man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

snprintf(3C)

Name

printf, fprintf, sprintf, snprintf, asprintf - print formatted output

Synopsis

#include <stdio.h>

int printf(const char *restrict format,
     /* args*/ ...);
int fprintf(FILE *restrict stream, const char *restrict format,
     /* args*/ ...);
int sprintf(char *restrict s, const char *restrict format,
     /* args*/ ...);
int snprintf(char *restrict s, size_t n,
     const char *restrict format, /* args*/ ...);
int asprintf(char ** ret, const char *restrict format,
     /* args*/ ...);

Description

The printf() function places output on the standard output stream stdout.

The fprintf() function places output on on the named output stream stream.

The sprintf() function places output, followed by the null byte (\0), in consecutive bytes starting at s; it is the user's responsibility to ensure that enough storage is available.

The snprintf() function is identical to sprintf() with the addition of the argument n, which specifies the size of the buffer referred to by s. If n is 0, nothing is written and s can be a null pointer. Otherwise, output bytes beyond the n-1st are discarded instead of being written to the array and a null byte is written at the end of the bytes actually written into the array.

The asprintf() function is the same as the sprintf() function except that it returns, in the ret argument, a pointer to a buffer sufficiently large to hold the output string. This pointer should be passed to free(3C) to release the allocated storage when it is no longer needed. If sufficient space cannot be allocated, the asprintf() function returns -1 and sets ret to be a NULL pointer.

Each of these functions converts, formats, and prints its arguments under control of the format. The format is a character string, beginning and ending in its initial shift state, if any. The format is composed of zero or more directives: ordinary characters, which are simply copied to the output stream and conversion specifications, each of which results in the fetching of zero or more arguments. The results are undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments are evaluated but are otherwise ignored.

Conversions can be applied to the nth argument after the format in the argument list, rather than to the next unused argument. In this case, the conversion specifier % (see below) is replaced by the sequence % n$, where n is a decimal integer in the range [1, NL_ARGMAX], giving the position of the argument in the argument list. This feature provides for the definition of format strings that select arguments in an order appropriate to specific languages (see the EXAMPLES section).

In format strings containing the %n$ form of conversion specifications, numbered arguments in the argument list can be referenced from the format string as many times as required.

In format strings containing the % form of conversion specifications, each argument in the argument list is used exactly once.

All forms of the printf() functions allow for the insertion of a language-dependent radix character in the output string. The radix character is defined by the program's locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character defaults to a period (.).

Conversion Specifications

Each conversion specification is introduced by the % character or by the character sequence %n$, after which the following appear in sequence:

  • An optional field, consisting of a decimal digit string followed by a $, specifying the next argument to be converted. If this field is not provided, the args following the last argument converted will be used.

  • Zero or more flags (in any order), which modify the meaning of the conversion specification.

  • An optional minimum field width. If the converted value has fewer bytes than the field width, it will be padded with spaces by default on the left; it will be padded on the right, if the left-adjustment flag (-), described below, is given to the field width. The field width takes the form of an asterisk (*), described below, or a decimal integer.

    If the conversion specifier is s, a standard-conforming application (see standards(5)) interprets the field width as the minimum number of bytes to be printed; an application that is not standard-conforming interprets the field width as the minimum number of columns of screen display. For an application that is not standard-conforming, %10s means if the converted value has a screen width of 7 columns, 3 spaces would be padded on the right.

    If the format is %ws, then the field width should be interpreted as the minimum number of columns of screen display.

  • An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversions (the field is padded with leading zeros); the number of digits to appear after the radix character for the a, A, e, E, f, and F conversions, the maximum number of significant digits for the g and G conversions; or the maximum number of bytes to be printed from a string in s and S conversions. The precision takes the form of a period (.) followed either by an asterisk (*), described below, or an optional decimal digit string, where a null digit string is treated as 0. If a precision appears with any other conversion specifier, the behavior is undefined.

