The printf family of functions in the C programming language are a set of functions that take a format string as input among a variable sized list of other values and produce as output a string that corresponds to the format specifier and given input values. The string is written in a simple template language: characters are usually copied literally into the function's output, but format specifiers, which start with a % character, indicate the location and method to translate a piece of data (such as a number) to characters. The design has been copied to expose similar functionality in other programming languages.

"printf" is the name of one of the main C output functions, and stands for "print formatted". printf format strings are complementary to scanf format strings, which provide formatted input (lexing aka. parsing). In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible template engines or lexers/parsers, but are sufficient for many purposes.

Many languages other than C copy the printf format string syntax closely or exactly in their own I/O functions.

Mismatches between the format specifiers and type of the data can cause crashes and other vulnerabilities. The format string itself is very often a string literal, which allows static analysis of the function call. However, it can also be the value of a variable, which allows for dynamic formatting but also a security vulnerability known as an uncontrolled format string exploit.

History

1950s: Fortran

Early programming languages like Fortran used special statements with different syntax from other calculations to build formatting descriptions.^[1] In this example, the format is specified on line 601, and the PRINT^[a] command refers to it by line number:

      PRINT 601, IA, IB, AREA
 601  FORMAT (4H A= ,I5,5H  B= ,I5,8H  AREA= ,F10.2, 13H SQUARE UNITS)

Hereby:

• 4H indicates a string of 4 characters " A= " (H means Hollerith Field);
• I5 indicates an integer field of width 5;
• F10.2 indicates a floating-point field of width 10 with 2 digits after the decimal point.

An output with input arguments 100, 200, and 1500.25 might look like this:

 A=   100  B=   200  AREA=    1500.25 SQUARE UNITS

1960s: BCPL and ALGOL 68

In 1967, BCPL appeared.^[2] Its library included the writef routine.^[3] An example application looks like this:

WRITEF("%I2-QUEENS PROBLEM HAS %I5 SOLUTIONS*N", NUMQUEENS, COUNT)

Hereby:

• %I2 indicates an integer of width 2 (the order of the format specification's field width and type is reversed compared to C's printf);
• %I5 indicates an integer of width 5;
• *N is a BCPL language escape sequence representing a newline character (for which C uses the escape sequence \n).

In 1968, ALGOL 68 had a more function-like API, but still used special syntax (the $ delimiters surround special formatting syntax):

printf(($"Color "g", number1 "6d,", number2 "4zd,", hex "16r2d,", float "-d.2d,", unsigned value"-3d"."l$,
         "red", 123456, 89, BIN 255, 3.14, 250));

In contrast to Fortran, using normal function calls and data types simplifies the language and compiler, and allows the implementation of the input/output to be written in the same language. These advantages outweigh the disadvantages (such as a complete lack of type safety in many instances) and in most newer languages I/O is not part of the syntax.

1970s: C

In 1973, printf is included as a C routine as part of Version 4 Unix.^[4]

1990s: Shell command

In 1990, a printf shell command is attested as part of 4.3BSD-Reno. It is modeled after the standard library function.^[5]

In 1991, a printf command is bundled with GNU shellutils (now part of GNU Core Utilities).

Format placeholder specification

Formatting takes place via placeholders within the format string. For example, if a program wanted to print out a person's age, it could present the output by prefixing it with "Your age is ", and using the signed decimal specifier character d to denote that we want the integer for the age to be shown immediately after that message, we may use the format string:

printf("Your age is %d", age);

Syntax

The syntax for a format placeholder is

%[parameter][flags][width][.precision][length]type

Parameter field

This is a POSIX extension and not in C99. The Parameter field can be omitted or can be:

This feature mainly sees its use in localization, where the order of occurrence of parameters vary due to the language-dependent convention.

On the non-POSIX Microsoft Windows, support for this feature is placed in a separate printf_p function.

Flags field

The Flags field can be zero or more (in any order) of:

Width field

The Width field specifies a minimum number of characters to output and is typically used to pad fixed-width fields in tabulated output, where the fields would otherwise be smaller, although it does not cause truncation of oversized fields.

