Warning(s):

source_file.c: In function ‘main’:
source_file.c:7:9: warning: implicit declaration of function ‘cos’ [-Wimplicit-function-declaration]
         f += cos(i);
         ^
source_file.c:7:14: warning: incompatible implicit declaration of built-in function ‘cos’ [enabled by default]
         f += cos(i);
              ^
source_file.c:12:5: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
     printf("%.4s%.4s%.4s\n", &a, &b, &c);
     ^
source_file.c:12:5: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
source_file.c:12:5: warning: format ‘%s’ expects argument of type ‘char *’, but argument 2 has type ‘int *’ [-Wformat=]
source_file.c:12:5: warning: format ‘%s’ expects argument of type ‘char *’, but argument 3 has type ‘int *’ [-Wformat=]
source_file.c:12:5: warning: format ‘%s’ expects argument of type ‘char *’, but argument 4 has type ‘int *’ [-Wformat=]