Version 2 (modified by Vojtech Horky, 12 years ago) ( diff )

More examples, improve wording

Using printf in a safe way

This page describes which type specifiers you shall use when printing special types, such as integers of fixed width or handles returned by various libraries. Do not use %d for everything that looks like an integer. Special types requires special handling. HelenOS system library libc provides macros that hides differences between various architectures and allows you to print special types in platform independent way.

The macros have names similar to PRIu32 (for uint32_t in decimal) or PRIxn (for native-size integer in hexadecimal) and you use them instead of the type specifier (see examples below).

The PRI* macros are defined in two places /common.h (yes, in the root directory of HelenOS source) and in /uspace/lib/c/arch/ARCHITECTURE/include/inttypes.h. common.h is a generated file that appears after you configure your build.

Integers of fixed width

The macro is of form PRI<kind><width>. <width> is one of 8, 16, 32 or 64 or n for native width. <kind> is summarized in following table.

<kind> Meaning
u unsigned integer, decimal
d signed integer, decimal
x unsigned integer, hexadecimal
o unsigned integer, octal

For example, to print uint16_t in octal, use PRIo16.

Special integer types

Type Directive
size_t "%zu"
sysarg_t PRIun / PRIxn
uintptr_t "%#" PRIxn


Although handles in HelenOS have their own typedef, they are usually backed by integer or pointer. Integers are usually used for handles passed from servers to clients (e.g. devices, files). Typically, they use unsigned integer of the native size. Pointers are used for structures used only locally (same application) and are used to hide implementation details.

Thus, for these handles one should use PRIun (makes more sense for integer backed handles) or PRIxn (usually for pointers).

The table below is not exhaustive and serves as a list of hints.

Type Directive
fid_t (fibril id) PRIxn
devman_handle_t (device handle) PRIun
thread_id_t (thread id) PRIu64
ipc_callid_t (IPC call id) PRIxn
aid_t (IPC message id) PRIxn

Annotating your own printf-like functions

If you are creating your own printf-like function, do not forget to add PRINTF_ATTRIBUTE() to it to force checking of the parameters.

void my_logging_function(int level, const char *format, ...)
// 2 .. second argument is the format string
// 3 .. the variadic arguments are here (third parameter)


devman_handle_t device_handle;
printf("Handle = %" PRIun ".\n", device_handle);

fid_t fibril_handle;
printf("Fibril handle = %#" PRIxn ".\n", fibril_handle);

uint32_t pci_register;
printf("Register at 0x%08" PRIx32 ".\n", pci_register);

int16_t small_value;
printf("16bits: %" PRId16 ".\n", small_value);

sysarg_t ipc_argument;
printf("IPC  arg1 = %20" PRIun ".\n", ipc_argument);

size_t buffer_size;
printf("Buffer has %zuB.\n", buffer_size);

uintptr_t pointer_as_number;
printf("Data at %p or at %#" PRIxn ".\n", (void *) pointer_as_number, pointer_as_number);

void *pointer;
printf("Pointer is built-in: %p.\n", pointer);
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