sprintf misbehaving with floats

["Kline, Matthew" <matthew dot kline at flukenetworks dot com>]

Hello all,

I'm using newlib for some firmware I'm developing on an ARM Cortex-M4
(a STM32F427ZIT6, FWIW), and I'm running into a bit of trouble.

printf and family (sprintf, etc.) seem to misbehave when given floating point
values. The following produces the intended result:

static char foo[1024];
sprintf(foo, "%d is an int", 42);

But the following

sprintf(foo, "%f is a float", 8675.309f);

produces garbage similar to

00000000: 0000 0320 2e49 3802 08d1 0320 6973 2061  ... .I8.... is a
00000010: 2066 6c6f 6174 0000 0000 0000 0000 0000   float..........

Oddly enough, it seems to return the correct value and write a string of the
right length, but the characters where the float should go are badly messed up.

I'm aware that newlib can be built without support for printing and scanning
floats using --disable-newlib-io-float, but my distro package that provides
newlib isn't being built with that flag. The package build script can be found
at https://projects.archlinux.org/svntogit/community.git/tree/trunk/PKGBUILD?h=packages/arm-none-eabi-newlib
and is being built with the following flags:

--disable-newlib-supplied-syscalls \
--disable-nls \
--enable-newlib-io-long-long \
--enable-newlib-register-fini

I'm currently working on building it myself with debug symbols so I can pick
things apart more, but in the meantime, does anybody have an idea of what could
be going on?

Thanks,
Matt Kline