Format specification syntax: printf and wprintf functions

The various printf and wprintf functions take a format string and optional arguments and produce a formatted sequence of characters for output. The format string contains zero or more directives, which are either literal characters for output or encoded conversion specifications that describe how to format an argument in the output. This topic describes the syntax used to encode conversion specifications in the format string. For a listing of these functions, see Stream I/O.

A conversion specification consists of optional and required fields in this form:

%[flags][width][.precision][size]type

Each field of the conversion specification is a character or a number that signifies a particular format option or conversion specifier. The required type field specifies the kind of conversion to be applied to an argument. The optional flags, width, and precision fields control additional format aspects such as leading spaces or zeroes, justification, and displayed precision. The size field specifies the size of the argument consumed and converted.

A basic conversion specification contains only the percent sign and a type character. For example, %s specifies a string conversion. To print a percent-sign character, use %%. If a percent sign is followed by a character that has no meaning as a format field, the invalid parameter handler is invoked. For more information, see Parameter Validation.

Important

For security and stability, ensure that conversion specification strings are not user-defined. For example, consider a program that prompts the user to enter a name and stores the input in a string variable that's named user_name. To print user_name, do not do this:

printf( user_name ); /* Danger! If user_name contains "%s", program will crash */

Instead, do this:

printf( "%s", user_name );

Type conversion specifier

The type conversion specifier character specifies whether to interpret the corresponding argument as a character, a string, a pointer, an integer, or a floating-point number. The type character is the only required conversion specification field, and it appears after any optional fields.

The arguments that follow the format string are interpreted according to the corresponding type character and the optional size prefix. Conversions for character types char and wchar_t are specified by using c or C, and single-byte and multi-byte or wide character strings are specified by using s or S, depending on which formatting function is being used. Character and string arguments that are specified by using c and s are interpreted as char and char* by printf family functions, or as wchar_t and wchar_t* by wprintf family functions. Character and string arguments that are specified by using C and S are interpreted as wchar_t and wchar_t* by printf family functions, or as char and char* by wprintf family functions. This behavior is Microsoft specific.

Integer types such as short, int, long, long long, and their unsigned variants, are specified by using d, i, o, u, x, and X. Floating-point types such as float, double, and long double, are specified by using a, A, e, E, f, F, g, and G. By default, unless they are modified by a size prefix, integer arguments are coerced to int type, and floating-point arguments are coerced to double. On 64-bit systems, an int is a 32-bit value; therefore, 64-bit integers will be truncated when they are formatted for output unless a size prefix of ll or I64 is used. Pointer types that are specified by p use the default pointer size for the platform.

Note

Microsoft Specific The Z type character, and the behavior of the c, C, s, and S type characters when they are used with the printf and wprintf functions, are Microsoft extensions. The ISO C standard uses c and s consistently for narrow characters and strings, and C and S for wide characters and strings, in all formatting functions.

