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crecursionreturn-valuecall

what happens when function doesnt explicitly return a value? can't figure out how the output of the code is coming

I came across a question in a test series, which when I solved manually according to my knowledge of C programming, should give an output which wasn't matching with any of the options given.

My output = ' ++++ '

Question: Output of following c program is?

 #include <stdio.h>

 int f(int x) 
 {
     if(x==2){ return 2; } 
     else{ printf("+"); f(x-1); }
 }
 int main() 
 {
     int n = f(6); 
     printf("%d",n); 
     return 0; 
 }

Options:

  1. '++++2' (correct option acc to answer key)
  2. '+++++2'
  3. '+++++'
  4. '2'

My logic: Because in the end f(6) doesn't explicitly return anything [only f(2) returns the value 2 to f(3)], the output should only contain the 4 times '+' due to each call f(6), f(5), f(4) and f(3).

Below are some test code and their outputs screenshots i tried on online c compilers - 'codechef' and 'onlinegdb' - but i couldnt make sense of their outputs either. Please help!

codechef

codechef

onlinegdb 1

onlinegdb 1

onlinegdb 2

onlinegdb 2

Solution

  • If a function is defined to return a value and it doesn't, then attempting to use the returned value results in undefined behavior.

    This is documented in section 6.9.1p12 of the C standard:

    If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

    This basically means that the result is not predictable and/or consistent.