    If the conversion specifier is s or S, a standard-conforming application (see standards(5)) interprets the precision as the maximum number of bytes to be written; an application that is not standard-conforming interprets the precision as the maximum number of columns of screen display. For an application that is not standard-conforming, %.5s would print only the portion of the string that would display in 5 screen columns. Only complete characters are written.

    For %ws, the precision should be interpreted as the maximum number of columns of screen display. The precision takes the form of a period (.) followed by a decimal digit string; a null digit string is treated as zero. Padding specified by the precision overrides the padding specified by the field width.

  • An optional length modifier that specified the size of the argument.

  • A conversion specifier that indicates the type of conversion to be applied.

A field width, or precision, or both can be indicated by an asterisk (*) . In this case, an argument of type int supplies the field width or precision. Arguments specifying field width, or precision, or both must appear in that order before the argument, if any, to be converted. A negative field width is taken as a − flag followed by a positive field width. A negative precision is taken as if the precision were omitted. In format strings containing the %n$ form of a conversion specification, a field width or precision may be indicated by the sequence *m$, where m is a decimal integer in the range [1, NL_ARGMAX] giving the position in the argument list (after the format argument) of an integer argument containing the field width or precision, for example:

printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

The format can contain either numbered argument specifications (that is, %n$ and *m$), or unnumbered argument specifications (that is, % and *), but normally not both. The only exception to this is that %% can be mixed with the %n$ form. The results of mixing numbered and unnumbered argument specifications in a format string are undefined. When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N–1)th, are specified in the format string.

Flag Characters

The flag characters and their meanings are:

'

The integer portion of the result of a decimal conversion (%i, %d, %u, %f, %F, %g, or %G) will be formatted with thousands' grouping characters. For other conversions the behavior is undefined. The non-monetary grouping character is used.

The result of the conversion will be left-justified within the field. The conversion will be right-justified if this flag is not specified.

+

The result of a signed conversion will always begin with a sign (+ or –). The conversion will begin with a sign only when a negative value is converted if this flag is not specified.

space

If the first character of a signed conversion is not a sign or if a signed conversion results in no characters, a space will be placed before the result. This means that if the space and + flags both appear, the space flag will be ignored.

#

The value is to be converted to an alternate form. For c, d, i, s, and u conversions, the flag has no effect. For an o conversion, it increases the precision (if necessary) to force the first digit of the result to be a zero. For x or X conversion, a non-zero result will have 0x (or 0X) prepended to it. For a, A, e, E, f, F, g, and G conversions, the result will always contain a radix character, even if no digits follow the radix character. Without this flag, the radix character appears in the result of these conversions only if a digit follows it. For g and G conversions, trailing zeros will not be removed from the result as they normally are.

0

For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the 0 and flags both appear, the 0 flag will be ignored. For d , i, o, u, x, and X conversions, if a precision is specified, the 0 flag will be ignored. If the 0 and ' flags both appear, the grouping characters are inserted before zero padding. For other conversions, the behavior is undefined.

Length Modifiers

The length modifiers and their meanings are:

hh

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer promotions, but its value will be converted to signed char or unsigned char before printing); or that a following n conversion specifier applies to a pointer to a signed char argument.

h

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short or unsigned short argument (the argument will have been promoted according to the integer promotions, but its value will be converted to short or unsigned short before printing); or that a following n conversion specifier applies to a pointer to a short argument.

l (ell)

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long or unsigned long argument; that a following n conversion specifier applies to a pointer to a long argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion specifier.

ll (ell-ell)

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long long or unsigned long long argument; or that a following n conversion specifier applies to a pointer to a long long argument.

j

Specifies that a following d, i, o, u, x, or X conversion specifier applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to an intmax_t argument. See NOTES.

z

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to size_t argument.

t

Specifies that a following d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.

