/in/foo.c: In function 'mystrcat':
/in/foo.c:12:11: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  int tlen=strlen(t);
           ^~~~~~
/in/foo.c:12:11: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:12:11: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:15:4: error: 'i' undeclared (first use in this function)
  t[i+tlen-1]=0;
    ^
/in/foo.c:15:4: note: each undeclared identifier is reported only once for each function it appears in