE] = {
/* 0*/ 0, 0, 0, 0, /* 1*/ 0, 1, 0, 0, /* 2*/ 1, 0, 0, 0, /* 3*/ 0, 0, 0, 0,
/* 4*/ 0, 0, 1, 0, /* 5*/ 0, 1, 1, 0, /* 6*/ 1, 0, 1, 0, /* 7*/ 0, 0, 1, 0,
/* 8*/ 0, 0, 0, 0, /* 9*/ 0, 1, 0, 0, /*10*/ 1, 1, 0, 0, /*11*/ 1, 0, 0, 0,
};