.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
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.\" modification, are permitted provided that the following conditions
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.\" notice, this list of conditions and the following disclaimer.
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.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
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.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
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.\" @(#)printf.3 6.14 (Berkeley) 7/30/91
.Nd formatted output conversion
.Fn printf "const char *format" ...
.Fn fprintf "FILE *stream" "const char *format" ...
.Fn sprintf "char *str" "const char *format" ...
.Fn snprintf "char *str" "size_t size" "const char *format" ...
.\" .Fn smprintf "const char *format" ...
.Fn vprintf "const char *format" "va_list ap"
.Fn vfprintf "FILE *stream" "const char *format" "va_list ap"
.Fn vsprintf "char *str" "char *format" "va_list ap"
.Fn vsnprintf "char *str" "size_t size" "const char *format" "va_list ap"
.\" .Fn vsmprintf "const char *format" "va_list ap"
family of functions produces output according to a
the standard output stream;
write output to the given output
write to the character string
.\" dynamically allocate a new string with
These functions write the output under the control of a
string that specifies how subsequent arguments
(or arguments accessed via the variable-length argument facilities of
are converted for output.
.\" all of these functions return
the number of characters printed
(not including the trailing
used to end output to strings).
.\" return a pointer to a string of an appropriate length;
.\" this pointer should be passed to
.\" to release the associated storage
.\" when it is no longer needed.
.\" If sufficient space is not avaliable,
of the characters printed into the output string
character then gets the terminating
if the return value is greater than or equal to the
argument, the string was too short
and some of the printed characters were discarded.
effectively assume an infinte
The format string is composed of zero or more directives:
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 arguments must correspond properly (after type promotion)
with the conversion specifier.
the following appear in sequence:
Zero or more of the following flags:
.Bl -hyphen -offset indent
specifying that the value should be converted to an ``alternate form''.
conversions, this option has no effect.
conversions, the precision of the number is increased to force the first
character of the output string to a zero (except if a zero value is printed
with an explicit precision of zero).
conversions, a non-zero result has the string
conversions) prepended to it.
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).
conversions, trailing zeros are not removed from the result as they
character specifying zero padding.
For all conversions except
the converted value is padded on the left with zeros rather than blanks.
If a precision is given with a numeric conversion
A negative field width flag
indicates the converted value is to be left adjusted on the field boundary.
conversions, the converted value is padded on the right with blanks,
rather than on the left with blanks or zeros.
A space, specifying that a blank should be left before a positive number
produced by a signed conversion
character specifying that a sign always be placed before a
number produced by a signed conversion.
overrides a space if both are used.
An optional decimal digit string 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) to fill out
An optional precision, in the form of a period
optional digit string. If the digit string is omitted, the precision
is taken as zero. This gives the minimum number of digits to appear for
conversions, the number of digits to appear after the decimal-point for
conversions, the maximum number of significant digits for
conversions, or the maximum number of characters to be printed from a
specifying that a following
conversion corresponds to a
argument, or that a following
conversion corresponds to a pointer to a
(ell) specifying that a following
conversion applies to a pointer to a
argument, or that a following
conversion corresponds to a pointer to a
specifying that a following
conversion corresponds to a
argument (but note that long double values are not currently supported
A character that specifies the type of conversion to be applied.
A field width or precision, or both, may be indicated by
argument supplies the field width or precision.
A negative field width is treated as a left adjustment flag followed by a
positive field width; a negative precision is treated as though it were
The conversion specifiers and their meanings are:
(or appropriate variant) argument is converted to signed decimal
The precision, if any, gives the minimum number of digits that must
appear; if the converted value requires fewer digits, it is padded on
argument is converted to signed decimal, unsigned octal, or unsigned
decimal, as if the format had been
These conversion characters are deprecated, and will eventually disappear.
argument is rounded and converted in the style
.Pf [\-]d Cm \&. No ddd Cm e No \\*(Pmdd
where there is one digit before the
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.
conversion uses the letter
to introduce the exponent.
The exponent always contains at least two digits; if the value is zero,
argument is rounded and converted to decimal notation in the style
.Pf [-]ddd Cm \&. No 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.
argument is converted in style
The precision specifies the number of significant digits.
If the precision is missing, 6 digits are given; if the precision is zero,
is used if the exponent from its 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
decimal point appears only if it is followed by at least one digit.
argument is converted to an
and the resulting character is written.
argument is expected to be a pointer to an array of character type (pointer
Characters from the array are written up to (but not including)
if a precision is specified, no more than the number specified are
If a precision is given, no null character
need be present; if the precision is not specified, or is greater than
the size of the array, the array must contain a terminating
pointer argument is printed in hexadecimal (as if by
The number of characters written so far is stored into the
(or variant) pointer argument.
No argument is converted.
is written. No argument is converted. The complete conversion specification
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 expanded to contain the conversion result.
To print a date and time in the form `Sunday, July 3, 10:02',
.Bd -literal -offset indent
fprintf(stdout, "%s, %s %d, %.2d:%.2d\en",
weekday, month, day, hour, min);
.Bd -literal -offset indent
fprintf(stdout, "pi = %.5f\en", 4 * atan(1.0));
To allocate a 128 byte string and print into it:
.Bd -literal -offset indent
char *newfmt(const char *fmt, ...)
if ((p = malloc(128)) == NULL)
(void) vsnprintf(p, 128, fmt, ap);
are provided only for backward compatibility.
The effect of padding the
format with zeros (either by the
flag or by specifying a precision), and the benign effect (i.e., none)
conversions, as well as other
nonsensical combinations such as
are not standard; such combinations
assume an infinitely long string,
callers must be careful not to overflow the actual space;
this is often impossible to assure.
For safety, programmers should use the
Unfortunately, this interface is not portable.