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Man Pages

Manual Reference Pages  -  PRINT (3)


print, fprint, sprint, snprint, seprint, smprint, runesprint, runesnprint, runeseprint, runesmprint, vfprint, vsnprint, vseprint, vsmprint, runevsnprint, runevseprint, runevsmprint - print formatted output


See Also


#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(Rune *format, va_list v)


Print writes text to the standard output. Fprint writes to the named output file descriptor: a buffered form is described in bio(3). Sprint places text followed by the NUL 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 transmitted (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 indicated 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(3).

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. They return a rune count rather than a byte count.

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.

The numeric verbs d, o, b, x, and X format their arguments in decimal, octal, binary, hexadecimal, and upper case hexadecimal. Each interprets the flags 0, h, hh, l, u, +, -, ,, and # to mean pad with zeros, short, byte, long, 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 two l flags are given, then the argument is interpreted as a vlong (usually an 8-byte, sometimes a 4-byte integer). If precision is not omitted, the number is padded on the left with zeros until at least precision digits appear. If precision is explicitly 0, and the number is 0, no digits are generated, and alternate formatting does not apply. Then, if alternate format is specified, for o conversion, the number is preceded 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 0, L +, -, and # to mean pad with zeros, long double argument, 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 maximum 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 conversion 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 decimal 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(7)); the runes are converted to UTF before output.

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 pointer value. At the moment, it is a synonym for x, but that will change if pointers and integers are different sizes.

The r verb takes no arguments; it copies the error string returned by a call to errstr(3).

Custom verbs may be installed using fmtinstall(3).


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);
      write(2, buf, out-buf);
      exits("fatal error"); }




fmtinstall(3), fprintf(3), utf(7)


Routines that write to a file descriptor or call malloc set errstr.


The formatting is close to that specified for ANSI fprintf(3); the main difference is that b and r are 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.

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