PRINT(2) PRINT(2)
NAME
print, fprint, sprint, snprint, seprint, smprint,
runesprint, runesnprint, runeseprint, runesmprint, vfprint,
vsnprint, vseprint, vsmprint, runevsnprint, runevseprint,
runevsmprint - print formatted output
SYNOPSIS
#include <u.h>
#include <libc.h>
int print(char *format, ...)
int fprint(int fd, char *format, ...)
int sprint(char *s, char *format, ...)
int snprint(char *s, int len, char *format, ...)
char* seprint(char *s, char *e, char *format, ...)
char* smprint(char *format, ...)
int runesprint(Rune *s, char *format, ...)
int runesnprint(Rune *s, int len, char *format, ...)
Rune* runeseprint(Rune *s, Rune *e, char *format, ...)
Rune* runesmprint(char *format, ...)
int vfprint(int fd, char *format, va_list v)
int vsnprint(char *s, int len, char *format, va_list v)
char* vseprint(char *s, char *e, char *format, va_list v)
char* vsmprint(char *format, va_list v)
int runevsnprint(Rune *s, int len, char *format, va_list
v)
Rune* runevseprint(Rune *s, Rune *e, char *format, va_list
v)
Rune* runevsmprint(char *format, va_list v)
DESCRIPTION
Print writes text to the standard output. Fprint writes to
the named output file descriptor; a buffered form is
described in bio(2). Sprint places text followed by the NUL
Page 1 Plan 9 (printed 12/21/24)
PRINT(2) PRINT(2)
character (\0) in consecutive bytes starting at s; it is the
user's responsibility to ensure that enough storage is
available. Each function returns the number of bytes trans-
mitted (not including the NUL in the case of sprint), or a
negative value if an output error was encountered.
Snprint is like sprint, but will not place more than len
bytes in s. Its result is always NUL-terminated and holds
the maximal number of complete UTF-8 characters that can
fit. Seprint is like snprint, except that the end is indi-
cated by a pointer e rather than a count and the return
value points to the terminating NUL of the resulting string.
Smprint is like sprint, except that it prints into and
returns a string of the required length, which is allocated
by malloc(2).
The routines runesprint, runesnprint, runeseprint, and
runesmprint are the same as sprint, snprint, seprint and
smprint except that their output is rune strings instead of
byte strings.
Finally, the routines vfprint, vsnprint, vseprint, vsmprint,
runevsnprint, runevseprint, and runevsmprint are like their
v-less relatives except they take as arguments a va_list
parameter, so they can be called within a variadic function.
The Example section shows a representative usage.
Each of these functions converts, formats, and prints its
trailing arguments under control of a format string. The
format contains two types of objects: plain characters,
which are simply copied to the output stream, and conversion
specifications, each of which results in fetching of zero or
more arguments. The results are undefined if there are
arguments of the wrong type or too few arguments for the
format. If the format is exhausted while arguments remain,
the excess is ignored.
Each conversion specification has the following format:
% [flags] verb
The verb is a single character and each flag is a single
character or a (decimal) numeric string. Up to two numeric
strings may be used; the first is called width, the second
precision. A period can be used to separate them, and if the
period is present then width and precision are taken to be
zero if missing, otherwise they are `omitted'. Either or
both of the numbers may be replaced with the character *,
meaning that the actual number will be obtained from the
argument list as an integer. The flags and numbers are
arguments to the verb described below.
Page 2 Plan 9 (printed 12/21/24)
PRINT(2) PRINT(2)
The numeric verbs d, o, b, x, and X format their arguments
in decimal, octal, binary, hexadecimal, and upper case hex-
adecimal. Each interprets the flags 0, h, hh, l, ll, u, +,
-, ,, and # to mean pad with zeros, short, byte, long,
vlong, unsigned, always print a sign, left justified, commas
every three digits, and alternate format. Also, a space
character in the flag position is like +, but prints a space
instead of a plus sign for non-negative values. If neither
short nor long is specified, then the argument is an int.
If unsigned is specified, then the argument is interpreted
as a positive number and no sign is output. If precision is
not omitted, the number is padded on the left with zeros
until at least precision digits appear. Then, if alternate
format is specified, for o conversion, the number is pre-
ceded by a 0 if it doesn't already begin with one; for x
conversion, the number is preceded by 0x; for X conversion,
the number is preceded by 0X. Finally, if width is not
omitted, the number is padded on the left (or right, if left
justification is specified) with enough blanks to make the
field at least width characters long.
The floating point verbs f, e, E, g, and G take a double
argument. Each interprets the flags +, -, and # to mean
always print a sign, left justified, and alternate format.
Width is the minimum field width and, if the converted value
takes up less than width characters, it is padded on the
left (or right, if `left justified') with spaces. Precision
is the number of digits that are converted after the decimal
place for e, E, and f conversions, and precision is the max-
imum number of significant digits for g and G conversions.
The f verb produces output of the form [-]digits[.digits].
E conversion appends an exponent E[-]digits, and e conver-
sion appends an exponent e[-]digits. The g verb will output
the argument in either e or f with the goal of producing the
smallest output. Also, trailing zeros are omitted from the
fraction part of the output, and a trailing decimal point
appears only if it is followed by a digit. The G verb is
similar, but uses E format instead of e. When alternate
format is specified, the result will always contain a deci-
mal point, and for g and G conversions, trailing zeros are
not removed.
The s verb copies a nul-terminated string (pointer to char)
to the output. The number of characters copied (n) is the
minimum of the size of the string and precision. These n
characters are justified within a field of width characters
as described above. If a precision is given, it is safe for
the string not to be nul-terminated as long as it is at
least precision characters (not bytes!) long. The S verb is
similar, but it interprets its pointer as an array of runes
(see utf(6)); the runes are converted to UTF before output.
Page 3 Plan 9 (printed 12/21/24)
PRINT(2) PRINT(2)
The c verb copies a single char (promoted to int) justified
within a field of width characters as described above. The
C verb is similar, but works on runes.
The p verb formats a single pointer or pointer-sized integer
(uintptr, see intro(2)) in hexadecimal.
The r verb takes no arguments; it copies the error string
returned by a call to errstr(2).
Custom verbs may be installed using fmtinstall(2).
EXAMPLE
This function prints an error message with a variable number
of arguments and then quits.
void fatal(char *msg, ...)
{
char buf[1024], *out;
va_list arg;
out = seprint(buf, buf+sizeof(buf), "Fatal error: ");
va_start(arg, msg);
out = vseprint(out, buf+sizeof(buf), msg, arg);
va_end(arg);
write(2, buf, out-buf);
exits("fatal error");
}
SOURCE
/sys/src/libc/fmt
SEE ALSO
fmtinstall(2), fprintf(2), utf(6), errstr(2)
DIAGNOSTICS
Routines that write to a file descriptor or call malloc set
errstr.
BUGS
The formatting is close to that specified for ANSI
fprintf(2); the main difference is that b is not in ANSI and
u is a flag here instead of a verb. Also, and distinctly
not a bug, print and friends generate UTF rather than ASCII.
There is no runeprint, runefprint, etc. because runes are
byte-order dependent and should not be written directly to a
file; use the UTF output of print or fprint instead. Also,
sprint is deprecated for safety reasons; use snprint,
seprint, or smprint instead. Safety also precludes the
existence of runesprint.
Page 4 Plan 9 (printed 12/21/24)