/in/foo.c: In function 'merge':
/in/foo.c:17:18: warning: operation on 'k' may be undefined [-Wsequence-point]
    a3[k++] = a1[k++];
                 ~^~
/in/foo.c:19:18: warning: operation on 'k' may be undefined [-Wsequence-point]
    a3[k++] = a2[k++];
                 ~^~
/in/foo.c:22:17: warning: operation on 'k' may be undefined [-Wsequence-point]
   a3[k++] = a1[k++];
                ~^~
/in/foo.c:24:17: warning: operation on 'k' may be undefined [-Wsequence-point]
   a3[k++] = a2[k++];
                ~^~
/in/foo.c:26:17: warning: operation on 'k' may be undefined [-Wsequence-point]
   a2[k++] = a3[k++];
                ~^~
/in/foo.c: In function 'main':
/in/foo.c:34:2: error: too few arguments to function 'merge'
  merge(a1,3,a2,2, a3);
  ^~~~~
/in/foo.c:10:6: note: declared here
 void merge(int a1[], int n1, int a2[], int n2, int a3[],int n3)
      ^~~~~