printf

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An example of the printf function.

The class of printf functions (which stands for "print formatted") is a class of functions, typically associated with curly bracket programming languages, that accept a string parameter (called the format string) which specifies a method for rendering a number of other parameters (of which there typically may be arbitrarily many, of a variety of types) into a string. Usually this string is then printed on the standard output stream, but variants exist that perform other tasks with the result. Characters in the format string are usually copied literally into the function's output, with the other parameters being rendered into the resulting text at points marked by format specifiers, which are typically introduced by a % character. A literal percent sign can be copied into the output using the escape sequence %%.

[edit] Timeline

Several programming languages implement a printf function, to output a formatted string. It originated from the C programming language, where it has a prototype similar to the following:

int printf(const char *format, ...)

The string constant format provides a description of the output, with placeholders marked by "%" escape characters, to specify both the relative location and the type of output that the function should produce.

[edit] 1960s: BCPL, ALGOL 68, Multics PL/I

C's variadic printf has its origins in BCPL's writef function.

ALGOL 68 Draft and Final report had the functions inf and outf, subsequently these were revised out of the original language and replaced with the now more familiar readf and printf/writef.

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

Multics has a standard function called ioa_ with a wide variety of control codes.

call ioa_ ("Hello, ^a", "world");

[edit] 1970s: C, Lisp

printf("Color %s, number1 %d, number2 %05d, hex %x, float %5.2f, unsigned value %u.\n",
             "red", 123456, 89, 255, 3.14, 250);

will print following line (including new-line character, \n):

Color red, number1 123456, number2 00089, hex ff, float  3.14, unsigned value 250.

The printf function returns the number of characters printed, or a negative value if an output error occurred.

Common Lisp has the format function.

(format t "Hello, ~a" "world")

prints "Hello, world" on the standard output stream. If the first argument is nil, format returns the string to its caller. The first argument can also be any output stream. format was introduced into ZetaLisp at M.I.T. in 1978, based on the Multics ioa_, and later adopted into the Common Lisp standard.

[edit] 1980s: perl

Perl also has a printf function. Common Lisp has a format function which acts according to the same principles as printf, but uses different characters for output conversion. The GLib library contains g_print, an implementation of printf.

Some Unix systems have a printf program for use in shell scripts. This can be used instead of echo in situations where the latter is not portable. For example:

echo -n -e "$FOO\t$BAR"

may be rewritten portably as:

printf "%s\t%s" "$FOO" "$BAR"

[edit] 1990s: PHP, Python, Java & Javascript

1991: Python's % operator hearkens to printf's syntax when interpolating the contents of a tuple. This operator can, for example, be used with the print statement:

print "%s\t%s" % (foo,bar)

1995: PHP also has the printf function, with the same specifications and usage as that in C/C++. MATLAB does not have printf, but does have its two extensions sprintf and fprintf which use the same formatting strings.

1995: Java supported printf from version 1.5 onwards as a member of the PrintStream class, giving it the functionality of both the printf and fprintf functions. This method is identical to the format method in the same classes.

System.out.printf("%s, %s", "Hello", "World!"); // Writes "Hello, World!" to standard output

The String class also includes a format method that returns a new string containing the formatted text, similar to the functionality of asprintf. Unlike most other implementations, Java's implementation of printf throws an exception on encountering a malformed format string.

1995: JavaScript does not have a printf function, despite it being a curly bracket programming language.

[edit] Derivative functions

The C Standard specifies a number of derivative functions to further leverage the printf functionality:

[edit] fprintf

int fprintf(FILE *stream, const char *format, ...)

fprintf enables printf output to be written to any file. Programmers frequently use it to print errors, by writing to the standard error device, but it can operate with any file opened with the fopen function.

[edit] sprintf

int sprintf (char *str, const char *format, ...)

sprintf prints to a string (char array) instead of to standard output. Users of sprintf must ensure, via calculation or via a guard page, that the resulting string will not be larger than the memory allocated for str. Failure to ensure this can allow a buffer overflow to occur.

In higher-level languages such as PHP the sprintf function does not have the str argument. Instead, it returns the formatted output string. The prototype in PHP is like this:

string sprintf (const string format, ...)

