Printf(), Unsigned Char, And %hhd - Stack Overflow

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Learn more about Labs printf(), unsigned char, and %hhd Ask Question Asked 6 years, 5 months ago Modified 6 years, 5 months ago Viewed 7k times 1

The program:

#include <stdio.h> #include <limits.h> int main( void ) { #ifdef __CHAR_UNSIGNED__ printf( "%d\n", __CHAR_UNSIGNED__ ); #endif printf( "%d\n", CHAR_MAX ); printf( "%d\n", CHAR_MIN ); printf( "%hhd\n", CHAR_MAX ); }

Output (on my x86_64 desktop):

127 -128 127

That is as expected. Now, I ran the same on a Raspberry Pi (ARM):

1 255 0 -1

So... apparently I have misunderstood some step CHAR_MAX is taking in its way to output, as the output I was expecting in that last line -- passing CHAR_MAX to %hhd on a machine with char being unsigned -- would have been 255.

(And if you should ask, the result is the same for (char)CHAR_MAX, and (unsigned char)CHAR_MAX.)

What am I missing?

This happened to me during regression testing my own printf() implementation on the Raspberry Pi -- which, by the way, does print 255. So... only one implementation is doing it right, and I have the sinking feeling it's not mine...

• c
• printf

Share Improve this question Follow edited Jul 2, 2018 at 17:47 DevSolar asked Jul 2, 2018 at 17:41 DevSolarDevSolar 70k2121 gold badges137137 silver badges215215 bronze badges 5

• 1 "Output (on my x86_64 desktop):..." You're missing a line. A very important line. – Mad Physicist Commented Jul 2, 2018 at 17:42
• 1 Think about what that __CHAR_UNSIGNED__ being non-zero could mean... And remember that it's implementation specific if char is signed or unsigned. – Some programmer dude Commented Jul 2, 2018 at 17:44
• What is __CHAR_UNSIGNED__ ??? – Stargateur Commented Jul 2, 2018 at 17:44
• @MadPhysicist: Actually I was missing two lines in my program, which I added after the x86_64 compiler barked at me. ;-) – DevSolar Commented Jul 2, 2018 at 17:44
• @Someprogrammerdude: I am thinking about what __CHAR_UNSIGNED__ means. That was why I was expecting the ARM output for %hhd to read 255 instead of -1 for CHAR_MAX... – DevSolar Commented Jul 2, 2018 at 17:45

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1 Answer 1

Sorted by: Reset to default Highest score (default) Trending (recent votes count more) Date modified (newest first) Date created (oldest first) 8

The format "%hhd" is for a signed byte. If you pass an unsigned byte then you technically have undefined behavior. To print an unsigned byte you need to use "%hhu".

What really happens is because of how two's complement integers work, and that 0xff is equal to -1.

Share Improve this answer Follow answered Jul 2, 2018 at 17:47 Some programmer dudeSome programmer dude 408k3535 gold badges412412 silver badges642642 bronze badges 6

• head -> desk. Of course. {Cursing voice fading into the background...} – DevSolar Commented Jul 2, 2018 at 17:48
• So now I may debug my printf() to figure out what went wrong - wrong in there. :-D Geeeeeez.... – DevSolar Commented Jul 2, 2018 at 17:48
• 2 Hmmm... on second thought... if passing an unsigned value to %hhd is UB, does it really matter that my implementation gives a different result...? (Just talking to myself here, feel free to not answer. ;-) ) – DevSolar Commented Jul 2, 2018 at 17:53
• @DevSolar is your implementation give 0 for printf( "%hhu\n", 256); ? I suspect not ;) – Stargateur Commented Jul 2, 2018 at 19:36
• @Stargateur: It does, actually. ;-) – DevSolar Commented Jul 2, 2018 at 20:03

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