If you built with sanitize, it should provide diagnostics with the buffer overflow, e.g. 

=================================================================
==99096==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000101938fe0 at pc 0x00010169d0b8 bp 0x00016f1f2240 sp 0x00016f1f2238
READ of size 1 at 0x000101938fe0 thread T0
#0 0x10169d0b4 in f_s fmt.c:98
#1 0x10169e358 in pars_f fmt.c:342
#2 0x10169fc9c in c_si iio.c:53
#3 0x1016a0220 in s_rsfi iio.c:85
#4 0x100c14380 in get_token__ parser.c:1113
#5 0x100c12f04 in parser_ parser.c:215
#6 0x100c25608 in PARSER_generate_code parser_int.c:59


This information is what is used to understand your problem.