L

Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

Conversion Specifiers

Each conversion specifier results in fetching zero or more arguments. The results are undefined if there are insufficient arguments for the format. If the format is exhausted while arguments remain, the excess arguments are ignored.

The conversion specifiers and their meanings are:

d, i

The int argument is converted to a signed decimal in the style []dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

o

The unsigned int argument is converted to unsigned octal format in the style dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

u

The unsigned int argument is converted to unsigned decimal format in the style dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

x

The unsigned int argument is converted to unsigned hexadecimal format in the style dddd; the letters abcdef are used. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting 0 with an explicit precision of 0 is no characters.

X

Behaves the same as the x conversion specifier except that letters ABCDEF are used instead of abcdef.

f, F

The double argument is converted to decimal notation in the style []ddd.ddd, where the number of digits after the radix character (see setlocale(3C)) is equal to the precision specification. If the precision is missing it is taken as 6; if the precision is explicitly 0 and the # flag is not specified, no radix character appears. If a radix character appears, at least 1 digit appears before it. The converted value is rounded to fit the specified output format according to the prevailing floating point rounding direction mode. If the conversion is not exact, an inexact exception is raised.

For the f specifier, a double argument representing an infinity or NaN is converted in the style of the e conversion specifier, except that for an infinite argument, “infinity” or “Infinity” is printed when the precision is at least 8 and “inf” or “Inf” is printed otherwise.

For the F specifier, a double argument representing an infinity or NaN is converted in the SUSv3 style of the E conversion specifier, except that for an infinite argument, “INFINITY” is printed when the precision is at least 8 and or “INF” is printed otherwise.

e, E

The double argument is converted to the style []d.ddde±dd, where there is one digit before the radix character (which is non-zero if the argument is non-zero) and the number of digits after it is equal to the precision. When the precision is missing it is taken as 6; if the precision is 0 and the # flag is not specified, no radix character appears. The E conversion specifier will produce a number with E instead of e introducing the exponent. The exponent always contains at least two digits. The converted value is rounded to fit the specified output format according to the prevailing floating point rounding direction mode. If the conversion is not exact, an inexact exception is raised.

Infinity and NaN values are handled in one of the following ways:

SUSv3

For the e specifier, a double argument representing an infinity is printed as “[−]infinity”, when the precision for the conversion is at least 7 and as “[−]inf” otherwise. A double argument representing a NaN is printed as “[−]nan”. For the E specifier, “INF”, “INFINITY”, and “NAN” are printed instead of “inf”, “infinity”, and “nan”, respectively. Printing of the sign follows the rules described above.

Default

A double argument representing an infinity is printed as “[−]Infinity”, when the precision for the conversion is at least 7 and as “[−]Inf” otherwise. A double argument representing a NaN is printed as “[−]NaN”. Printing of the sign follows the rules described above.

g, G

The double argument is printed in style f or e (or in style E in the case of a G conversion specifier), with the precision specifying the number of significant digits. If an explicit precision is 0, it is taken as 1. The style used depends on the value converted: style e (or E) will be used only if the exponent resulting from the conversion is less than –4 or greater than or equal to the precision. Trailing zeros are removed from the fractional part of the result. A radix character appears only if it is followed by a digit.

A double argument representing an infinity or NaN is converted in the style of the e or E conversion specifier, except that for an infinite argument, “infinity”, “INFINITY”, or “Infinity” is printed when the precision is at least 8 and “inf”, “INF”, or “Inf” is printed otherwise.

a, A

A double argument representing a floating-point number is converted in the style “[-]0xh.hhhhp±d”, where the single hexadecimal digit preceding the radix point is 0 if the value converted is zero and 1 otherwise and the number of hexadecimal digits after it is equal to the precision; if the precision is missing, the number of digits printed after the radix point is 13 for the conversion of a double value, 16 for the conversion of a long double value on x86, and 28 for the conversion of a long double value on SPARC; if the precision is zero and the '#' flag is not specified, no decimal-point character will appear. The letters “abcdef” are used for a conversion and the letters “ ABCDEF” for A conversion. The A conversion specifier produces a number with 'X' and 'P' instead of 'x' and 'p'. The exponent will always contain at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent is zero.

