John Smith John Smith - 9 months ago 47
C Question

Warning: type of ‘numRest’ defaults to ‘int’ (in function 'sleep')

I get a warning In function ‘sleep’: warning: type of ‘numRest’ defaults to ‘int’ and I have no idea why. It runs perfectly fine but apparently I got this warning. Does anyone else get this warning when they run it?

void sleep(numRest){

if ((numRest >= 0) && (numRest <=4)){
printf("Sleep deprived!");

else if ((numRest > 4) && (numRest < 6)){
printf("You need more sleep.");

else if ((numRest >= 6) && (numRest < 8)){
printf("Not quite enough.");

printf("Well done!");


int main()
int numSleep = -1;

if (numSleep == -1){
printf("Test 1\n");
printf("Input: -1\n");
printf("Expected Result: Error, you cannot have a negative number of hours of sleep.\n");
printf("Actual Result: ");

numSleep = 4.5;
printf("Test 2\n");
printf("Input: 4.5\n");
printf("Expected Result: You need more sleep.\n");
printf("Actual Result: ");


return 0;

Answer Source

The issue is with the function signature definition.

 void sleep(numRest) {

should be

void sleep(int numRest) {

Otherwise, the compiler will "assume" (now obsolete by latest standard) that the missing datatype is int.

Related, quoting from C11, Major changes (over previous versions) list

  • remove implicit int

That said,

  • sleep() is a library function already, prototyped in unistd.h, do not try to use the same for for user-defined functions.
  • int main() should be int main(void), at least for hosted environments to conform to the standard.