return value of a function in C when it does not return any value

return value of a function in C when it does not return any value

what does a function in C return when it does not have any return value ...
for example this code:
#include<stdio.h>
int x;
f1(void);
f2(void);
main()
{
x=10;
printf("x = %d, ",x);
printf("x = %d, ",f1());
printf("x = %d, ",f2());
}
f1(void)
{
}
f2(void)
{
}