FMTINSTALL(2) FMTINSTALL(2)
NAME
fmtinstall, dofmt, dorfmt, fmtprint, fmtvprint, fmtrune,
fmtstrcpy, fmtrunestrcpy, fmtfdinit, fmtfdflush, fmtstrinit,
fmtstrflush, runefmtstrinit, runefmtstrflush, errfmt -
support for user-defined print formats and output routines
SYNOPSIS
#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;
};
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,
FmtFlag = FmtComma << 1
};
int fmtfdinit(Fmt *f, int fd, char *buf, int nbuf);
int fmtfdflush(Fmt *f);
int fmtstrinit(Fmt *f);
char* fmtstrflush(Fmt *f);
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int runefmtstrinit(Fmt *f);
Rune* runefmtstrflush(Fmt *f);
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);
DESCRIPTION
The interface described here allows the construction of cus-
tom 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 rou-
tines 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 inter-
nally 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 rou-
tines described here. There are two main cases: output to
an open file descriptor and output to a string.
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FMTINSTALL(2) FMTINSTALL(2)
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 allo-
cated 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 charac-
ters.
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 preci-
sion, 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 conver-
sion routine. Fmtinstall returns 0 if the installation suc-
ceeds, -1 if it fails.
Fmtprint and fmtvprint may be called to help prepare output
in custom conversion routines. These functions will pre-
serve width, precision, and flags. Both functions return 0
for success and -1 for failure.
The functions dofmt and dorfmt are the underlying format-
ters; they use the existing contents of Fmt and should be
called only by sophisticated conversion routines. These
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FMTINSTALL(2) FMTINSTALL(2)
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. Con-
version routines that call these functions will work prop-
erly regardless of whether the output is bytes or runes.
8c(1) describes the C directive #pragma varargck that can be
used to provide type-checking for custom print verbs and
output routines.
EXAMPLES
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
void
fatal(char *fmt, ...)
{
Fmt f;
char buf[64];
va_list arg;
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;
#pragma varargck type "X" Complex
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);
}
main(...)
{
Complex x = (Complex){ 1.5, -2.3 };
fmtinstall('X', Xfmt);
print("x = %X\n", x);
}
SOURCE
/sys/src/libc/fmt
SEE ALSO
print(2), utf(6), errstr(2)
DIAGNOSTICS
These routines return negative numbers or nil for errors and
set errstr.
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