The width field may be omitted, or a numeric integer value, or a dynamic value when passed as another argument when indicated by an asterisk *.^[6] For example, printf("%*d", 5, 10) will result in 10 being printed, with a total width of 5 characters.

Though not part of the width field, a leading zero is interpreted as the zero-padding flag mentioned above, and a negative value is treated as the positive value in conjunction with the left-alignment - flag also mentioned above.

Precision field

The Precision field usually specifies a maximum limit on the output, depending on the particular formatting type. For floating-point numeric types, it specifies the number of digits to the right of the decimal point that the output should be rounded. For the string type, it limits the number of characters that should be output, after which the string is truncated.

The precision field may be omitted, or a numeric integer value, or a dynamic value when passed as another argument when indicated by an asterisk *. For example, printf("%.*s", 3, "abcdef") will result in abc being printed.

Length field

The Length field can be omitted or be any of:

Additionally, several platform-specific length options came to exist prior to widespread use of the ISO C99 extensions:

ISO C99 includes the inttypes.h header file that includes a number of macros for use in platform-independent printf coding. These must be outside double-quotes, e.g. printf("%" PRId64 "\n", t);

Example macros include:

Type field

The Type field can be any of:

Custom format placeholders

There are a few implementations of printf-like functions that allow extensions to the escape-character-based mini-language, thus allowing the programmer to have a specific formatting function for non-builtin types. One of the most well-known is the (now deprecated) glibc's register_printf_function(). However, it is rarely used due to the fact that it conflicts with static format string checking. Another is Vstr custom formatters, which allows adding multi-character format names.

Some applications (like the Apache HTTP Server) include their own printf-like function, and embed extensions into it. However these all tend to have the same problems that register_printf_function() has.

The Linux kernel printk function supports a number of ways to display kernel structures using the generic %p specification, by appending additional format characters.^[11] For example, %pI4 prints an IPv4 address in dotted-decimal form. This allows static format string checking (of the %p portion) at the expense of full compatibility with normal printf.

Most languages that have a printf-like function work around the lack of this feature by just using the %s format and converting the object to a string representation.

Vulnerabilities

Invalid conversion specifications

If there are too few function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are undefined and the program may crash. Implementations are inconsistent about whether syntax errors in the string consume an argument and what type of argument they consume. Excess arguments are ignored. In a number of cases, the undefined behavior has led to "Format string attack" security vulnerabilities. In most C or C++ calling conventions arguments may be passed on the stack, which means in the case of too few arguments printf will read past the end of the current stackframe, thus allowing the attacker to read the stack.

Some compilers, like the GNU Compiler Collection, will statically check the format strings of printf-like functions and warn about problems (when using the flags -Wall or -Wformat). GCC will also warn about user-defined printf-style functions if the non-standard "format" __attribute__ is applied to the function.

Tabular output

Using only field widths to provide for tabulation, as with a format like %8d%8d%8d for three integers in three 8-character columns, will not guarantee that field separation will be retained if large numbers occur in the data:

1234567 1234567 1234567 
123     123     123     
123     12345678123     

Loss of field separation can easily lead to corrupt output. In systems which encourage the use of programs as building blocks in scripts, such corrupt data can often be forwarded into and corrupt further processing, regardless of whether the original programmer expected the output would only be read by human eyes. Such problems can be eliminated by including explicit delimiters, even spaces, in all tabular output formats. Simply changing the dangerous example from before to %7d %7d %7d addresses this, formatting identically until numbers become larger, but then explicitly preventing them from becoming merged on output due to the explicitly included spaces:

1234567 1234567 1234567 
123     123     123     
123     12345678 123     

Similar strategies apply to string data.

Memory write

Although an outputting function on the surface, printf allows writing to a memory location specified by an argument via %n. This functionality is occasionally used as a part of more elaborate format-string attacks.^[12]

The %n functionality also makes printf accidentally Turing-complete even with a well-formed set of arguments. A game of tic-tac-toe written in the format string is a winner of the 27th IOCCC.^[13]

Programming languages with printf

Not included in this list are languages that use format strings that deviate from the style in this article (such as AMPL and Elixir), languages that inherit their implementation from the JVM or other environment (such as Clojure and Scala), and languages that do not have a standard native printf implementation but have external libraries which emulate printf behavior (such as JavaScript).