man(1) Manual page archive

     PRINT(2)                                                 PRINT(2)

          print, fprint, sprint, snprint, seprint, fmtinstall,
          strconv, Strconv, numbconv, fltconv, doprint - print
          formatted output

          #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, ...)

          int   fmtinstall(int c, int (*f)(va_list*, Fconv*))

          void  strconv(char *s, Fconv *fp)

          void  Strconv(Rune *s, Fconv *fp)

          int   numbconv(va_list *arg, Fconv *fp)

          int   fltconv(va_list *arg, Fconv *fp)

          char* doprint(char *s, char *es, char *format, va_list arg)

          extern int printcol;

          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
          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.

     PRINT(2)                                                 PRINT(2)

          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 f1, the second f2.
          A period can be used to separate them, and if the period is
          present then f1 and f2 are taken to be zero if missing, oth-
          erwise 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, x, and X format their arguments in
          decimal, octal, hexadecimal, and upper case hexadecimal.
          Each interprets the flags h, l, u, +, -, and # to mean
          short, long, unsigned, always print a sign, left justified,
          and alternate format.  If neither short nor long is speci-
          fied, then the argument is an int.  If unsigned is speci-
          fied, 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 (a 4-byte or sometimes
          8-byte integer).  If f2 is not omitted, the number is padded
          on the left with zeros until at least f2 digits appear.
          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
          f1 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 f1 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.
          F1 is the minimum field width and, if the converted value
          takes up less than f1 characters, it is padded on the left
          (or right, if `left justified') with spaces.  F2 is the num-
          ber of digits that are converted after the decimal place for
          e, E, and f conversions, and f2 is the maximum number of

     PRINT(2)                                                 PRINT(2)

          significant digits for g and G conversions.  The f verb pro-
          duces 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 out-
          put.  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 string (pointer to char) to the output.
          The number of characters copied (n) is the minimum of the
          size of the string and f2. These n characters are justified
          within a field of f1 characters as described above.  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

          The c verb copies a single char (promoted to int) justified
          within a field of f1 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 ux, but that will change once pointers and inte-
          gers are different sizes.

          The r verb takes no arguments; it copies the error string
          returned by a call to errstr(2).

          Fmtinstall is used to install custom verbs and flags labeled
          by character c, which must have value less than 512.  Fn
          should be declared as

               int   fn(va_list *arg, Fconv *fp)

          Fp->chr is the flag or verb character to cause fn to be
          called.  In fn, fp->f1 and fp->f2 are the decoded flags in
          the conversion.  A missing fp->f1 is denoted by the value
          zero.  A missing fp->f2 is denoted by a negative number.
          Fp->f3 is the bitwise OR of all the flags seen since the
          most recent `%'.  The standard flags values are: 1 (+), 2
          (-), 4 (#), 8 (l), 16 (h), 32 (u), and 64 (ll).  Fn is
          passed a pointer arg to the argument list.  If fp->chr is a
          verb, fn should use va_arg to fetch its argument from the
          list, then format it, and return zero.  If fp->chr is a
          flag, fn should return a negative value: the negation of one
          of the above flag values, or some otherwise unused power of
          two.  All interpretation of fp->f1, fp->f2, and fp->f3 is
          left up to the conversion routine.  Fmtinstall returns 0 if
          the installation succeeds, -1 if it fails.

     PRINT(2)                                                 PRINT(2)

          Sprint, snprint, and seprint are re-entrant; they may be
          called to help prepare output in custom conversion routines.

          The function strconv formats a UTF string.  S is the string,
          fp has the same meaning as above.  The strconv routine
          interprets the `-' flag in fp->f3 as left-justification.
          The function Strconv (with a capital S) is like strconv, but
          its input is a rune string, which is converted to UTF on

          Printcol indicates the position of the next output charac-
          ter.  Tabs, backspaces and carriage returns are interpreted

          Numbconv and fltconv are used to implement the integer and
          floating verbs.  Their arguments are like those of the func-
          tion argument to fmtinstall. Both numbconv and fltconv use
          strconv to put their results into the current print buffer.

          One of strconv or Strconv must be called to produce output;
          no other routine puts characters in the output buffer.

          Doprint formats the argument list arg arg into the buffer
          starting at s, but it writes no characters after the address
          es. It returns a pointer to the NUL terminating the format-
          ted string.

          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 = doprint(buf, buf+sizeof(buf), "Fatal error: ");
                     va_start(arg, msg);
                     out = doprint(out, buf+sizeof(buf), msg, arg);
                     write(2, buf, out-buf);
                     exits("fatal error");

          This example adds a verb to print complex numbers.

               struct {
                     double      r, i;
               } Complex;


     PRINT(2)                                                 PRINT(2)

               Xconv(va_list *arg, Fconv *fp)
                     char str[50];
                     Complex c;

                     c = va_arg(*arg, Complex);
                     sprint(str, "(%g,%g)", c.r, c.i);
                     strconv(str, fp);
                     return 0;

                     Complex x = (Complex){ 1.5, -2.3 };

                     fmtinstall('X', Xconv);
                     print("x = %X\n", x);


          fprintf(2), utf(6), errstr(2)

          Print and fprint set errstr.

          The formatting is close to that specified for ANSI
          fprintf(2); the differences are:

               the - flag doesn't work

               u is a flag here instead of a verb

               there are no 0 or space flags here

               there are no P or n verbs here

          Also, and distinctly not a bug, print and friends generate
          UTF rather than ASCII.