Type field characters

Type character Argument Output format
c Character When used with printf functions, specifies a single-byte character; when used with wprintf functions, specifies a wide character.
C Character When used with printf functions, specifies a wide character; when used with wprintf functions, specifies a single-byte character.
d Integer Signed decimal integer.
i Integer Signed decimal integer.
o Integer Unsigned octal integer.
u Integer Unsigned decimal integer.
x Integer Unsigned hexadecimal integer; uses "abcdef."
X Integer Unsigned hexadecimal integer; uses "ABCDEF."
e Floating-point Signed value that has the form [-]d.dddddd[d] where d is one decimal digit, dddd is one or more decimal digits depending on the specified precision, or six by default, and dd[d] is two or three decimal digits depending on the output format and size of the exponent.
E Floating-point Identical to the e format except that E rather than e introduces the exponent.
f Floating-point Signed value that has the form [-]dddd.dddd, where dddd is one or more decimal digits. The number of digits before the decimal point depends on the magnitude of the number, and the number of digits after the decimal point depends on the requested precision, or six by default.
F Floating-point Identical to the f format except that infinity and nan output is capitalized.
g Floating-point Signed values are displayed in f or e format, whichever is more compact for the given value and precision. The e format is used only when the exponent of the value is less than -4 or greater than or equal to the precision argument. Trailing zeros are truncated, and the decimal point appears only if one or more digits follow it.
G Floating-point Identical to the g format, except that E, rather than e, introduces the exponent (where appropriate).
a Floating-point Signed hexadecimal double-precision floating-point value that has the form [-]0xh.hhhhdd, where h.hhhh are the hex digits (using lower case letters) of the mantissa, and dd are one or more digits for the exponent. The precision specifies the number of digits after the point.
A Floating-point Signed hexadecimal double-precision floating-point value that has the form [-]0Xh.hhhhdd, where h.hhhh are the hex digits (using capital letters) of the mantissa, and dd are one or more digits for the exponent. The precision specifies the number of digits after the point.
n Pointer to integer Number of characters that are successfully written so far to the stream or buffer. This value is stored in the integer whose address is given as the argument. The size of the integer pointed to can be controlled by an argument size specification prefix. The n specifier is disabled by default; for information see the important security note.
p Pointer type Displays the argument as an address in hexadecimal digits.
s String When used with printf functions, specifies a single-byte or multi-byte character string; when used with wprintf functions, specifies a wide-character string. Characters are displayed up to the first null character or until the precision value is reached.
S String When used with printf functions, specifies a wide-character string; when used with wprintf functions, specifies a single-byte or multi-byte character string. Characters are displayed up to the first null character or until the precision value is reached.
Z ANSI_STRING or UNICODE_STRING structure When the address of an ANSI_STRING or UNICODE_STRING structure is passed as the argument, displays the string contained in the buffer pointed to by the Buffer field of the structure. Use a size modifier prefix of w to specify a UNICODE_STRING argument—for example, %wZ. The Length field of the structure must be set to the length, in bytes, of the string. The MaximumLength field of the structure must be set to the length, in bytes, of the buffer.

Typically, the Z type character is used only in driver debugging functions that use a conversion specification, such as dbgPrint and kdPrint.

Starting in Visual Studio 2015, if the argument that corresponds to a floating-point conversion specifier (a, A, e, E, f, F, g, G) is infinite, indefinite, or NaN, the formatted output conforms to the C99 standard. This table lists the formatted output:

Value Output
infinity inf
Quiet NaN nan
Signalling NaN nan(snan)
Indefinite NaN nan(ind)

Any of these values may be prefixed by a sign. If a floating-point type conversion specifier character is a capital letter, then the output is also formatted in capital letters. For example, if the format specifier is %F instead of %f, an infinity is formatted as INF instead of inf. The scanf functions can also parse these strings, so these values can make a round-trip through printf and scanf functions.

Before Visual Studio 2015, the CRT used a different, non-standard format for output of infinite, indefinite, and NaN values:

Value Output
+ infinity 1.#INF random-digits
- infinity -1.#INF random-digits
Indefinite (same as quiet NaN) digit .#IND random-digits
NaN digit .#NAN random-digits

Any of these may have been prefixed by a sign, and may have been formatted slightly differently depending on field width and precision, sometimes with unusual effects. For example, printf("%.2f\n", INFINITY) would print 1.#J because the #INF would be "rounded" to a precision of 2 digits.

Note

If the argument that corresponds to %s or %S, or the Buffer field of the argument that corresponds to %Z, is a null pointer, "(null)" is displayed.

Note

In all exponential formats, the minimum number of digits of exponent to display is two, using three only if necessary. By using the _set_output_format function, you can set the number of digits displayed to three for backward compatibility with code written for Visual Studio 2013 and before.

Important

Because the %n format is inherently insecure, it is disabled by default. If %n is encountered in a format string, the invalid parameter handler is invoked, as described in Parameter Validation. To enable %n support, see _set_printf_count_output.