[edit] Safer alternatives to sprintf

As an alternative, many environments offer the snprintf function:

int snprintf(char *str, size_t size, const char *format, ...)

snprintf is guaranteed not to write more than size bytes into str, so use of it can help avoid the risk of a buffer overflow, as in the following code fragment:

#define BUFFER_SIZE 50
char buf[BUFFER_SIZE];
int n;
 
...
 
n = snprintf(buf, BUFFER_SIZE, "Your name is %s.\n", username);
if (n < 0 || n >= BUFFER_SIZE)
   /* Handle error */

If username in the above example exceeds 34 characters in length, the function will limit the string that gets saved in buf by cutting off final characters (truncating). This may seem undesirable, but it is usually preferable to having a security vulnerability, which buffer overflows often cause. Additionally, the return code of snprintf indicates how many characters the function would have written to the string had enough space existed. Systems can use this information to allocate a new (larger) buffer if they require the whole string.

snprintf does not form part of the widely implemented ANSI C standard, as sprintf does. However, it came into the language for the later C99 standard and often existed in C libraries before that.

Another safe sprintf alternative is asprintf:

int asprintf(char **ret, const char *format, ...)

asprintf automatically allocates enough memory to hold the final string. It sets *ret to a pointer to the resulting string, or to an undefined value if an error occurred (GLibc is notable in being the only implementation that doesn't always set *ret to NULL on error). The programmer using asprintf has the responsibility of freeing the allocated memory after use. Though not part of any standard, asprintf comes in the C libraries of several operating systems (including OpenBSD, FreeBSD, and NetBSD) and on other platforms in the libiberty library.

GLib provides yet another safe alternative: g_strdup_printf, which allocates enough memory, but, unlike asprintf, returns the resulting string as its return value rather than via the first argument.

[edit] C/C++ alternatives to sprintf for numeric conversion

The standard method for string formatting and the conversion of other types to strings in C++ is iostream. Unlike printf, the iostream standard library is type-safe and extensible.

A common programming task is convert a numeric type into a string (char buffer). The sprintf family, while useful, in many applications seems like overkill for such a simple task.

A number of alternative means in C/C++ have been developed:

• Boost::lexical_cast
• Boost::format
• modp_numtoa
• itoa

[edit] vprintf, vfprintf, vsprintf, vsnprintf, and vasprintf

/* va_list versions of above */
int vprintf(const char *format, va_list ap);
int vfprintf(FILE *stream, const char *format, va_list ap);
int vsprintf(char *str, const char *format, va_list ap);
int vsnprintf(char *str, size_t size, const char *format, va_list ap);
int vasprintf(char **ret, const char *format, va_list ap);

These are analogous to the above functions without the vs, except that they use variable argument lists. These functions offer the ability for programmers to essentially create their own printf variants. For instance, a programmer could write a function

void fatal_error(const char *format, ...)

which would use the va_start macro to obtain a va_list variable from the extra parameters, print a message on the standard error device using vfprintf, clean up after the va_list variable with the va_end macro, and finally perform the necessary tasks to cleanly shut down the program.

Another common application of these functions is to write a custom printf that prints to a different target than a file. For instance, a graphical library might provide a printf-like function with X and Y coordinates:

int graphical_printf(int x, int y, const char *format, ...)

This would work by temporarily saving the string to a private buffer using vsnprintf or vasprintf.

[edit] printf format placeholders

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 ". To denote that we want the integer for the age to be shown immediately after that message, we may use the format string:

"Your age is %d."

The syntax for a format placeholder is "%[parameter][flags][width][.precision][length]type".

Parameter can be omitted or can be:

• 'n$' Where n is the number of the parameter to display using this format specifier, allowing the parameters provided to be output multiple times, using varying format specifiers or in different orders. This is a POSIX extension and not in C99.

