/in/foo.c:1:1: error: unknown type name 'def'
 def isPrime(x):
 ^~~
/in/foo.c: In function 'isPrime':
/in/foo.c:1:15: error: expected declaration specifiers before ':' token
 def isPrime(x):
               ^
/in/foo.c:1:5: warning: type of 'x' defaults to 'int' [-Wimplicit-int]
 def isPrime(x):
     ^~~~~~~
/in/foo.c:7:5: warning: control reaches end of non-void function [-Wreturn-type]
     return 1
     ^~~~~~