C static variables initialization

Question

I learned some C and came across an explanation for static variables. They showed this code:

#include<stdio.h>
int fun()
{
  static int count = 0;
  count++;
  return count;
}
  
int main()
{
  printf("%d ", fun());
  printf("%d ", fun());
  return 0;
}

I can't understand why calling the function twice is fine, because the line

static int count = 0;

actually runs twice... I can't understand how is that possible... Can you actually declare it twice or does the compiler just ignore it the second time?

Static variable initialization only happens the first time the function is called. — Barmar
– Barmar, Commented Dec 24, 2021 at 18:44

Petr Skocik · Accepted Answer · 2021-12-25 02:07:38Z

2

This (statics/globals) is where an initializing definition is really different from an uninitialized definition followed by an assignment. Historically, the former even used to have different syntax (int count /*no '=' here*/ 0;).

When you do:

int fun() {
  static int count = 0;
  //...
}

then except for the different scopes (but not lifetimes) of count, it's equivalent to:

static int count = 0; //wider scope, same lifetime
int fun() {
  
  //...
}

In both cases, the static variable becomes initialized at load time, typically en-masse with other statics and globals in the executable.

edited Dec 25, 2021 at 2:07

answered Dec 25, 2021 at 1:11

Petr Skocik

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Comments

Mureinik · Accepted Answer · 2021-12-25 05:51:53Z

1

static variables are initialized on program startup, not every time the function is called.

edited Dec 25, 2021 at 5:51

answered Dec 24, 2021 at 18:47

Mureinik

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2 Comments

Eric Postpischil Over a year ago

C 2018 5.1.2 1 says “All objects with static storage duration shall be initialized (set to their initial values) before program startup”, emphasis added.

Employed Russian Over a year ago

This answer is only half-correct. Please update it or delete it.

chux · Accepted Answer · 2021-12-25 01:21:13Z

0

... because the line static int count = 0; actually runs twice.

No. Just once, right before main() is called.

answered Dec 25, 2021 at 1:21

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