/in/foo.c: In function 'main':
/in/foo.c:20:16: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  for(int i=0;i<strlen(sentences);i++)
                ^~~~~~
/in/foo.c:20:16: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:20:16: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:37:5: error: 't' undeclared (first use in this function)
  if(t->top==-1)
     ^
/in/foo.c:37:5: note: each undeclared identifier is reported only once for each function it appears in