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#11

Bug reports / Re: bug report. setvbuf() func...

Last post by TimoVJL - March 10, 2026, 12:24:39 PM

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#include <stdio.h>

int main(void)
{
    int size = 0;
    if (setvbuf(stdout, NULL, _IOFBF, 5000))
        fprintf(stdout, "setvbuf error\n");
    else {
        for (int i = 0; i <= 309; i++)
            size += fprintf(stdout, "'%d'\n", i);
        fprintf(stdout, "size: %d\n", size);
    }
    return 0;
}


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'307'
'08'
'309'
size: 1750

#12

Bug reports / Re: bug report. setvbuf() func...

Last post by John Z - March 10, 2026, 02:57:04 AM

Great point!  Checking the return shows my 'fix' only works because setvbuf fails return is -1! so no buffer at all.

When 'fix' is removed setvbuf passes return is 0 but then errors on 7003.  In fact multiple errors in the return stream:
 
267? 0
268? 0
269?
270? 0
271? 0
-------
523? 0
524? 0
525?
526? 0
527? 0
-------
779? 0
780? 0
781?
782? 0
783? 0
-------
1011? 0
1012? 0
1013? 3? 0
1014? 0
1015? 0
-------
1239? 0
1240? 0
141? 0
1242? 0
1243? 0

Yikes something going on . . . . almost every 256th but some at 228

John Z

[setvbuf()]

#13

Bug reports / Re: bug report. setvbuf() func...

Last post by TimoVJL - March 10, 2026, 01:10:28 AM

how about check function setvbuf() error too ?

#14

Bug reports / Re: bug report. setvbuf() func...

Last post by John Z - March 09, 2026, 09:59:50 PM

Test this - works for all buffer size setting I tried -

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#include <stdio.h>

int main(void)
{   // all problems go away when next line is added
    printf("test result");// initializes stdout I/O ?
   
    setvbuf(stdout, NULL, _IOFBF, 7003);
    for (int i = 0; i <= 1351; i++)
        printf("%d?\n", i);
   
    return 0;
}
Maybe because of this statement in the help file?
"The setvbuf function may be used only after the stream pointed to by stream has been associated with an open file and before any other operation (other than an unsuccessful call to setvbuf) is performed on the stream. "

I am thinking that printf is initializing the stdout stream before setvbuf is called on stdout so no problem.

John Z

#15

Bug reports / Re: bug report. setvbuf() func...

Last post by TimoVJL - March 09, 2026, 05:31:09 PM

simpler check

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#include <stdio.h>

int main(void)
{
    setvbuf(stdout, NULL, _IOFBF, 7000);
    for (int i = 0; i <= 1351; i++)
        printf("%d?\n", i);
   
    return 0;
}
output:

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...
1328?
329?
1330?
1331?
1332?
1333?
1334?
1335?
1336?
1337?
1338?
1339?
1340?
1341?
1342?
1343?
1344?
1345?
1346?
1347?
1348?
1349?
1350?
13511351?

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'1157'
'1151158'
'1159'size: 7003

#16

Bug reports / Re: bug report. setvbuf() func...

Last post by ander_cc - March 09, 2026, 02:26:45 PM

https://en.cppreference.com/w/c/io/setvbuf
buffer size should be at least multiple of 2

I have read it last week. "the actual buffer size is usually rounded down to a multiple of 2". Just NOT use some bytes, so any number is OK.

I try it at home, my laptop. see picture.
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#17

Bug reports / Re: stdckdint.h bug report

Last post by TimoVJL - March 09, 2026, 12:10:59 PM

with dataset:

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    int a = 655350;
    int b = 10;
    short c = 0;
Clang output:

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true  c = -100
true  c = 0
true  c = -20

#18

Bug reports / Re: bug report. setvbuf() func...

Last post by TimoVJL - March 09, 2026, 11:54:49 AM

https://en.cppreference.com/w/c/io/setvbuf
buffer size should be at least multiple of 2

#19

Bug reports / Re: bug report. setvbuf() func...

Last post by ander_cc - March 09, 2026, 10:10:37 AM

Hi ander,

I could not reproduce this.  With the setvbuf 1000, 5000, 7000, or not even use setvbuf it always ends the loop on 20000 .  I just pasted your code in. 

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19998
19999
20000
Press any key to continue...
John Z

thank you for test.
I changed the number and try it again, and got different result, I got a new line "00".
Windows 11 Pro 25H2 simplified chinese. Only for chinese windows? I will try it on my laptop at home later

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#20

Bug reports / Re: bug report. setvbuf() func...

Last post by John Z - March 09, 2026, 09:56:30 AM

Hi ander,

I could not reproduce this.  With the setvbuf 1000, 5000, 7000, or not even use setvbuf it always ends the loop on 20000 .  I just pasted your code in. 

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19998
19999
20000
Press any key to continue...
John Z