.\" Copyright (c) 1990 The Regents of the University of California.
.\" This code is derived from software contributed to Berkeley by
.\" %sccs.include.redist.man%
.\" @(#)printf.3 6.9 (Berkeley) %G%
printf, fprintf, sprintf, snprintf, vprintf, vfprintf,
vsprintf, vsnprintf \- formatted output conversion
.B fprintf(stream, format
.B snprintf(str, size, format
.\" .B char *smprintf(format
.B vfprintf(stream, format, ap)
.B vsprintf(str, format, ap)
.B vsnprintf(str, size, format, ap)
.\" .B char *vsmprintf(format, ap)
family of functions produces output according to a
write the standard output stream
write to the character string
.\" dynamically allocate a new string with
Each function converts, formats, and prints the arguments following the
(or accessed via the variable-length argument facilities of
.\" all of these functions return
the number of characters printed
(not including the trailing `\e0' 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 `\e0');
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 is a string comprised of two types of objects:
plain characters, which are simply copied to the output stream,
and conversion specifications,
each of which causes conversion and printing of the next successive
These arguments must correspond properly (after type promotion)
with the conversion specifier.
Each conversion is introduced by the
(percent sign) character.
The rest of the conversion includes, in the following order,
Zero or more of the following flags:
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
A zero `0' 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 minus sign (`-') specifying left adjustment of the converted value
conversions, 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, specifying that a blank should be left before a positive number
produced by a signed conversion
a `+' character specifying that a sign always be placed before a
number produced by a signed conversion.
A `+' overrides a space if both are used.
An optional digit string specifying a field width.
If the converted value has fewer characters than the field width, it will
be padded on the left (or right, if the left-adjustment flag is used) to
An optional precision, in the form of a period (`.') followed by an
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 corresponds 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
by the \s-2VAX\s0 and Tahoe compilers).
A character which indicates the type of conversion to be applied.
A field width or precision may be an asterisk (`*') instead of a
argument supplies the value.
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 characters and their meanings are:
(or appropriate variant) argument is converted to signed decimal
notation respectively. The letters
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
`[\fB\-\fR]d\fB.\fRddd\fBe\fR\(+-dd' where there is one digit before the
decimal point and the number after is equal to the precision specification
If the precision is missing, 6 digits are given; if the precision is
explicitly zero, no decimal point 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
`[\fB\-\fR]ddd.ddd' where the number of digits after the decimal point
is equal to the precision.
If the precision is missing, 6 digits are given; if the precision is
explicitly 0, no digits and no decimal point are printed.
If a decimal point appears, at least one digit appears before it.
argument is printed 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 printed.
argument is taken to be a string (character pointer).
Characters from the string are printed until a null character is reached,
or until the number of characters indicated by the precision have been
printed, whichever occurs first; if a precision is given, no null character
pointer argument is printed in hexadecimal (as if by `%#x' or `%#lx').
The number of characters written so far is stored into the
(or variant) pointer argument.
No argument is converted.
Prints a `%'; no argument is converted.
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 it.
Similarly, padding takes place only if the specified field width exceeds
To print a date and time in the form `Sunday, July 3, 10:02',
are pointers to null-terminated strings:
printf("%s, %s %d, %02d:%.2d", weekday, month, day, hour, min);
printf("pi = %.5f", 4*atan(1.0));
To allocate a 128 byte string and print into it:
char *p, *malloc(), *fmt;
if ((p = malloc(128)) == NULL)
fmt = va_arg(ap, char *);
(void) vsprintf(p, fmt, ap);
The conversion formats `%D', `%O', and `%U' are not standard and
are provided only for backward compatibility.
The effect of padding the `%p' format with zeros (either by the `0'
flag or by specifying a precision), and the benign effect (i.e., none)
of the `#' flag on `%n' and `%p' conversions, as well as other
nonsensical combinations such as `%Ld', 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.
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