[SOLVED] Strange interaction between sleep() and printf strings: GCC/Debian

BruceV · 11-16-2022, 06:47 AM

I encountered a strange problem in the very early stages of prototyping a test routine in GCC under Debian. The program follows, it should compile and run, please excuse any syntactical or style shortcomings, cleanup will come later.

Sorry, my indents are being removed by the site. Can't find a way to put them in.

The program loops with a 1 second sleep delay at the end of the loop. As presented, what I saw is that the sleep delay was being bypassed for certain values of the counter, two of the Entering statements would occur together. When I removed the ‘\n’s from the last two printf statements, the problem disappeared.

I wasn’t aware that there would be any interaction between control characters in printf strings and the sleep function. Can anyone explain what's going on?

Thanks for any info.

/* Test program for ......
November 22
*/

#include <stdio.h>
#include <malloc.h>

int main()
{
int counter, ledstate ;

ledstate = 0 ;
counter = 4 ;

while (1)
{
printf ("\nEntering %d", counter) ;

if (counter > 0) counter = counter - 1 ;

if (ledstate == 1)
{
ledstate = 0 ;
counter = 6 ;
printf ("\nLED OFF, dividers set") ;
}

else if (ledstate == 0 && counter == 0)
{
ledstate = 1 ;
printf ("\nLED ON, MCCNT = 14000") ;
}

sleep(1) ;
}
return 0 ;
}

pan64 · 11-16-2022, 07:05 AM

would be nice to use code tags for your code to keep formatting.
In general the print output can be buffered, probably you need to switch it off.

smallpond · 11-16-2022, 08:05 AM

put your \n at the end of the string if you want it to be printed right away. By default, output is buffered until \n.

computersavvy · 11-16-2022, 06:11 PM

The only thing I see is the location of the \n as previously noted, and a warning about implicit declaration of the sleep(1) function.
Your program seems to work for me and the only mod I made was to relocate the \n to the end of the string in each of those 3 printf statements.

rclark · 11-16-2022, 10:09 PM

Code:

/* 
   Test program for ......
   November 22 
*/

#include <stdio.h>
#include <malloc.h> // not needed, should comment out or delete
#include <unistd.h> // for sleep() get rid of implicit warning

int main()
   {
   int counter, ledstate;

   ledstate = 0;
   counter  = 4;
   while (1)
      {
      printf("\nEntering %d", counter);

      if (counter > 0) 
         counter = counter - 1;

      if (ledstate == 1)
         {
         ledstate = 0;
         counter  = 6;
         printf("\nLED OFF, dividers set");
         }
      else if ((ledstate == 0) && (counter == 0))
         {
         ledstate = 1;
         printf("\nLED ON, MCCNT = 14000");
         }
      sleep(1);
      }
   return 0;
   }

output:

Code:

Entering 4
Entering 3
Entering 2
Entering 1
LED ON, MCCNT = 14000
Entering 0
LED OFF, dividers set
Entering 6
Entering 5
Entering 4
Entering 3
Entering 2
Entering 1
LED ON, MCCNT = 14000
Entering 0
LED OFF, dividers set
Entering 6
Entering 5
Entering 4
Entering 3
Entering 2
Entering 1
LED ON, MCCNT = 14000
...

Works fine here.

computersavvy · 11-17-2022, 12:38 PM

With the original code as posted I added a single printf statement

Code:

printf ("sleeping\n") ;

immediately before the sleep(1) statement and this is the result of the compile and test.

Yes, it seemed to work fine with no changes, but the location of the \n at the beginnint of the print line causes the extra spacing in the output lines as soon as one additional printf statement is added that ends with a \n. This clearly shows that the output is buffered until it receives the \n (newline) output.

Code:

$ gcc -o testitprint test.c
test.c: In function ‘main’:
test.c:34:1: warning: implicit declaration of function ‘sleep’ [-Wimplicit-function-declaration]
   34 | sleep(1) ;
      | ^~~~~
[jvian@eagle test]$ ./testitprint 

Entering 4sleeping

Entering 3sleeping

Entering 2sleeping

Entering 1
LED ON, MCCNT = 14000sleeping

Entering 0
LED OFF, dividers setsleeping

Entering 6sleeping

I then modified the 3 original printf statements to move the \n to the end of the line and this was the result.

Code:

$ gcc -o testitprint2 test2.c
test.c: In function ‘main’:
test.c:34:1: warning: implicit declaration of function ‘sleep’ [-Wimplicit-function-declaration]
   34 | sleep(1) ;
      | ^~~~~
[jvian@eagle test]$ ./testitprint2 
Entering 4
sleeping
Entering 3
sleeping
Entering 2
sleeping
Entering 1
LED ON, MCCNT = 14000
sleeping
Entering 0
LED OFF, dividers set
sleeping

Modified program with the original printf statements altered and the added printf statement.

Code:

$ cat test2.c
/* Test program for ......
November 22
*/

#include <stdio.h>
#include <malloc.h>

int main()
{
int counter, ledstate ;

ledstate = 0 ;
counter = 4 ;

while (1)
{
printf ("Entering %d\n", counter) ;

if (counter > 0) counter = counter - 1 ;

if (ledstate == 1)
{
ledstate = 0 ;
counter = 6 ;
printf ("LED OFF, dividers set\n") ;
}

else if (ledstate == 0 && counter == 0)
{
ledstate = 1 ;
printf ("LED ON, MCCNT = 14000\n") ;
}
printf ("sleeping\n") ;
sleep(1) ;
}
return 0 ;
}