Thread: I don't get what cause the following while loop to terminated

  1. #1
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    I don't get what cause the following while loop to terminated

    The following...

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <utmp.h>
    
    int main(void) {
      FILE *f;
      int n;
      char buffer[1024];
    
      f=fopen(_PATH_UTMP,"rb");
      if (f==NULL) exit(0);
    
      while (1) { /* trademark bartc loop */
    
        n=fread(buffer,1,1024,f);
        printf("N = %d EOF = %d\n",n,feof(f));
        if (n<1024) {
          printf("no more\n");
          break;
        }
      }
    
      fclose(f);
    
    }
    produces..

    [cd@localhost oakland]$ gcc -g bart.c -o bart
    [cd@localhost oakland]$ ./bart
    N = 1024 EOF = 0
    N = 1024 EOF = 0
    N = 1024 EOF = 0
    N = 1024 EOF = 0
    N = 1024 EOF = 0
    N = 1024 EOF = 0
    N = 1024 EOF = 0
    N = 512 EOF = 1
    no more
    [cd@localhost oakland]$
    The question is, what is while being compared against to make this loop terminate. I mean,

    Code:
    while(1){ }
    Is just

    Code:
    while(1 != 0){ }
    So what is getting compared against not zero in this case?

  2. #2
    Registered User
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    It's the "break". That will terminate the (innermost) loop you're in, regardless of whether the condition is true or false.

  3. #3
    Malum in se abachler's Avatar
    Join Date
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    3,195
    whats actually happening is it terminates when it fails to read an entire 1024 bytes from the file, in which case it executes the break statement which exits the current loop.

  4. #4
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    Okay, I see it now. Thanks.

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