/in/foo.c: In function 'plus':
/in/foo.c:9:17: error: 'i' undeclared (first use in this function)
   rev[k]=is[len-i-1];
                 ^
/in/foo.c:9:17: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c: In function 'main':
/in/foo.c:37:6: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  len=strlen(strs);
      ^~~~~~
/in/foo.c:37:6: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:37:6: note: include '<string.h>' or provide a declaration of 'strlen'