Order of evaluation of parameters...

[gtoal]

printf("Compressed (%i): %u, decompressed: (%u)\n", i, 
 file_lz77_compress("./lz.c", "lz.c.z77", 10000000, i),
 file_lz77_decompress("./lz.c.z77", "lz-2.c"))

C evaluates parameters right-to-left, so decompress gets called before compress, with the result that the correct values are only ever printed if you run the code twice in a row.