prog.c: In function ‘main’:
prog.c:6:3: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
printf("Hello World!%d/n",a);
^~~~~~
prog.c:6:3: warning: incompatible implicit declaration of built-in function ‘printf’
prog.c:6:3: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
prog.c:7:9: error: ‘num’ undeclared (first use in this function)
num++;
^~~
prog.c:7:9: note: each undeclared identifier is reported only once for each function it appears in