/in/foo.c: In function 'main':
/in/foo.c:8:5: warning: implicit declaration of function 'sort' [-Wimplicit-function-declaration]
     sort(a,a+n);
     ^~~~
/tmp/cc64d6Zb.o: In function `main':
foo.c:(.text.startup+0x78): undefined reference to `sort'
collect2: error: ld returned 1 exit status