Where type can be any of:

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

• '+' : Causes printf to always denote the sign '+' or '-' of a number (the default is to omit the sign for positive numbers). Only applicable to numeric types.
• '-' : Causes printf to left-align the output of this placeholder (the default is to right-align the output).
• '#' : Alternate form. For 'g' and 'G', trailing zeros are not removed. For 'f', 'F', 'e', 'E', 'g', 'G', the output always contains a decimal point. For 'o', 'x', and 'X', a 0, 0x, and 0X, respectively, is prepended to non-zero numbers.
• a number : Causes printf to left-pad the output with spaces until the required length of output is attained. If combined with '0' (see below), it will cause the sign to become a space when positive, but the remaining characters will be zero-padded
• '0' : Causes printf to use '0' (instead of spaces) to left fill a fixed length field. For example (assume i = 3) printf("%2d", i) results in " 3", while printf("%02d", i) results in "03"

Width can be omitted or be any of:

• a number : Causes printf to pad the output of this placeholder with spaces until it is at least number characters wide. If number has a leading '0', then padding is done with '0' characters.
• '*' : Causes printf to pad the output until it is n characters wide, where n is an integer value stored in the a function argument just preceding that represented by the modified type. For example printf("%*d", 5, 10) will result in "10" being printed with a width of 5.

Precision can be omitted or be any of:

• a number : For non-integral numeric types, causes the decimal portion of the output to be expressed in at least number digits. For the string type, causes the output to be truncated at number characters.
• '*' : Same as the above, but uses an integer value in the intaken argument to determine the number of decimal places or maximum string length. For example, printf("%.*s", 3, "abcdef") will result in "abc" being printed.

If the precision is zero, nothing is printed for the corresponding argument.

Length can be omitted or be any of:

• 'hh' : For integer types, causes printf to expect an int sized integer argument which was promoted from a char.
• 'h' : For integer types, causes printf to expect a int sized integer argument which was promoted from a short.
• 'l' : (ell) For integer types, causes printf to expect a long sized integer argument.
• 'll' : (ell ell) For integer types, causes printf to expect a long long sized integer argument.
• 'L' : For floating point types, causes printf to expect a long double argument.
• 'z' : For integer types, causes printf to expect a size_t sized integer argument.
• 'j' : For integer types, causes printf to expect a intmax_t sized integer argument.
• 't' : For integer types, causes printf to expect a ptrdiff_t sized integer argument.

Also note that common implementation extensions are:

• 'I' : (eye) size_t or ptrdiff_t: __int32 on Win32; __int64 on Win64. Microsoft C/C++ Runtime only.
• 'I32' : (eye-thirty-two) __int32. Microsoft C/C++ Runtime only.
• 'I64' : (eye-sixty-four) __int64. Microsoft C/C++ Runtime only.
• 'q' : (quad) Same as 'll'.

If the syntax of a conversion specification is invalid, behavior remains undefined, and in fact can cause program termination. 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 also undefined. Excess arguments are ignored. In a number of cases, the undefined behavior has led to "Format string attack" security vulnerabilities. Note that some compilers, like the GNU Compiler Collection, will statically check the format strings of printf-like functions and warn about problems (specially by using the flags -Wall or -Wformat)

[edit] Risks of using field width versus explicit delimiters in 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. 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. Similar strategies apply to string data.

[edit] Custom printf 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 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, and can work with static format checkers.

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.

Most non-C 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. C++ offers a notable exception, in that it has a printf function inherited from its C history, but also has a completely different mechanism that is preferred.

[edit] Programming languages with printf

• AMPL
• awk
• Bourne shell (sh) and derivatives such as Korn shell (ksh), Bourne again shell (bash), or Z shell (zsh)
• C programming language, and subsequently C++ and Obj-C (C++ also provides overloaded shift operators and manipulators as an alternative for formatted output - see iostream and iomanip)
• GNU Octave
• Java programming language (since version 1.5)
• Mathematica
• MATLAB
• Objective Caml
• PHP programming language, web-based inflected form of C
• Python programming language (using the % operator)
• Perl
• Ruby programming language

[edit] See also

• scanf
• C standard library
• Format string attack
• iostream

[edit] External links

• printf format specifications quick reference
• printf: print formatted output – System Interfaces Reference, The Single UNIX® Specification, Issue 6 from The Open Group
• The Formatter specification in Java 1.5
• GNU Bash printf(1) builtin
• C printf comparison page
• printf(3): formatted output conversion – OpenBSD Library Functions Manual
• sprintf() for JavaScript - a lightweight and easy to use implementation