Printf tricks

Printf Tricks

It may be old-fashioned, but I still find printf (and sprintf and _vsnprintf) incredibly useful, both for printing debug output and for generating formatted strings.

Here are a few lesser-known formats that I use again and again. See MSDN for the full reference.

%04x - 4-digit hex number with leading zeroes

A quick review of some of the basics.

%x prints an int in hexadecimal.

%4x prints a hex int, right-justified to 4 places. If it's less than 4 digits, it's preceded by spaces. If it's more than 4 digits, you get the full number.

%04x prints a hex int, right-justified to 4 places. If it's less than 4 digits, it's preceded by zeroes. If it's more than 4 digits, you get the full number, but no leading zeroes.

Similarly, %d prints a signed int in decimal, and %u prints an unsigned int in decimal.

Not so similarly, %c prints a character and %s prints a string. For wide (Unicode) strings, prefix with l (ell, or w): %lc and %ls.

Note: For the Unicode variants, such as wprintf and friends, %c and %s print wide strings. To force a narrow string, no matter which variant, use the %h size prefix, and to force a wide string, use the %l size prefix; e.g., %hs and %lc.

%p - pointer

The wrong way to print a pointer is to use %x. The right way is to use %p. It's portable to Win64, as well as to all other operating systems.

Everyone should know this one, but many don't.

%I64d, %I64u, %I64x - 64-bit integers

To print 64-bit numbers (__int64), use the I64 size prefix.

%Iu, %Id, %Ix - ULONG_PTR

ULONG_PTR, LONG_PTR, and DWORD_PTR are numeric types that are as wide as a pointer. In other words, they map to ULONG, LONG, and DWORD respectively on Win32, and ULONGLONG, LONGLONG, and ULONGLONG on Win64.

The I size prefix (capital-i, not lowercase-L) is what you need to print *LONG_PTR on Win32 and Win64.

%*d - runtime width specifier

If you want to calculate the width of a field at runtime, you can use %*. This says the next argument is the width, followed by whatever type you want to print.

For example, the following can be used to print a tree:

 void Tree::Print(Node* pNode, int level)
 {
     if (NULL != pNode)
     {
         Print(pNode->Left, level+1);
         printf("%*d%s\n", 2 * level, pNode->Key);
         Print(pNode->Right, level+1);
     }
 }

%.*s - print a substring

With a variable precision, you can print a substring, or print a non-NUL-terminated string, if you know its length. printf("%.*s\n", sublen, str) prints the first sublen characters of str.

[2005/7/19: fixed a typo in previous sentence (%.s -> %.*s). A little elaboration on the syntax: . in a printf format specification is followed by the precision. For strings, the precision specificies how many characters will be printed. A precision of * indicates that the precision is the next argument on the stack. If the precision is zero, then nothing is printed. If a string has a precision specification, its length is ignored.]

%.0d - print nothing for zero

I've occasionally found it useful to suppress output when a number is zero, and %.0d is the way to do it. (If you attempt to print a non-zero number with this zero-precision specifier, it will be printed.) Similarly, %.0s swallows a string.

%#x - print a leading 0x

If you want printf to automatically generate 0x before hex numbers, use %#x instead of %x.

Other tricks

See the documentation for other useful tricks.

Security

Never use an inputted string as the format argument: printf(str). Instead, use printf("%s", str). The former is a stack smasher waiting to happen.

%n is dangerous and disabled by default in VS2005.

Don't use sprintf. Use the counted version, _snprintf or _vsnprintf instead. Better still, use the StrSafe.h functions, StringCchPrintf and StringCchVPrintf, to guarantee that your strings are NUL-terminated.

[Update: 2008/01/25: See also Printf %n.]