Simple I/O Messing Up

Basic I/O should be simple. I guess you agree. And I guess that’s way many C++ or Java programmers that look back to humble printf with some nostalgia. In fact it is hard to beat the conciseness and the clarity of something like:

When it comes to create formatted output C printf is usually one of the best tool available.
Unfortunately things are not so simple. One of the first limitations acknowledged for this little gem is that it lacks of robustness, or, put from a different perspective, it doesn’t type check.
What happens if ‘x’ in the above example is a float number? or a pointer? Or worst if the format string specifies a string pointer and an integer is passed?
This problem is mostly overcome in the GNU compiler via a custom extension that allows the compiler to check for consistency between format string and arguments.
The mechanism is enough flexible to be applied to user defined functions. Suppose you have a logging function that behaves like printf, something like

That’s handy so you don’t have to perform string processing to build your message to log when you want just log. If you use Gcc and declare the function like:

Then the compiler will kindly check all the invocation of function log in your code to ensure that specifiers and arguments match.
So far so good, but enter C99. In the old days there was an integer type (int) with two available modifier (short and long). That was reflected in printf specifier/modifier: %d is for straightforward ints, %hd for shorts and %ld for longs.
And this is fine until you work on the same compiler and platform. If your code needs to be portable, then some complications are ready for you.
The last standard (C99) mandates a header, namely stdint.h, where a number of typedefs provide a wealth of integer types: grouped by size and by efficiency, you have (if I count correctly) some 30 types.
From one side this is jolly good since poses an end to the critics against C for not having an integer type with a declared bit size valid for all platforms (like Java has).
Unfortunately, on the other size printf is not able to autodetect types and thus you have to write a different format string whether your int32_t is defined as long int, or just int.
To leave the nightmare behind C99 mandates another header file – inttypes.h that provides the proper specifier for each one of those 30 integer types. For example, if you want to print an int32_t, you have to write:

As you can see it relies on the C preprocessor that merges two consecutive strings into one.
That does the job, but, IMO some simplicity of the original idea is lost.

Leave a Comment