Why cast is needed in printf?

Question

To print a number of type off_t it was recommended to use the following piece of code:

off_t a;
printf("%llu\n", (unsigned long long)a);

• Why the format string is not enough?
• What will be the problem if it were not casted?

Answer 1

The format string doesn't tell the compiler to perform a cast to unsigned long long, it just tells printf that it's going to receive an unsigned long long. If you pass in something that's not an unsigned long long (which off_t might not be), then printf will simply misinterpret it, with surprising results.

The reason for this is that the compiler doesn't have to know anything about format strings. A good compiler will give you a warning message if you write printf("%d", 3.0), but what can a compiler do if you write printf(s, 3.0), with s being a string determined dynamically at run-time?

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Edited to add: As Keith Thompson points out in the comments below, there are many places where the compiler can perform this sort of implicit conversion. printf is rather exceptional, in being one case where it can't. But if you declare a function to accept an unsigned long long, then the compiler will perform the conversion:

#include <stdio.h>
#include <sys/types.h>


int print_llu(unsigned long long ull)
{
  return printf("%llu\n", ull); // O.K.; already converted
}


int main()
{
  off_t a;

  printf("%llu\n", a); // WRONG! Undefined behavior!
  printf("%llu\n", (unsigned long long) a); // O.K.; explicit conversion
  print_llu((unsigned long long) a); // O.K.; explicit conversion
  print_llu(a); // O.K.; implicit conversion

  return 0;
}

The reason for this is that printf is declared as int printf(const char *format, ...), where the ... is a "variadic" or "variable-arguments" notation, telling the compiler that it can accept any number and types of arguments after the format. (Obviously printf can't really accept any number and types of arguments: it can only accept the number and types that you tell it to, using format. But the compiler doesn't know anything about that; it's left to the programmer to handle it.)

Even with ..., the compiler does do some implicit conversions, such as promoting char to int and float to double. But these conversions are not specific to printf, and they do not, and cannot, depend on the format string.