printf

Synopsis

#include <stdio.h>

int dprintf(int _fildes_, const char *restrict format, ...);

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

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

int snprintf(char *restrict s, size_t n,

const char *restrict format, ...);

int sprintf(char *restrict s, const char *restrict format, ...);

Status

Partially implemented

Conformance

IEEE Std 1003.1-2017

Description

The fprintf() function shall place output on the named output stream. The printf() function shall place output on the standard output stream stdout. The sprintf() function shall place output followed by the null byte, \0, in consecutive bytes starting at *s; it is the user’s responsibility to ensure that enough space is available.

The dprintf() function shall be equivalent to the fprintf() function, except that dprintf() shall write output to the file associated with the file descriptor specified by the fildes argument rather than place output on a stream.

The snprintf() function shall be equivalent to sprintf(), with the addition of the n argument which states the size of the buffer referred to by s. If n is zero, nothing shall be written and s may be a null pointer. Otherwise, output bytes beyond the n-1st shall be 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.

If copying takes place between objects that overlap as a result of a call to sprintf() or snprintf(), the results are undefined.

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 shall result 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 shall be evaluated but are otherwise ignored.

In format strings containing the % form of conversion specification, each conversion specification uses the first unused argument in the argument list.

All forms of the fprintf() functions allow for the insertion of a language-dependent radix character in the output string. The radix character is defined in the current locale (category LC_NUMERIC). In the POSIX locale, or in a locale where the radix character is not defined, the radix character shall default to a <period> (.).

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

  • 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 shall be padded with <space> characters by default on the left; it shall 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 a <asterisk> (*), described below, or a decimal integer.

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

An optional length modifier that specifies the size of the argument.

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

A field width, or precision, or both, may be indicated by a <asterisk> (*). In this case an argument of type int supplies the field width or precision. Applications shall ensure that arguments specifying field width, or precision, or both 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.

The flag characters and their meanings are:

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

  • + - The result of a signed conversion shall always begin with a sign (+ or -). The conversion shall 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> shall be prefixed to the result. This means that if the <space> and + flags both appear, the <space> flag shall be ignored.

  • # Specifies that the value is to be converted to an alternative form. For o conversion, it shall increase the precision, if and only if necessary, to force the first digit of the result to be a zero (if the value and precision are both 0, a single 0 is printed). For x or X conversion specifiers, a non-zero result shall have 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G conversion specifiers, the result shall always contain a radix character, even if no digits follow the radix character. Without this flag, a radix character appears in the result of these conversions only if a digit follows it. For g and G conversion specifiers, trailing zeros shall not be removed from the result as they normally are. For other conversion specifiers, the behavior is undefined.

  • 0 For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversion specifiers, leading zeros (following any indication of sign or base) are used to pad to the field width rather than performing space padding, except when converting an infinity or NaN. If the 0 and - flags both appear, the 0 flag is ignored. For d, i, o, u, x, and X conversion specifiers, if a precision is specified, the 0 flag shall be ignored. If the 0 and <apostrophe> flags both appear, the grouping characters are inserted before zero padding. For other conversions, the behavior is undefined.

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 shall 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 shall 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; 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 a intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to a intmax_t argument.

  • 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 a 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.

The conversion specifiers and their meanings are:

  • d, i - The int argument shall be 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 shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

  • o - The unsigned argument shall be 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 shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

  • u - The unsigned argument shall be 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 shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

  • x - The unsigned argument shall be 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 shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be no characters.

  • X - Equivalent to the x conversion specifier, except that letters ABCDEF are used instead of abcdef.

  • f, F - The double argument shall be converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the radix character is equal to the precision specification. If the precision is missing, it shall be taken as 6; if the precision is explicitly zero and no # flag is present, no radix character shall appear. If a radix character appears, at least one digit appears before it. The low-order digit shall be rounded in an implementation-defined manner. A double argument representing an infinity shall be converted in one of the styles [-]inf or [-]infinity; which style is implementation-defined. A double argument representing a NaN shall be converted in one of the styles [-]nan(n-char-sequence) or [-]nan; which style, and the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.

  • e, E - The double argument shall be converted in 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; if the precision is missing, it shall be taken as 6; if the precision is zero and no ‘#’ flag is present, no radix character shall appear. The low-order digit shall be rounded in an implementation-defined manner. The E conversion specifier shall produce a number with E instead of e introducing the exponent. The exponent shall always contain at least two digits. If the value is zero, the exponent shall be zero. A double argument representing an infinity or NaN shall be converted in the style of a f or F conversion specifier.

  • g, G The double argument representing a floating-point number shall be converted in the style f or e (or in the style F or E in the case of a G conversion specifier), depending on the value converted and the precision. Let P equal the precision if non-zero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of X: - If P > X>=-4, the conversion shall be with style f (or F) and precision P-(X+1). - Otherwise, the conversion shall be with style e (or E) and precision P-1. Finally, unless the # flag is used, any trailing zeros shall be removed from the fractional portion of the result and the decimal-point character shall be removed if there is no fractional portion remaining. A double argument representing an infinity or NaN shall be converted in the style of a f or F conversion specifier.

