glpk creating correct linear program but returning incorrect values after its resolution

Hello, 

I'm currently trying to resolve a cutting stock problem using the glpk c librairy. The problem uses the values found in this page: 

http://www.unc.edu/depts/stat-or/courses/provan/STOR614_web/lect12_cutstock.pdf 

I managed to execute the first step of the resolution, applying the simplex method to a reduce matrix and retrieving the dual values, but when it comes to column generation, I'm lost. I'm using a glpk's integer problem resolution, the column generation being akin to a knapsack problem, but the values returned seem very illogical to me. I especially obtain a negative value while I set a lower bound of 0 to all my columns. I used the glp_write_lp() function on my knapsack problem to see if the linear program created is correct, and it seems to me that it is. 

I think I either failed in setting the lower 0 bound, or in using the correct functions to retrieve my results. 

Could you help me identify the problem? You will find a zip joined to this message with the main.cpp, a makefile for compilation, a lp.txt file (linear program of the original problem), and a knapsack.txt file (the problematic linear program, that seems right to me but from which I can't seem to extract the results) 

Thanks in advance.

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