/in/foo.c: In function 'main':
/in/foo.c:7:10: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  int len=strlen(a);
          ^~~~~~
/in/foo.c:7:10: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:7:10: note: include '<string.h>' or provide a declaration of 'strlen'