fmtinstall, dofmt, dorfmt, fmtprint, fmtvprint, fmtrune, fmtstrcpy, fmtrunestrcpy, fmtfdinit, fmtfdflush, fmtstrinit, fmtstrflush, runefmtstrinit, runefmtstrflush, errfmt – support for user-defined print formats and output routines


#include <u.h>
#include <libc.h>

typedef struct Fmt Fmt; struct Fmt{ uchar runes; /* output buffer is runes or chars? */ void *start; /* of buffer */ void *to; /* current place in the buffer */ void *stop; /* end of the buffer; overwritten if flush fails */ int (*flush)(Fmt*); /* called when to == stop */ void *farg; /* to make flush a closure */ int nfmt; /* num chars formatted so far */ va_list args; /* args passed to dofmt */ int r; /* % format Rune */ int width; int prec; ulong flags; };

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enum{ FmtWidth = 1, FmtLeft = FmtWidth << 1, FmtPrec = FmtLeft << 1, FmtSharp = FmtPrec << 1, FmtSpace = FmtSharp << 1, FmtSign = FmtSpace << 1, FmtZero = FmtSign << 1, FmtUnsigned = FmtZero << 1, FmtShort = FmtUnsigned << 1, FmtLong = FmtShort << 1, FmtVLong = FmtLong << 1, FmtComma = FmtVLong << 1,

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FmtFlag = FmtComma << 1 };

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int fmtfdinit(Fmt *f, int fd, char *buf, int nbuf);

int fmtfdflush(Fmt *f);

int fmtstrinit(Fmt *f);

char* fmtstrflush(Fmt *f);

int runefmtstrinit(Fmt *f);

Rune* runefmtstrflush(Fmt *f);

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int fmtinstall(int c, int (*fn)(Fmt*));

int dofmt(Fmt *f, char *fmt);

int dorfmt(Fmt*, Rune *fmt);

int fmtprint(Fmt *f, char *fmt, ...);

int fmtvprint(Fmt *f, char *fmt, va_list v);

int fmtrune(Fmt *f, int r);

int fmtstrcpy(Fmt *f, char *s);

int fmtrunestrcpy(Fmt *f, Rune *s);

int errfmt(Fmt *f);


The interface described here allows the construction of custom print(2) verbs and output routines. In essence, they provide access to the workings of the formatted print code.

The print(2) suite maintains its state with a data structure called Fmt. A typical call to print(2) or its relatives initializes a Fmt structure, passes it to subsidiary routines to process the output, and finishes by emitting any saved state recorded in the Fmt. The details of the Fmt are unimportant to outside users, except insofar as the general design influences the interface. The Fmt records whether the output is in runes or bytes, the verb being processed, its precision and width, and buffering parameters. Most important, it also records a flush routine that the library will call if a buffer overflows. When printing to a file descriptor, the flush routine will emit saved characters and reset the buffer; when printing to an allocated string, it will resize the string to receive more output. The flush routine is nil when printing to fixed-size buffers. User code need never provide a flush routine; this is done internally by the library.

Custom output routines

To write a custom output routine, such as an error handler that formats and prints custom error messages, the output sequence can be run from outside the library using the routines described here. There are two main cases: output to an open file descriptor and output to a string.

To write to a file descriptor, call fmtfdinit to initialize the local Fmt structure f, giving the file descriptor fd, the buffer buf, and its size nbuf. Then call fmtprint or fmtvprint to generate the output. These behave like fprint (see print(2)) or vfprint except that the characters are buffered until fmtfdflush is called and the return value is either 0 or –1. A typical example of this sequence appears in the Examples section.

The same basic sequence applies when outputting to an allocated string: call fmtstrinit to initialize the Fmt, then call fmtprint and fmtvprint to generate the output. Finally, fmtstrflush will return the allocated string, which should be freed after use. To output to a rune string, use runefmtstrinit and runefmtstrflush. Regardless of the output style or type, fmtprint or fmtvprint generates the characters.

Custom format verbs

Fmtinstall is used to install custom verbs and flags labeled by character c, which may be any non-zero Unicode character. Fn should be declared as


int	fn(Fmt*)

Fp->r is the flag or verb character to cause fn to be called. In fn, fp->width, fp->prec are the width and precision, and fp->flags the decoded flags for the verb (see print(2) for a description of these items). The standard flag values are: FmtSign (+), FmtLeft (-), FmtSpace (’ ’), FmtSharp (#), FmtComma (,), FmtLong (l), FmtShort (h), FmtUnsigned (u), and FmtVLong (ll). The flag bits FmtWidth and FmtPrec identify whether a width and precision were specified.

Fn is passed a pointer to the Fmt structure recording the state of the output. If fp->r is a verb (rather than a flag), fn should use Fmt->args to fetch its argument from the list, then format it, and return zero. If fp->r is a flag, fn should return one. All interpretation of fp->width, fp->prec, and fp->flags is left up to the conversion routine. Fmtinstall returns 0 if the installation succeeds, –1 if it fails.

Fmtprint and fmtvprint may be called to help prepare output in custom conversion routines. However, these functions clear the width, precision, and flags. Both functions return 0 for success and –1 for failure.

The functions dofmt and dorfmt are the underlying formatters; they use the existing contents of Fmt and should be called only by sophisticated conversion routines. These routines return the number of characters (bytes of UTF or runes) produced.

Some internal functions may be useful to format primitive types. They honor the width, precision and flags as described in print(2). Fmtrune formats a single character r. Fmtstrcpy formats a string s; fmtrunestrcpy formats a rune string s. Errfmt formats the system error string. All these routines return zero for successful execution. Conversion routines that call these functions will work properly regardless of whether the output is bytes or runes.

2c(1) describes the C directive #pragma varargck that can be used to provide type-checking for custom print verbs and output routines.


This function prints an error message with a variable number of arguments and then quits. Compared to the corresponding example in print(2), this version uses a smaller buffer, will never truncate the output message, but might generate multiple write system calls to produce its output.


#pragma	varargck	argpos	fatal	1

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void fatal(char *fmt, ...) { Fmt f; char buf[64]; va_list arg;

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fmtfdinit(&f, 1, buf, sizeof buf); fmtprint(&f, "fatal: "); va_start(arg, fmt); fmtvprint(&f, fmt, arg); va_end(arg); fmtprint(&f, "\n"); fmtfdflush(&f); exits("fatal error"); }

This example adds a verb to print complex numbers.


typedef struct {
	double	r, i;
} Complex;

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#pragma varargck type "X" Complex

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int Xfmt(Fmt *f) { Complex c;

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c = va_arg(f->args, Complex); return fmtprint(f, "(%g,%g)", c.r, c.i); }

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

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




print(2), utf(6), errstr(2)


These routines return negative numbers or nil for errors and set errstr.