printf (3)

Table of Contents


printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vdprintf, vsprintf, vsnprintf - formatted output conversion


#include <stdio.h>int printf(const char *format, ...);int fprintf(FILE *stream,
const char *format, ...);int dprintf(int fd, const char *format, ...);int sprintf(char
*str, const char *format, ...);int snprintf(char *str, size_t size, const
char *format, ...);#include <stdarg.h>
int vprintf(const char *format, va_list
ap);int vfprintf(FILE *stream, const char *format, va_list ap);int vdprintf(int
fd, 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 *formatva_list
" ap );

Feature Test Macro Requirements for glibc (see feature_test_macros(7) ):

snprintf(), vsnprintf():

_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE || || /* Glibc versions <= 2.19: */ _BSD_SOURCE

dprintf(), vdprintf():

Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:


The functions in the printf() family produce output according to a format as described below. The functions printf() and vprintf() write output to stdout, the standard output stream; fprintf() and vfprintf() write output to the given output stream; sprintf(), snprintf(), vsprintf() and vsnprintf() write to the character string str.

The function dprintf() is the same as fprintf() except that it outputs to a file descriptor, fd, instead of to a stdio stream.

The functions snprintf() and vsnprintf() write at most size bytes (including the terminating null byte (aq\0aq)) to str.

The functions vprintf(), vfprintf(), vdprintf(), vsprintf(), vsnprintf() are equivalent to the functions printf(), fprintf(), dprintf(), sprintf(), snprintf(), respectively, except that they are called with a va_list instead of a variable number of arguments. These functions do not call the va_end macro. Because they invoke the va_arg macro, the value of ap is undefined after the call. See stdarg(3) .

All of these functions write the output under the control of a format string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of stdarg(3) ) are converted for output.

C99 and POSIX.1-2001 specify that the results are undefined if a call to sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause copying to take place between objects that overlap (e.g., if the target string array and one of the supplied input arguments refer to the same buffer). See NOTES.

Format of the format string

The format string is a character string, beginning and ending in its initial shift state, if any. The format string is composed of zero or more directives: ordinary characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments. Each conversion specification is introduced by the character %, and ends with a conversion specifier. In between there may be (in this order) zero or more flags, an optional minimum field width, an optional precision and an optional length modifier.

The arguments must correspond properly (after type promotion) with the conversion specifier. By default, the arguments are used in the order given, where each aq*aq (see Field width and Precision below) and each conversion specifier asks for the next argument (and it is an error if insufficiently many arguments are given). One can also specify explicitly which argument is taken, at each place where an argument is required, by writing "%m$" instead of aq%aq and "*m$" instead of aq*aq, where the decimal integer m denotes the position in the argument list of the desired argument, indexed starting from 1. Thus,

printf("%*d", width, num);


printf("%2$*1$d", width, num);

are equivalent. The second style allows repeated references to the same argument. The C99 standard does not include the style using aq$aq, which comes from the Single UNIX Specification. If the style using aq$aq is used, it must be used throughout for all conversions taking an argument and all width and precision arguments, but it may be mixed with "%%" formats, which do not consume an argument. There may be no gaps in the numbers of arguments specified using aq$aq; for example, if arguments 1 and 3 are specified, argument 2 must also be specified somewhere in the format string.

For some numeric conversions a radix character ("decimal point") or thousands’ grouping character is used. The actual character used depends on the LC_NUMERIC part of the locale. (See setlocale(3) .) The POSIX locale uses as radix character, and does not have a grouping character. Thus,

printf("%aq.2f", 1234567.89);

results in "1234567.89" in the POSIX locale, in "1234567,89" in the nl_NL locale, and in "1.234.567,89" in the da_DK locale.

Flag characters

The character % is followed by zero or more of the following flags:
The value should be converted to an "alternate form". For o conversions, the first character of the output string is made zero (by prefixing a 0 if it was not zero already). For x and X conversions, a nonzero result has the string "0x" (or "0X" for X conversions) prepended to it. For a, A, e, E, f, F, g, and G conversions, the result will always contain a decimal point, even if no digits follow it (normally, a decimal point appears in the results of those conversions only if a digit follows). For g and G conversions, trailing zeros are not removed from the result as they would otherwise be. For other conversions, the result is undefined.
  • The value should be zero padded. For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, the converted value is padded on the left with zeros rather than blanks. If the 0 and - flags both appear, the 0 flag is ignored. If a precision is given with a numeric conversion (d, i, o, u, x, and X), the 0 flag is ignored. For other conversions, the behavior is undefined.
  • -
    The converted value is to be left adjusted on the field boundary. (The default is right justification.) The converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A - overrides a 0 if both are given.
    aq aq
    (a space) A blank should be left before a positive number (or empty string) produced by a signed conversion.
    A sign (+ or -) should always be placed before a number produced by a signed conversion. By default, a sign is used only for negative numbers. A + overrides a space if both are used.

    The five flag characters above are defined in the C99 standard. The Single UNIX Specification specifies one further flag character.

    For decimal conversion (i, d, u, f, F, g, G) the output is to be grouped with thousands’ grouping characters if the locale information indicates any. (See setlocale(3) .) Note that many versions of gcc(1) cannot parse this option and will issue a warning. (SUSv2 did not include %aqF, but SUSv3 added it.)

    glibc 2.2 adds one further flag character.

    For decimal integer conversion (i, d, u) the output uses the locale’s alternative output digits, if any. For example, since glibc 2.2.3 this will give Arabic-Indic digits in the Persian ("fa_IR") locale.

    Field width

    An optional decimal digit string (with nonzero first digit) specifying a minimum field width. If the converted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if the left-adjustment flag has been given). Instead of a decimal digit string one may write "*" or "*m$" (for some decimal integer m) to specify that the field width is given in the next argument, or in the m-th argument, respectively, which must be of type int. A negative field width is taken as a aq-aq flag followed by a positive field width. In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.


    An optional precision, in the form of a period ( followed by an optional decimal digit string. Instead of a decimal digit string one may write "*" or "*m$" (for some decimal integer m) to specify that the precision is given in the next argument, or in the m-th argument, respectively, which must be of type int. If the precision is given as just, the precision is taken to be zero. A negative precision is taken as if the precision were omitted. This gives the minimum number of digits to appear for d, i, o, u, x, and X conversions, the number of digits to appear after the radix character for a, A, e, E, f, and F conversions, the maximum number of significant digits for g and G conversions, or the maximum number of characters to be printed from a string for s and S conversions.

    Length modifier

    Here, "integer conversion" stands for d, i, o, u, x, or X conversion.
    A following integer conversion corresponds to a signed char or unsigned char argument, or a following n conversion corresponds to a pointer to a signed char argument.
    A following integer conversion corresponds to a short int or unsigned short int argument, or a following n conversion corresponds to a pointer to a short int argument.
    (ell) A following integer conversion corresponds to a long int or unsigned long int argument, or a following n conversion corresponds to a pointer to a long int argument, or a following c conversion corresponds to a wint_t argument, or a following s conversion corresponds to a pointer to wchar_t argument.