Printf format string refers to a control parameter used by a class of functions typically associated with some types of programming languages. The format string specifies a method for rendering an arbitrary number of varied data type parameter(s) into a string. This string is then by default 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
Percent sign
The percent sign is the symbol used to indicate a percentage .Related signs include the permille sign ‰ and the permyriad sign , which indicate that a number is divided by one thousand or ten thousand respectively...
.
Timeline
Many programming languages implement a printffunction
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
A string literal is the representation of a string value within the source code of a computer program. There are numerous alternate notations for specifying string literals, and the exact notation depends on the individual programming language in question...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
A function prototype in C, Perl or C++ is a declaration of a function that omits the function body but does specify the function's return type, name, arity and argument types...
similar to the following: int printf(const char *format, ...)
The string constant
Variable (programming)
In computer programming, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents...
format provides a description of the output, with placeholder
Free variables and bound variables
In mathematics, and in other disciplines involving formal languages, including mathematical logic and computer science, a free variable is a notation that specifies places in an expression where substitution may take place...
In computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters...
s, to specify both the relative location and the type of output that the function should produce. The return value yields the number of printed characters.
In computer programming, a variadic function is a function of indefinite arity, i.e., one which accepts a variable number of arguments. Support for variadic functions differs widely among programming languages....
PRINT statement referenced a non-executable FORMAT statement.
PRINT 601, 123456, 1000.0, 3.1415, 250
601 FORMAT (8H RED NUM,I7,4H EXP,E8.1,5H REAL,F5.2,4H VAL,I4)
will print the following (on a new line, because of the leading blank character):
RED NUM 123456 EXP 1.0E 03 REAL 3.14 VAL 250
COBOL provided formatting via hierarchical data structure specification:
01 out-rec.
02 out-name picture x(20).
02 out-amount picture $9,999.99.
...
move me to out-name.
move amount to out-amount.
write out-rec.
In computer programming, a variadic function is a function of indefinite arity, i.e., one which accepts a variable number of arguments. Support for variadic functions differs widely among programming languages....
BCPL is a procedural, imperative, and structured computer programming language designed by Martin Richards of the University of Cambridge in 1966.- Design :...
ALGOL 68 isan imperative computerprogramming language that was conceived as a successor to theALGOL 60 programming language, designed with the goal of a...
Draft and Final report had the functions inf and outf, subsequently these were revised out of the original language and replaced with the now more familiar readf/getf and printf/putf.
printf(($"Color "g", number1 "6d,", number2 "4zd,", hex "16r2d,", float "-d.2d,", unsigned value"-3d"."l$,
"red", 123456, 89, BIN 255, 3.14, 250));
Multics was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts...
has a standard function called ioa_ with a wide variety of control codes. It was based on a machine-language facility from Multics's BOS (Bootstrap Operating System).
call ioa_ ("Hello, ^a", "World!");
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
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 MIT
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
Format is a function in Common Lisp that can produce formatted text and is normally used in a manner analogous to printf in C and other curly bracket programming languages...
function which acts according to the same principles as printf, but uses different characters for output conversion. The GLib
GLib
GLib is a cross-platform software utility library that began as part of the GTK+ project. However, before releasing version 2 of GTK+, the project's developers decided to separate non-GUI-specific code from the GTK+ platform, thus creating GLib as a separate product...
library contains g_print, an implementation of printf.
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
systems have a printf program for use in shell script
Shell script
A shell script is a script written for the shell, or command line interpreter, of an operating system. It is often considered a simple domain-specific programming language...
s. 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"
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
's % operator harkens to printf's syntax when interpolating the contents of a tuple. This operator can, for example, be used with the print function: print("%s\t%s" % (foo,bar))
Version 2.6 of Python included the str.format which is preferred to the obsolete % which may go away in future versions of Python:
print("If you multiply five and six you get {0}.".format(5*6))
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
also has the printf function, with the same specifications and usage as that in C/C++. MATLAB
MATLAB
MATLAB is a numerical computing environment and fourth-generation programming language. Developed by MathWorks, MATLAB allows matrix manipulations, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs written in other languages,...
does not have printf, but does have its two extensions sprintf and fprintf which use the same formatting strings. sprintf returns a formatted string instead of producing a visual output.
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
supported printf from version 1.5 onwards as a member of the PrintStreamclass
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
, giving it the functionality of both the printf and fprintf functions. At the same time sprintf-like functionality was added to the String class by adding the format(String, Object... args) method.