  • a, A - A double argument representing a floating-point number shall be converted in the style [-]0xh.hhhhp±d, where there is one hexadecimal digit (which shall be non-zero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point character and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision shall be sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision shall be sufficient to distinguish values of type double, except that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no decimal-point character shall appear. The letters abcdef shall be 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 shall 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 shall be zero. A double argument representing an infinity or NaN shall be converted in the style of a f or F conversion specifier.

  • c - The int argument shall be converted to a unsigned char, and the resulting byte shall be written.

  • s - The argument shall be a pointer to an array of char. Bytes from the array shall be written up to (but not including) any terminating null byte. If the precision is specified, no more than that many bytes shall be written. If the precision is not specified or is greater than the size of the array, the application shall ensure that the array contains a null byte.

  • p - The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printable characters, in an implementation-defined manner.

  • n - The argument shall be a pointer to an integer into which is written the number of bytes written to the output so far by this call to one of the fprintf() functions. No argument is converted.

  • % - Print a % character; no argument is converted. The complete conversion specification shall be %%.

If a conversion specification does not match one of the above forms, the behavior is undefined. If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

In no case shall 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 shall be expanded to contain the conversion result. Characters generated by fprintf() and printf() are printed as if fputc() had been called.

For the a and A conversion specifiers, if FLT_RADIX is a power of 2, the value shall be correctly rounded to a hexadecimal floating number with the given precision.

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

For the e, E, f, F, g, and G conversion specifiers, if the number of significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded. If the number of significant decimal digits is more than DECIMAL_DIG, but the source value is exactly representable with DECIMAL_DIG digits, then the result should be an exact representation with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that the error should have a correct sign for the current rounding direction.

The last data modification and last file status change timestamps of the file shall be marked for update:

  • Between the call to a successful execution of fprintf() or printf() and the next successful completion of a call to fflush() or fclose() on the same stream or a call to exit() or abort()

  • Upon successful completion of a call to dprintf()

Return value

Upon successful completion, the dprintf(), fprintf(), and printf() functions shall return the number of bytes transmitted.

Upon successful completion, the sprintf() function shall return the number of bytes written to s, excluding the terminating null byte.

Upon successful completion, the snprintf() function shall return the number of bytes that would be written to s had n been sufficiently large excluding the terminating null byte.

If an output error was encountered, these functions shall return a negative value and set errno to indicate the error.

If the value of n is zero on a call to snprintf(), nothing shall be written, the number of bytes that would have been written had n been sufficiently large excluding the terminating null shall be returned, and s may be a null pointer.

Errors

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

In addition, all forms of fprintf() shall fail if:

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

  • [EOVERFLOW] - The value to be returned is greater than INT_MA.

The dprintf() function may fail if:

  • [EBADF] - The fildes argument is not a valid file descriptor.

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

  • [ENOMEM] - Insufficient storage space is available.

The snprintf() function shall fail if:

  • [EOVERFLOW] - The value of n is greater than INT_MA.

POSIX deviations

IEEE Std 1003.1-2017 defines numbered argument conversion specifications (%n$ and *m$). The libphoenix library does not support them: a conversion specification in the %n$ form is not interpreted as a conversion and no argument is selected by position. Only the % character and the $ character are written to the output, the digits before $ are taken as a field width and dropped, and the characters after $ are written as ordinary characters (for example %3$d is written as %$d). The printf() core of libphoenix is intentionally kept compact to suit resource-constrained targets, so selecting arguments by position is not provided.

The ' (<apostrophe>) flag, which requests thousands’ grouping characters in the integer portion of a decimal conversion, is not supported: a conversion specification containing it is not interpreted as a conversion. Only the % character and the ' character are written to the output, any flags, field width, precision and length modifier before the ' character are dropped, and the characters after it are written as ordinary characters (for example %5'd is written as %'d). Grouping is defined by the LC_NUMERIC category of the current locale and libphoenix does not implement locales, since the locale machinery does not fit the footprint of the target devices.

A long double argument of a floating-point conversion specifier (with the L length modifier) is converted to double before it is formatted, so the precision and the range of the printed value are limited to those of double. For the a and A conversion specifiers the result is the hexadecimal representation of the converted double value. Printing of long double values is only partially supported by the compact printf() core of libphoenix, which is kept small to suit resource-constrained targets.

Wide characters are not supported by libphoenix. With the l length modifier, the c conversion specifier converts its wint_t argument to unsigned char and writes that single byte without any multibyte conversion, and the s conversion specifier treats its argument as a pointer to an array of char, so a wide-character string is not converted. The C and S conversion specifiers (equivalent to lc and ls) are not recognized: a conversion specification using them is not interpreted as a conversion, only the % character and the conversion specifier character are written to the output, and any flags, field width, precision and length modifier before it are dropped (for example %5C is written as %C).