For example, the variable gap is defined as char in nqpd.c, but referenced in nqp3.c as short:

nqp3.c:6:21: warning: type of ‘gap’ does not match original declaration [-Wlto-type-mismatch]
    6 |     chsdim, chpdim, gap;
      |                     ^
nqd.c:24:72: note: type ‘char’ should match type ‘short int’
   24 |     outf1[80], outf2[80], outfd[80], outcopy[80], act, ch1, crel, cfm, gap;
      |                                                                        ^
nqd.c:24:72: note: ‘gap’ was previously declared here

Redefining exp (of <math.h> fame) as a variable could cause problems as well:

pcp.c:10:31: warning: built-in function ‘exp’ declared as non-function [-Wbuiltin-declaration-mismatch]
   10 | short      npt, np, npt1, nb, exp, prime;
      |                               ^~~

Defining functions as returning short and calling them through an implicit function declaration (which has an implied return type of int) is undefined, too:

pcscfns.c:21:11: warning: type of ‘image’ does not match original declaration [-Wlto-type-mismatch]
   21 |     addsv(image(p[base[i]]), svptr[i]);
      |           ^
permfns.c:53:7: note: return value type mismatch
   53 | short image(short pt)
      |       ^
permfns.c:53:7: note: type ‘short int’ should match type ‘int’
permfns.c:53:7: note: ‘image’ was previously declared here

These are just examples. I suggest to collect all extern declarations in the "defs.h" header, and then make sure that the types match.

Reproducible: Always