#2242 printf error?

[El Condor]

Hello

It seems that the same function, with different input values, return the first value.
This program compiled with gcc (GCC) 4.9.1

#include <stdio.h>
char * Time(long ms) {  // from start time
    static char timeOfDay[9] = "00:00:00";
    char s = (ms/1000) % 60;
    long h = ms / 3600000;
    int m = (ms/1000 - h*3600)/60;
    timeOfDay[0] = 48+h/10;
    timeOfDay[1] = 48+h%10;
    timeOfDay[3] = 48+m/10;
    timeOfDay[4] = 48+m%10;
    timeOfDay[6] = 48+s/10;
    timeOfDay[7] = 48+s%10;
    return timeOfDay;
}
int main(int argc, char * argv[]) {
    printf("7 %s 0 %s \n",Time(7000),Time(0));
    printf(" 0 %s \n",Time(0));
}

outputs

7 00:00:07 0 00:00:07
0 00:00:00

My be it is my error because this happens also when compiled with cint 5.16.19

Best regards

John Rossati

[Keith Marshall]

Yes, it is your error: you are printing the content of the same static buffer twice, after your function to populate it has been called for the second time; you have no sequence point to ensure that printf() will ever see its intermediate state, between the two calls to Time().

To clarify, here's what happens when your printf() call is executed:

1. All function calls, to determine its arguments, are invoked in unspecified order, and their return values, (the pointer to the same static buffer, in the case of your two Time() calls), are pushed on to the stack. At this point, you have two copies of the same pointer on the stack, and the static buffer content is as left by the second call to Time().

2. Control is then passed to printf(); it reads its arguments from the stack, and finds the two identical pointers, so it prints the same content twice. Your results suggest that the two Time() calls were processed in right-to-left order, which is logical, but by no means guaranteed; it doesn't matter: at the time when printf() interpolates the buffer content into the output stream, it can have only one value.

[Keith Marshall]

BTW, where did you get gcc-4.9.1? We haven't published any packages for this version yet, so it can't be ours; thus, were this not your bug, it certainly would not be ours.

[El Condor]

Thanks

I downloaded from http://nuwen.net (Stephan T. Lavavej).

John Rossati

----Messaggio originale----

Da: keithmarshall@users.sf.net

Data: 06/11/2014 12.41

A: "[mingw:bugs] "2242@bugs.mingw.p.re.sf.net

Ogg: [mingw:bugs] #2242 printf error?

BTW, where did you get gcc-4.9.1? We haven't published any packages for this version yet, so it can't be ours; thus, were this not your bug, it certainly would not be ours.

[bugs:#2242] printf error?

Status: closed

Group: OTHER

Labels: printf

Created: Thu Nov 06, 2014 08:51 AM UTC by El Condor

Last Updated: Thu Nov 06, 2014 11:23 AM UTC

Owner: nobody

Hello

It seems that the same function, with different input values, return the first value.

This program compiled with gcc (GCC) 4.9.1

include <stdio.h></stdio.h>

char * Time(long ms) { // from start time
static char timeOfDay[9] = "00:00:00";
char s = (ms/1000) % 60;
long h = ms / 3600000;
int m = (ms/1000 - h*3600)/60;
timeOfDay[0] = 48+h/10;
timeOfDay[1] = 48+h%10;
timeOfDay[3] = 48+m/10;
timeOfDay[4] = 48+m%10;
timeOfDay[6] = 48+s/10;
timeOfDay[7] = 48+s%10;
return timeOfDay;
}
int main(int argc, char * argv[]) {
printf("7 %s 0 %s \n",Time(7000),Time(0));
printf(" 0 %s \n",Time(0));
}

outputs

7 00:00:07 0 00:00:07

0 00:00:00

My be it is my error because this happens also when compiled with cint 5.16.19

Best regards

John Rossati

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