// Write "Hello, World!" to standard output (like printf)
System.out.printf("%s, %s", "Hello", "World!");
// create a String object with the value "Hello, World!" (like sprintf)
String myString = String.format("%s, %s", "Hello", "World!");
Unlike most other implementations, Java's implementation of printf throws an exception
Exception handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....
on encountering a malformed format string.
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:
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
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
C99 is a modern dialect of the C programming language. It extends the previous version with new linguistic and library features, and helps implementations make better use of available computer hardware and compiler technology.-History:...
always denote the sign '+' or '-' of a number (the default is to omit the sign for positive numbers). Only applicable to numeric types.
space
prefixes non-negative signed numbers with a space
-
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.
0
use 0 instead of spaces to pad a field when the width option is specified. For example, printf("%2d", 3) results in " 3", while printf("%02d", 3) results in "03".
Width 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. A leading zero in the width value 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 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.
Length can be omitted or be any of:
Character
Description
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 an int sized integer argument which was promoted from a short.
l
For integer types, causes printf to expect a long sized integer argument.
ll
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.
Additionally, several platform-specific length options came to exist prior to widespread use of the ISO C99 extensions:
Characters
Description
I
For signed integer types, causes printf to expect ptrdiff_t sized integer argument; for unsigned integer types, causes printf to expect size_t sized integer argument. Commonly found in Win32/Win64 platforms.
I32
For integer types, causes printf to expect a 32-bit (double word) integer argument. Commonly found in Win32/Win64 platforms.
I64
For integer types, causes printf to expect a 64-bit (quad word) integer argument. Commonly found in Win32/Win64 platforms.
q
For integer types, causes printf to expect a 64-bit (quad word) integer argument. Commonly found in BSD platforms.
ISO C99 includes the inttypes.h header file that includes a number of macros for use in platform-independent printf coding. Example macros include:
Characters
Description
PRId32
Typically equivalent to I32d (Win32/Win64) or d
PRId64
Typically equivalent to I64d (Win32/Win64), lld (32-bit platforms) or ld (64-bit platforms)
PRIi32
Typically equivalent to I32i (Win32/Win64) or i
PRIi64
Typically equivalent to I64i (Win32/Win64), lli (32-bit platforms) or li (64-bit platforms)
PRIu32
Typically equivalent to I32u (Win32/Win64) or u
PRIu64
Typically equivalent to I64u (Win32/Win64), llu (32-bit platforms) or lu (64-bit platforms)
PRIx64
Typically equivalent to I64x (Win32/Win64), llx (32-bit platforms) or lx (64-bit platforms)
The decimal numeral system has ten as its base. It is the numerical base most widely used by modern civilizations....
number. '%d' and '%i' are synonymous for output, but are different when used with scanf
Scanf
Scanf format string refers to a control parameter used by a class of functions typically associated with some types of programming languages. The format string specifies a method for reading a string into an arbitrary number of varied data type parameter...
In computing, a fixed-point number representation is a real data type for a number that has a fixed number of digits after the radix point...
) notation. 'f' and 'F' only differs in how the strings for an infinite number or NaN are printed ('inf', 'infinity' and 'nan' for 'f', 'INF', 'INFINITY' and 'NAN' for 'F').
e, E
double value in standard form ([-]d.ddd e[+/-]ddd). An E conversion uses the letter E (rather than e) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. In Windows, the exponent contains three digits by default, e.g. 1.5e002, but this can be altered by Microsoft-specific _set_output_format function.
g, G
double in either normal or exponential notation, whichever is more appropriate for its magnitude. 'g' uses lower-case letters, 'G' uses upper-case letters. This type differs slightly from fixed-point notation in that insignificant zeroes to the right of the decimal point are not included. Also, the decimal point is not included on whole numbers.
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...
number. 'x' uses lower-case letters and 'X' uses upper-case.
In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a null character...
.
c
char (character).
p
void * (pointer to void) in an implementation-defined format.
n
Print nothing, but write number of characters successfully written so far into an integer pointer parameter.
%
a literal '%' character (this type doesn't accept any flags, width, precision or length).
The width and precision formatting parameters may be omitted, or they can be a fixed number embedded in the format string, or passed as another function argument when indicated by an asterisk "*" in the format string. For example printf("%*d", 5, 10) will result in " 10" being printed, with a total width of 5 characters, and printf("%.*s", 3, "abcdef") will result in "abc" being printed.
