/in/foo.c: In function 'main':
/in/foo.c:17:12: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
     if (s1[strlen(s1) - 1] == '\n') {
            ^~~~~~
/in/foo.c:17:12: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:17:12: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:23:12: warning: incompatible implicit declaration of built-in function 'strlen'
     if (s2[strlen(s2) - 1] == '\n') {
            ^~~~~~
/in/foo.c:23:12: note: include '<string.h>' or provide a declaration of 'strlen'