The converted value is rounded to fit the specified output format according to the prevailing floating point rounding direction mode. If the conversion is not exact, an inexact exception is raised.

A double argument representing an infinity or NaN is converted in the SUSv3 style of an e or E conversion specifier.

c

The int argument is converted to an unsigned char, and the resulting byte is printed.

If an l (ell) qualifier is present, the wint_t argument is converted as if by an ls conversion specification with no precision and an argument that points to a two-element array of type wchar_t, the first element of which contains the wint_t argument to the ls conversion specification and the second element contains a null wide-character.

C

Same as lc.

wc

The int argument is converted to a wide character (wchar_t), and the resulting wide character is printed.

s

The argument must be a pointer to an array of char. Bytes from the array are written up to (but not including) any terminating null byte. If a precision is specified, a standard-conforming application (see standards(5)) will write only the number of bytes specified by precision; an application that is not standard-conforming will write only the portion of the string that will display in the number of columns of screen display specified by precision. If the precision is not specified, it is taken to be infinite, so all bytes up to the first null byte are printed. An argument with a null value will yield undefined results.

If an l (ell) qualifier is present, the argument must be a pointer to an array of type wchar_t. Wide-characters from the array are converted to characters (each as if by a call to the wcrtomb(3C) function, with the conversion state described by an mbstate_t object initialized to zero before the first wide-character is converted) up to and including a terminating null wide-character. The resulting characters are written up to (but not including) the terminating null character (byte). If no precision is specified, the array must contain a null wide-character. If a precision is specified, no more than that many characters (bytes) are written (including shift sequences, if any), and the array must contain a null wide-character if, to equal the character sequence length given by the precision, the function would need to access a wide-character one past the end of the array. In no case is a partial character written.

S

Same as ls.

ws

The argument must be a pointer to an array of wchar_t. Bytes from the array are written up to (but not including) any terminating null character. If the precision is specified, only that portion of the wide-character array that will display in the number of columns of screen display specified by precision will be written. If the precision is not specified, it is taken to be infinite, so all wide characters up to the first null character are printed. An argument with a null value will yield undefined results.

p

The argument must be a pointer to void. The value of the pointer is converted to a set of sequences of printable characters, which should be the same as the set of sequences that are matched by the %p conversion of the scanf(3C) function.

n

The argument must be a pointer to an integer into which is written the number of bytes written to the output standard I/O stream so far by this call to one of the printf() functions. No argument is converted.

%

Print a %; no argument is converted. The entire conversion specification must be %%.

If a conversion specification does not match one of the above forms, the behavior is undefined.

In no case does a non-existent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is simply expanded to contain the conversion result. Characters generated by printf() and fprintf() are printed as if the putc(3C) function had been called.

The st_ctime and st_mtime fields of the file will be marked for update between the call to a successful execution of printf() or fprintf() and the next successful completion of a call to fflush(3C) or fclose(3C) on the same stream or a call to exit(3C) or abort(3C).

Return Values

The printf(), fprintf(), sprintf(), and asprintf() functions return the number of bytes transmitted (excluding the terminating null byte in the case of sprintf() and asprintf()).

The snprintf() function returns the number of bytes that would have been written to s if n had been sufficiently large (excluding the terminating null byte.) If the value of n is 0 on a call to snprintf (), s can be a null pointer and the number of bytes that would have been written if n had been sufficiently large (excluding the terminating null byte) is returned.

Each function returns a negative value if an output error was encountered.

Errors

For the conditions under which printf() and fprintf() will fail and may fail, refer to fputc(3C) or fputwc(3C).

The snprintf() function will fail if:

EOVERFLOW

The value of n is greater than INT_MAX or the number of bytes needed to hold the output excluding the terminating null is greater than INT_MAX.

