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ccmakec99format-specifiersformat-string

%zu format specifier with C99 not working

I'm willing to print a size_t value using the %zu format specifier in my format string, however, I always get "zu" as an output, rather than the actual value in my size_t variable:

size_t val = 10;
printf("val: %zu\n", val);  // outputs "zu", not "10"

I've seen that for other people that faced a similar issue, the fix was to set the C standard to C99 or above.

I'm building my project with CMake, and I have the following line in it:

# Set the C standard to C11
set(CMAKE_C_STANDARD 11)

So, I'd assume that I'm good to go, but no, I'm still getting the same issue.

Could I be missing something?

I'm using the following stack:

  • CMake version 3.22
  • Cross compiling with the arm-none-eabi-gcc version 10.3.1 toolchain
  • compilation flags: -Os -g -ffunction-sections -fdata-sections -fno-common -fmessage-length=0 -mcpu=cortex-m4 -mthumb -mthumb-interwork -mlittle-endian -mfloat-abi=hard -mfpu=fpv4-sp-d16

I've also added the -std=c11 compilation option just in case. Still does not work.

Solution

  • The standard library (newlib) used in the toolchain (Arm GNU Toolchain 10.3.1 provided by arm) does not have the c99 I/O format support enabled. If _WANT_IO_C99_FORMATS is not defined, %zu cannot be used as a format specifier. You can compile your standard library with the --enable-newlib-io-c99-formats configuration to make it work.