GCC ARM C compiler not respecting -std=c99 for %llx printf formatting code

Question

I'm trying to print some 64-bit unsigned integers using something like this:

uint64_t x = 0xFFFFFFFFFFFFFFFF;
printf("Value: %016llx \n", x);

and I get this in response:

0000000000000000lx

If I change the formatting string to %016lx I get a compile warning for using the wrong type and it only prints the lower 32 bits:

00000000FFFFFFFF

I've got the -std=c99 string in my compiler options, which should enforce the ll option, right?

For reference, this is the arm-none-eabi-gcc compiler (v4.7.3) that ships with the latest version of Silicon Labs' Simplicity Studio IDE.

This isn't a characteristic of the compiler, it's a problem with your C library. Fix it, or use a different one? — Carl Norum, Oct 02 '14 at 18:53

soerium · Accepted Answer · 2014-10-02T19:22:22.457

1

Have you tried this ?

#include <stdio.h>
#include <inttypes.h>

int
main(void)
{
    uint64_t n = 0x123456789;

    printf("n = %#" PRIx64 "\n", n);

    return (0);
}

Implementation of printf or scanf depends on libc your're using and may not support long long. Please, check it by printing value of PRIx64 - I guess in your case it will be "lx" instead of "llx".

Pasting a part of inttype.h of ARM toolchain gcc-arm-embedded

/* 64-bit types */
#if __have_long64
#define __PRI64(x) __STRINGIFY(l##x)
#define __SCN64(x) __STRINGIFY(l##x)
#elif __have_longlong64
#define __PRI64(x) __STRINGIFY(ll##x)
#define __SCN64(x) __STRINGIFY(ll##x)
#else
#define __PRI64(x) __STRINGIFY(x)
#define __SCN64(x) __STRINGIFY(x)
#endif
...
#define PRIx64          __PRI64(x)
#define PRIX64          __PRI64(X)

edited Oct 02 '14 at 19:22

answered Oct 02 '14 at 17:50

soerium

573
4
12

+1, but could you perhaps add some explanation? That would make it a really great answer :) – Jens Gustedt Oct 02 '14 at 18:23
A C library that doesn't support `%llx` is unlikely to have `PRIx64` - presumably it would have to *be* `llx`. – Carl Norum Oct 02 '14 at 18:54
Yeah the library doesn't know what to do with PRIx64: `error: expected ')' before 'PRIx64'` – Mouser Williams Oct 03 '14 at 14:38
Aah, after adding the `#include ` it compiles and gives me `n = lx` – Mouser Williams Oct 03 '14 at 14:44
Based on that output I'd assume that I am unable to print long longs using the current C library I have, though I find this line in inttypes.h confusing: `#define __PRI64(x) __STRINGIFY(ll##x)` – Mouser Williams Oct 03 '14 at 14:53
Please note that long long int and long int are not the same type but they happen to be the same size. – soerium Oct 04 '14 at 06:41

GCC ARM C compiler not respecting -std=c99 for %llx printf formatting code

1 Answers1