C - printf() interchangeability of specifiers

Question

Is there any reason to choose

printf("%hhx", foo);

over

printf("%x", foo);

Here foo is an unsigned char. I've conducted some tests and they appear to be interchangeable, is that really the case? If so, which one should use when bearing portability in mind?

What is the type of `foo` ? Did you carefully read documentation of [printf](http://en.cppreference.com/w/c/io/fprintf) ? — Basile Starynkevitch, Mar 01 '18 at 07:09
https://stackoverflow.com/q/4586962/11683 probably answers your question. — GSerg, Mar 01 '18 at 07:12
Only makes a difference if `foo` is a `signed char` and has a negative value, e.g. `char foo = 0xee;` — user3386109, Mar 01 '18 at 07:14

score 4 · Accepted Answer · answered Mar 01 '18 at 07:21

4

In the specific case of unsigned char foo, there isn't a difference. If you tried:

#include <stdio.h>

int main(void)
{
      signed char bar = '\xFF';
    unsigned char foo = '\xFF';
    printf("%hhx %x\n", bar, bar);
    printf("%hhx %x\n", foo, foo);
    return 0;
}

You'd typically get the output:

ff ffffffff
ff ff

The reason is that in both cases, the character is converted to an int when it is passed to printf(), but the signed character is converted to a negative int, which is then processed by conversion to unsigned char when the length modifier is hh and is left at full size (4 bytes, I'm assuming) when the length modifier is omitted.

answered Mar 01 '18 at 07:21

Jonathan Leffler

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isn't `hh` a type modifier though? – Trey Mar 01 '18 at 07:25
1

In the standard ([C11 §7.21.6.1 The `printf` function ¶4 — also ¶7](https://port70.net/~nsz/c/c11/n1570.html#7.21.6.1p4)), the qualifiers such as `hh` are specifically called _length modifiers._ – Jonathan Leffler Mar 01 '18 at 07:28

C - printf() interchangeability of specifiers

1 Answers1