Flag directives

The first optional field in a conversion specification contains flag directives, zero or more flag characters that specify output justification and control output of signs, blanks, leading zeros, decimal points, and octal and hexadecimal prefixes. More than one flag directive may appear in a conversion specification, and the flag characters can appear in any order.

Flag characters

Flag Meaning Default
- Left align the result within the given field width. Right align.
+ Use a sign (+ or -) to prefix the output value if it is of a signed type. Sign appears only for negative signed values (-).
0 If width is prefixed by 0, leading zeros are added until the minimum width is reached. If both 0 and - appear, the 0 is ignored. If 0 is specified for an integer format (i, u, x, X, o, d) and a precision specification is also present—for example, %04.d—the 0 is ignored. If 0 is specified for the a or A floating-point format, leading zeros are prepended to the mantissa, after the 0x or 0X prefix. No padding.
blank (' ') Use a blank to prefix the output value if it is signed and positive. The blank is ignored if both the blank and + flags appear. No blank appears.
# When it's used with the o, x, or X format, the # flag uses 0, 0x, or 0X, respectively, to prefix any nonzero output value. No blank appears.
When it's used with the e, E, f, F, a or A format, the # flag forces the output value to contain a decimal point. Decimal point appears only if digits follow it.
When it's used with the g or G format, the # flag forces the output value to contain a decimal point and prevents the truncation of trailing zeros.

Ignored when used with c, d, i, u, or s.
Decimal point appears only if digits follow it. Trailing zeros are truncated.

Width specification

In a conversion specification, the optional width specification field appears after any flags characters. The width argument is a non-negative decimal integer that controls the minimum number of characters that are output. If the number of characters in the output value is less than the specified width, blanks are added to the left or the right of the values—depending on whether the left-alignment flag (-) is specified—until the minimum width is reached. If width is prefixed by 0, leading zeros are added to integer or floating-point conversions until the minimum width is reached, except when conversion is to an infinity or NaN.

The width specification never causes a value to be truncated. If the number of characters in the output value is greater than the specified width, or if width is not given, all characters of the value are output, subject to the precision specification.

If the width specification is an asterisk (*), an int argument from the argument list supplies the value. The width argument must precede the value that's being formatted in the argument list, as shown in this example:

printf("%0*f", 5, 3); /* 00003 is output */

A missing or small width value in a conversion specification does not cause the truncation of an output value. If the result of a conversion is wider than the width value, the field expands to contain the conversion result.

Precision specification

In a conversion specification, the third optional field is the precision specification. It consists of a period (.) followed by a non-negative decimal integer that, depending on the conversion type, specifies the number of string characters, the number of decimal places, or the number of significant digits to be output.

Unlike the width specification, the precision specification can cause either truncation of the output value or rounding of a floating-point value. If precision is specified as 0 and the value to be converted is 0, the result is no characters output, as shown in this example:

printf( "%.0d", 0 ); /* No characters output */

If the precision specification is an asterisk (*), an int argument from the argument list supplies the value. In the argument list, the precision argument must precede the value that's being formatted, as shown in this example:

printf( "%.*f", 3, 3.14159265 ); /* 3.142 output */

The type character determines either the interpretation of precision or the default precision when precision is omitted, as shown in the following table.

How Precision Values Affect Type

Type Meaning Default
a, A The precision specifies the number of digits after the point. Default precision is 13. If precision is 0, no decimal point is printed unless the # flag is used.
c, C The precision has no effect. Character is printed.
d, i, o, u, x, X The precision specifies the minimum number of digits to be printed. If the number of digits in the argument is less than precision, the output value is padded on the left with zeros. The value is not truncated when the number of digits exceeds precision. Default precision is 1.
e, E The precision specifies the number of digits to be printed after the decimal point. The last printed digit is rounded. Default precision is 6. If precision is 0 or the period (.) appears without a number following it, no decimal point is printed.
f, F The precision value specifies the number of digits after the decimal point. If a decimal point appears, at least one digit appears before it. The value is rounded to the appropriate number of digits. Default precision is 6. If precision is 0, or if the period (.) appears without a number following it, no decimal point is printed.
g, G The precision specifies the maximum number of significant digits printed. Six significant digits are printed, and any trailing zeros are truncated.
s, S The precision specifies the maximum number of characters to be printed. Characters in excess of precision are not printed. Characters are printed until a null character is encountered.