The printf(), fprintf(), sprintf(), and snprintf() functions may fail if:

EILSEQ

A wide-character code that does not correspond to a valid character has been detected.

EINVAL

There are insufficient arguments.

The printf(), fprintf(), and asprintf() functions may fail due to an underlying malloc(3C) failure if:

EAGAIN

Storage space is temporarily unavailable.

ENOMEM

Insufficient storage space is available.

Usage

If the application calling the printf() functions has any objects of type wint_t or wchar_t, it must also include the header wchar.h to have these objects defined.

Escape Character Sequences

It is common to use the following escape sequences built into the C language when entering format strings for the printf() functions, but these sequences are processed by the C compiler, not by the printf() function.

\a

Alert. Ring the bell.

\b

Backspace. Move the printing position to one character before the current position, unless the current position is the start of a line.

\f

Form feed. Move the printing position to the initial printing position of the next logical page.

\n

Newline. Move the printing position to the start of the next line.

\r

Carriage return. Move the printing position to the start of the current line.

\t

Horizontal tab. Move the printing position to the next implementation-defined horizontal tab position on the current line.

\v

Vertical tab. Move the printing position to the start of the next implementation-defined vertical tab position.

In addition, the C language supports character sequences of the form

\octal-number

and

\hex-number

which translates into the character represented by the octal or hexadecimal number. For example, if ASCII representations are being used, the letter 'a' may be written as '\141' and 'Z' as '\132'. This syntax is most frequently used to represent the null character as '\0'. This is exactly equivalent to the numeric constant zero (0). Note that the octal number does not include the zero prefix as it would for a normal octal constant. To specify a hexadecimal number, omit the zero so that the prefix is an 'x' (uppercase 'X' is not allowed in this context). Support for hexadecimal sequences is an ANSI extension. See standards(5).

Examples

Example 1 To print the language-independent date and time format, the following statement could be used:
printf (format, weekday, month, day, hour, min);

For American usage, format could be a pointer to the string:

"%s, %s %d, %d:%.2d\n"

producing the message:

Sunday, July 3, 10:02

whereas for German usage, format could be a pointer to the string:

"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"

producing the message:

Sonntag, 3. Juli, 10:02
Example 2 To print a date and time in the form Sunday, July 3, 10:02, where weekday and month are pointers to null-terminated strings:
printf("%s, %s %i, %d:%.2d", weekday, month, day, hour, min);
Example 3 To print pi to 5 decimal places:
printf("pi = %.5f", 4 * atan(1.0));
Example 4 The following example applies only to applications that are not standard-conforming. To print a list of names in columns which are 20 characters wide:
printf("%20s%20s%20s", lastname, firstname, middlename);

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
CSI
Enabled
Interface Stability
Committed
MT-Level
See below.
Standard
See below.

All of these functions can be used safely in multithreaded applications, as long as setlocale(3C) is not being called to change the locale. The sprintf () and snprintf() functions are Async-Signal-Safe.

See standards(5) for the standards conformance of printf(), fprintf(), sprintf(), and snprintf (). The asprintf() function is modeled on the one that appears in the FreeBSD, NetBSD, and GNU C libraries.

See Also

exit(2), lseek(2), write(2), abort(3C), ecvt(3C), exit(3C), fclose(3C), fflush(3C), fputwc(3C), free(3C), malloc(3C), putc(3C), scanf(3C), setlocale(3C), stdio(3C), vprintf(3C), wcstombs(3C), wctomb(3C), attributes(5), environ(5), standards(5)

Notes

If the j length modifier is used, 32-bit applications that were compiled using c89 on releases prior to Solaris 10 will experience undefined behavior.

The snprintf() return value when n = 0 was changed in the Solaris 10 release. The change was based on the SUSv3 specification. The previous behavior was based on the initial SUSv2 specification, where snprintf() when n = 0 returns an unspecified value less than 1.