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man_3_printf
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.TH man 3 "26 July 2021" "hk1.0" "_printf man page"
.SH AUTHOR "Henry Ugwoke" "Ken Nsofor"
.SH NAME
.B _printf
\- function to print formatted output of arguments
.SH SYNOPSIS
#include <stdio.h>
int _printf(const char *format, ...);
.B _printf
.RI {FORMAT}
.RI [ARGUMENTS]...
.SH DESCRIPTION
The functions in the _printf() family produce output according to a format as described below. The functions _printf() write output to stdout,
the standard output stream.
The function 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.
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 di‐
rectives: 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 '*' (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$"
instead of '%' and "*m$" instead of '*', 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);
and
_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 '$', which
comes from the Single UNIX Specification. If the style using '$' 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 argu‐
ments specified using '$'; 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("%'.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
result as they would otherwise be. For other conversions, the result is undefined.
0 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.
' ' (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 num‐
bers. 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 indi‐
cates any. (See setlocale(3).) Note that many versions of gcc(1) cannot parse this option and will issue a warning. (SUSv2 did not include
%'F, but SUSv3 added it.)
glibc 2.2 adds one further flag character.
I 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, respec‐
tively, which must be of type int. A negative field width is taken as a '-' 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.
Precision
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.
hh 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.
h 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.
l (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.
ll (ell-ell). A following integer conversion corresponds to a long long int or unsigned long long int argument, or a following n conversion cor‐
responds to a pointer to a long long int argument.
q A synonym for ll. This is a nonstandard extension, derived from BSD; avoid its use in new code.
L A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argument. (C99 allows %LF, but SUSv2 does not.)
j A following integer conversion corresponds to an intmax_t or uintmax_t argument, or a following n conversion corresponds to a pointer to an
intmax_t argument.
z A following integer conversion corresponds to a size_t or ssize_t argument, or a following n conversion corresponds to a pointer to a size_t
argument.
Z A nonstandard synonym for z that predates the appearance of z. Do not use in new code.
t A following integer conversion corresponds to a ptrdiff_t argument, or a following n conversion corresponds to a pointer to a ptrdiff_t argu‐
ment.
SUSv3 specifies all of the above, except for those modifiers explicitly noted as being nonstandard extensions. SUSv2 specified only the length modi‐
fiers h (in hd, hi, ho, hx, hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf, Lg, LG).
As a nonstandard extension, the GNU implementations treats ll and L as synonyms, so that one can, for example, write llg (as a synonym for the stan‐
dards-compliant Lg) and Ld (as a synonym for the standards compliant lld). Such usage is nonportable.
Conversion specifiers
A character that specifies the type of conversion to be applied. The conversion specifiers and their meanings are:
d, i The int argument is converted to signed decimal notation. The precision, if any, gives the minimum number of digits that must appear; if the
converted value requires fewer digits, it is padded on the left with zeros. The default precision is 1. When 0 is printed with an explicit
precision 0, the output is empty.
o, u, x, X
The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal (x and X) notation. The letters
abcdef are used for x conversions; the letters ABCDEF are used for X conversions. The precision, if any, gives the minimum number of digits
that must appear; if the converted value requires fewer digits, it is padded on the left with zeros. The default precision is 1. When 0 is
printed with an explicit precision 0, the output is empty.
e, E The double argument is rounded and converted in the style [-]d.ddde±dd where there is one digit (which is nonzero if the argument is nonzero)
before the decimal-point character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6;
if the precision is zero, no decimal-point character appears. 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.
f, F The double argument is rounded and converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point
character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly zero, no
decimal-point character appears. If a decimal point appears, at least one digit appears before it.
(SUSv2 does not know about F and says that character string representations for infinity and NaN may be made available. SUSv3 adds a specifi‐
cation for F. The C99 standard specifies "[-]inf" or "[-]infinity" for infinity, and a string starting with "nan" for NaN, in the case of f
conversion, and "[-]INF" or "[-]INFINITY" or "NAN" in the case of F conversion.)
g, G The double argument is converted in style f or e (or F or E for G conversions). The precision specifies the number of significant digits. If
the precision is missing, 6 digits are given; if the precision is zero, it is treated as 1. Style e is used if the exponent from its conver‐
sion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional part of the result; a decimal
point appears only if it is followed by at least one digit.
a, A (C99; not in SUSv2, but added in SUSv3) For a conversion, the double argument is converted to hexadecimal notation (using the letters abcdef)
in the style [-]0xh.hhhhp±; for A conversion the prefix 0X, the letters ABCDEF, and the exponent separator P is used. There is one hexadeci‐
mal digit before the decimal point, and the number of digits after it is equal to the precision. The default precision suffices for an exact
representation of the value if an exact representation in base 2 exists and otherwise is sufficiently large to distinguish values of type dou‐
ble. The digit before the decimal point is unspecified for nonnormalized numbers, and nonzero but otherwise unspecified for normalized num‐
bers.
c If no l modifier is present, the int argument is converted to an unsigned char, and the resulting character is written. If an l modifier is
present, the wint_t (wide character) argument is converted to a multibyte sequence by a call to the wcrtomb(3) function, with a conversion
state starting in the initial state, and the resulting multibyte string is written.
s If no l modifier is present: the const char * argument is expected to be a pointer to an array of character type (pointer to a string). Char‐
acters from the array are written up to (but not including) a terminating null byte ('\0'); if a precision is specified, no more than the num‐
ber specified are written. If a precision is given, no null byte need be present; if the precision is not specified, or is greater than the
size of the array, the array must contain a terminating null byte.
If an l modifier is present: the const wchar_t * argument is expected to be a pointer to an array of wide characters. Wide characters from
the array are converted to multibyte characters (each by a call to the wcrtomb(3) function, with a conversion state starting in the initial
state before the first wide character), up to and including a terminating null wide character. The resulting multibyte characters are written
up to (but not including) the terminating null byte. If a precision is specified, no more bytes than the number specified are written, but no
partial multibyte characters are written. Note that the precision determines the number of bytes written, not the number of wide characters
or screen positions. The array must contain a terminating null wide character, unless a precision is given and it is so small that the number
of bytes written exceeds it before the end of the array is reached.
C (Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.) Synonym for lc. Don't use.
S (Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.) Synonym for ls. Don't use.
p The void * pointer argument is printed in hexadecimal (as if by %#x or %#lx).
n The number of characters written so far is stored into the integer pointed to by the corresponding argument. That argument shall be an int *,
or variant whose size matches the (optionally) supplied integer length modifier. No argument is converted. (This specifier is not supported
by the bionic C library.) The behavior is undefined if the conversion specification includes any flags, a field width, or a precision.
m (Glibc extension; supported by uClibc and musl.) Print output of strerror(errno). No argument is required.
% A '%' is written. No argument is converted. The complete conversion specification is '%%'.
.SH RETURN VALUE
Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).
.B ARGUMENT
(s) according to
.B
.SH FORMAT
.SH Reporting Bugs
Report _printf bugs to https://github.com/HenryIyke/printf/issues
Code such as _printf(foo); often indicates a bug, since foo may contain a % character. If foo comes from untrusted user input, it may contain %n,
causing the _printf() call to write to memory and creating a security hole.
.SH Copyright
Copyright 2021
Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.