Argument size specification

In a conversion specification, the size field is an argument length modifier for the type conversion specifier. The size field prefixes to the type field—hh, h, j, l (lowercase L), L, ll, t, w, z, I (uppercase i), I32, and I64—specify the "size" of the corresponding argument—long or short, 32-bit or 64-bit, single-byte character or wide character—depending on the conversion specifier that they modify. These size prefixes are used with type characters in the printf and wprintf families of functions to specify the interpretation of argument sizes, as shown in the following table. The size field is optional for some argument types. When no size prefix is specified, the formatter consumes integer arguments—for example, signed or unsigned char, short, int, long, and enumeration types—as 32-bit int types, and float, double, and long double floating-point arguments are consumed as 64-bit double types. This matches the default argument promotion rules for variable argument lists. For more information about argument promotion, see Ellipses and Default Arguments in Postfix expressions. On both 32-bit and 64-bit systems, the conversion specification of a 64-bit integer argument must include a size prefix of ll or I64. Otherwise, the behavior of the formatter is undefined.

Some types are different sizes in 32-bit and 64-bit code. For example, size_t is 32 bits long in code compiled for x86, and 64 bits in code compiled for x64. To create platform-agnostic formatting code for variable-width types, you can use a variable-width argument size modifier. Alternatively, use a 64-bit argument size modifier and explicitly promote the variable-width argument type to 64 bits. The Microsoft-specific I (uppercase i) argument size modifier handles variable-width integer arguments, but we recommend the type-specific j, t, and z modifiers for portability.

Size Prefixes for printf and wprintf Format-Type Specifiers

To specify Use prefix With type specifier
char
unsigned char
hh d, i, o, u, x, or X
short int
short unsigned int
h d, i, o, u, x, or X
__int32
unsigned __int32
I32 d, i, o, u, x, or X
__int64
unsigned __int64
I64 d, i, o, u, x, or X
intmax_t
uintmax_t
j or I64 d, i, o, u, x, or X
long double l (lowercase L) or L a, A, e, E, f, F, g, or G
long int
long unsigned int
l (lowercase L) d, i, o, u, x, or X
long long int
unsigned long long int
ll (lowercase LL) d, i, o, u, x, or X
ptrdiff_t t or I (uppercase i) d, i, o, u, x, or X
size_t z or I (uppercase i) d, i, o, u, x, or X
Single-byte character h c or C
Wide character l (lowercase L) or w c or C
Single-byte character string h s, S, or Z
Wide-character string l (lowercase L) or w s, S, or Z

The ptrdiff_t and size_t types are __int32 or unsigned __int32 on 32-bit platforms, and __int64 or unsigned __int64 on 64-bit platforms. The I (uppercase i), j, t, and z size prefixes take the correct argument width for the platform.

In Visual C++, although long double is a distinct type, it has the same internal representation as double.

An hc or hC type specifier is synonymous with c in printf functions and with C in wprintf functions. An lc, lC, wc or wC type specifier is synonymous with C in printf functions and with c in wprintf functions. An hs or hS type specifier is synonymous with s in printf functions and with S in wprintf functions. An ls, lS, ws or wS type specifier is synonymous with S in printf functions and with s in wprintf functions.

Note

Microsoft Specific The I (uppercase i), I32, I64, and w argument size modifier prefixes are Microsoft extensions and are not ISO C-compatible. The h prefix when it's used with data of type char and the l (lowercase L) prefix when it's used with data of type double are Microsoft extensions.

See Also

printf, _printf_l, wprintf, _wprintf_l
printf_s, _printf_s_l, wprintf_s, _wprintf_s_l
printf_p Positional Parameters