/in/bigint.c: In function 'add':
/in/bigint.c:109:9: error: suggest parentheses around assignment used as truth value [-Werror=parentheses]
     if (up = 0)
         ^~
/in/bigint.c: In function 'mul':
/in/bigint.c:125:5: error: 'n' is used uninitialized in this function [-Werror=uninitialized]
     for (i = 0; i < n; i++)
     ^~~
/in/bigint.c:127:5: error: 'm' is used uninitialized in this function [-Werror=uninitialized]
     for (i = 0; i < m; i++)
     ^~~
/in/bigint.c:132:5: error: 'k' is used uninitialized in this function [-Werror=uninitialized]
     for (i = 0; i <= k; i++)
     ^~~
/in/bigint.c: In function 'scan':
/in/bigint.c:42:18: error: iteration 2000 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
         a.val[i] = d.val[d.len - i];
         ~~~~~~~~~^~~~~~~~~~~~~~~~~~
/in/bigint.c:40:5: note: within this loop
     for (int i = 0; i < d.len; i++)
     ^~~
/in/bigint.c: In function 'construct':
/in/bigint.c:62:20: error: iteration 2000 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
         big.val[i] = d.val[d.len - i];
         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
/in/bigint.c:60:5: note: within this loop
     for (int i = 0; i < d.len; i++)
     ^~~
/in/bigint.c: In function 'add':
/in/bigint.c:116:18: error: iteration 2000 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
         c.val[i] = d.val[c.len - i];
         ~~~~~~~~~^~~~~~~~~~~~~~~~~~
/in/bigint.c:114:5: note: within this loop
     for (int i = 0; i < c.len; i++)
     ^~~
cc1: all warnings being treated as errors