In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....
of a conversion specification is invalid, behavior is undefined, and 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
Format string attack
Uncontrolled format string is a type of software vulnerability, discovered around 1999, that can be used in security exploits. Previously thought harmless, format string exploits can be used to crash a program or to execute harmful code...
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
, 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.
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.
Custom format placeholders
There are a few implementations of printf-like functions that allow extensions to the escape-character
Escape character
In computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters...
In software development and domain engineering, a domain-specific language is a programming language or specification language dedicated to a particular problem domain, a particular problem representation technique, and/or a particular solution technique...
, 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, and can work with static format checkers.
The Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...
) 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++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
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.
AMPL, an acronym for "A Mathematical Programming Language", is an algebraic modeling language for describing and solving high-complexity problems for large-scale mathematical computation AMPL, an acronym for "A Mathematical Programming Language", is an algebraic modeling language for describing and...
The Bourne shell, or sh, was the default Unix shell of Unix Version 7 and most Unix-like systems continue to have /bin/sh - which will be the Bourne shell, or a symbolic link or hard link to a compatible shell - even when more modern shells are used by most users.Developed by Stephen Bourne at AT&T...
The Korn shell is a Unix shell which was developed by David Korn in the early 1980s and announced at USENIX on July 14, 1983. Other early contributors were AT&T Bell Labs developers Mike Veach, who wrote the emacs code, and Pat Sullivan, who wrote the vi code...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
In object oriented computer programming, operator overloading—less commonly known as operator ad-hoc polymorphism—is a specific case of polymorphism, where different operators have different implementations depending on their arguments...
shift operators and manipulators as an alternative for formatted output - see iostream
Iostream
iostream is a header file which is used for input/output in the C++ programming language. It is part of the C++ standard library. The name stands for Input/Output Stream. In C++ and its predecessor, the C programming language, there is no special syntax for streaming data input or output. Instead,...
Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...
Clojure |closure]]") is a recent dialect of the Lisp programming language created by Rich Hickey. It is a general-purpose language supporting interactive development that encourages a functional programming style, and simplifies multithreaded programming....
The D programming language is an object-oriented, imperative, multi-paradigm, system programming language created by Walter Bright of Digital Mars. It originated as a re-engineering of C++, but even though it is mainly influenced by that language, it is not a variant of C++...
The GNU Linear Programming Kit is a software package intended for solving large-scale linear programming , mixed integer programming , and other related problems. It is a set of routines written in ANSI C and organized in the form of a callable library...
GNU Octave is a high-level language, primarily intended for numerical computations. It provides a convenient command-line interface for solving linear and nonlinear problems numerically, and for performing other numerical experiments using a language that is mostly compatible with MATLAB...
Go is a compiled, garbage-collected, concurrent programming language developed by Google Inc.The initial design of Go was started in September 2007 by Robert Griesemer, Rob Pike, and Ken Thompson. Go was officially announced in November 2009. In May 2010, Rob Pike publicly stated that Go was being...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
Maple is a general-purpose commercial computer algebra system. It was first developed in 1980 by the Symbolic Computation Group at the University of Waterloo in Waterloo, Ontario, Canada....
MATLAB is a numerical computing environment and fourth-generation programming language. Developed by MathWorks, MATLAB allows matrix manipulations, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs written in other languages,...
OCaml , originally known as Objective Caml, is the main implementation of the Caml programming language, created by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Didier Rémy and others in 1996...
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
R is a programming language and software environment for statistical computing and graphics. The R language is widely used among statisticians for developing statistical software, and R is widely used for statistical software development and data analysis....
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
Vala is a programming language created with the goal of bringing modern language features to C, with no added runtime needs and with little overhead, by targeting the GObject object system. It is being developed by Jürg Billeter and Raffaele Sandrini. The syntax borrows heavily from C#...
The C Standard Library is the standard library for the programming language C, as specified in the ANSI C standard.. It was developed at the same time as the C POSIX library, which is basically a superset of it...
Uncontrolled format string is a type of software vulnerability, discovered around 1999, that can be used in security exploits. Previously thought harmless, format string exploits can be used to crash a program or to execute harmful code...
iostream is a header file which is used for input/output in the C++ programming language. It is part of the C++ standard library. The name stands for Input/Output Stream. In C++ and its predecessor, the C programming language, there is no special syntax for streaming data input or output. Instead,...
Scanf format string refers to a control parameter used by a class of functions typically associated with some types of programming languages. The format string specifies a method for reading a string into an arbitrary number of varied data type parameter...
