libc/stdio/scanf.3

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.Dd January 31, 1995
.Dt SCANF 3
.Os
.Sh NAME
.Nm scanf ,
.Nm fscanf ,
.Nm sscanf ,
.Nm vscanf ,
.Nm vsscanf ,
.Nm vfscanf
.Nd input format conversion
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft int
.Fn scanf "const char *format" ...
.Ft int
.Fn fscanf "FILE *stream" "const char *format" ...
.Ft int
.Fn sscanf "const char *str" "const char *format" ...
.Fd #include <stdarg.h>
.Ft int
.Fn vscanf "const char *format" "va_list ap"
.Ft int
.Fn vsscanf "const char *str" "const char *format" "va_list ap"
.Ft int
.Fn vfscanf "FILE *stream" "const char *format" "va_list ap"
.Sh DESCRIPTION
The
.Fn scanf
family of functions read input according to the given
.Fa format
as described below.
This format may contain
.Dq conversion specifiers ;
the results of such conversions, if any, are stored through a set of pointer
arguments.
.Pp
The
.Fn scanf
function reads input from the standard input stream
.Em stdin ,
.Fn fscanf
reads input from the supplied stream pointer
.Fa stream ,
and
.Fn sscanf
reads its input from the character string pointed to by
.Fa str .
.Pp
The
.Fn vfscanf
function is analogous to
.Xr vfprintf 3
and reads input from the stream pointer
.Fa stream
using a variable argument list of pointers (see
.Xr stdarg 3 ) .
The
.Fn vscanf
function scans a variable argument list from the standard input and the
.Fn vsscanf
function scans it from a string; these are analogous to the
.Fn vprintf
and
.Fn vsprintf
functions, respectively.
.Pp
Each successive
.Em pointer
argument must correspond properly with each successive conversion specifier
(but see
.Dq suppression
below).
All conversions are introduced by the
.Cm %
(percent sign) character.
The
.Fa format
string may also contain other characters.
Whitespace (such as blanks, tabs, or newlines) in the
.Fa format
string match any amount of whitespace, including none, in the input.
Everything else matches only itself.
Scanning stops when an input character does not match such a format character.
Scanning also stops when an input conversion cannot be made (see below).
.Sh CONVERSIONS
Following the
.Cm %
character introducing a conversion there may be a number of
.Em flag
characters, as follows:
.Bl -tag -width indent
.It Cm *
Suppresses assignment.
The conversion that follows occurs as usual, but no pointer is used;
the result of the conversion is simply discarded.
.It Cm h
Indicates that the conversion will be one of
.Cm dioux
or
.Cm n
and the next pointer is a pointer to a
.Li short int
(rather than
.Li int ) .
.It Cm l
Indicates either that the conversion will be one of
.Cm dioux
or
.Cm n
and the next pointer is a pointer to a
.Li long int
(rather than
.Li int ) ,
or that the conversion will be one of
.Cm efg
and the next pointer is a pointer to
.Li double
(rather than
.Li float ) .
.It Cm q
Indicates that the conversion will be one of
.Cm dioux
or
.Cm n
and the next pointer is a pointer to a
.Li quad_t
(rather than
.Li int ) .
.It Cm L
Indicates that the conversion will be
.Cm efg
and the next pointer is a pointer to
.Li long double .
.El
.Pp
In addition to these flags, there may be an optional maximum field width,
expressed as a decimal integer, between the
.Cm %
and the conversion.
If no width is given,
a default of
.Dq infinity
is used (with one exception, below);
otherwise at most this many characters are scanned in processing the
conversion.
Before conversion begins, most conversions skip whitespace;
this whitespace is not counted against the field width.
.Pp
The following conversions are available:
.Bl -tag -width XXXX
.It Cm %
Matches a literal `%'.
That is,
.Ql %\&%
in the format string matches a single input
.Ql %
character.
No conversion is done, and assignment does not occur.
.It Cm d
Matches an optionally signed decimal integer;
the next pointer must be a pointer to
.Li int .
.It Cm D
Equivalent to
.Cm ld ;
this exists only for backwards compatibility.
.It Cm i
Matches an optionally signed integer;
the next pointer must be a pointer to
.Li int .
The integer is read in base 16 if it begins
with
.Ql 0x
or
.Ql 0X ,
in base 8 if it begins with
.Ql 0 ,
and in base 10 otherwise.
Only characters that correspond to the base are used.
.It Cm o
Matches an octal integer;
the next pointer must be a pointer to
.Li unsigned int .
.It Cm O
Equivalent to
.Cm lo ;
this exists for backwards compatibility.
.It Cm u
Matches an optionally signed decimal integer;
the next pointer must be a pointer to
.Li unsigned int .
.It Cm x
Matches an optionally signed hexadecimal integer;
the next pointer must be a pointer to
.Li unsigned int .
.It Cm X
Equivalent to
.Cm x .
.It Cm f
Matches an optionally signed floating-point number;
the next pointer must be a pointer to
.Li float .
.It Cm e
Equivalent to
.Cm f .
.It Cm g
Equivalent to
.Cm f .
.It Cm E
Equivalent to
.Cm f .
.It Cm G
Equivalent to
.Cm f .
.It Cm s
Matches a sequence of non-whitespace characters;
the next pointer must be a pointer to
.Li char ,
and the array must be large enough to accept all the sequence and the
terminating
.Tn NUL
character.
The input string stops at whitespace
or at the maximum field width, whichever occurs first.
.It Cm c
Matches a sequence of
.Li width
count characters (default 1);
the next pointer must be a pointer to
.Li char ,
and there must be enough room for all the characters
(no terminating
.Tn NUL
is added).
The usual skip of leading whitespace is suppressed.
To skip whitespace first, use an explicit space in the format.
.It Cm \&[
Matches a nonempty sequence of characters from the specified set
of accepted characters;
the next pointer must be a pointer to
.Li char ,
and there must be enough room for all the characters in the string,
plus a terminating
.Tn NUL
character.
The usual skip of leading whitespace is suppressed.
The string is to be made up of characters in
(or not in)
a particular set;
the set is defined by the characters between the open bracket
.Cm [
character
and a close bracket
.Cm ]
character.
The set excludes those characters
if the first character after the open bracket is a circumflex
.Cm ^ .
To include a close bracket in the set,
make it the first character after the open bracket
or the circumflex;
any other position will end the set.
The hyphen character
.Cm -
is also special;
when placed between two other characters,
it adds all intervening characters to the set.
To include a hyphen,
make it the last character before the final close bracket.
For instance,
.Ql [^]0-9-]
means the set `everything except close bracket, zero through nine,
and hyphen'.
The string ends with the appearance of a character not in the
(or, with a circumflex, in) set
or when the field width runs out.
.It Cm p
Matches a pointer value (as printed by
.Ql %p
in
.Xr printf 3 ) ;
the next pointer must be a pointer to
.Li void .
.It Cm n
Nothing is expected;
instead, the number of characters consumed thus far from the input
is stored through the next pointer,
which must be a pointer to
.Li int .
This is
.Em not
a conversion, although it can be suppressed with the
.Cm *
flag.
.El
.Pp
For backwards compatibility, other conversion characters (except
.Ql \e0 )
are taken as if they were
.Ql %d
or, if uppercase,
.Ql %ld ,
and a `conversion' of
.Ql %\e0
causes an immediate return of
.Dv EOF .
.Sh RETURN VALUES
These functions return the number of input items assigned, which can be fewer
than provided for, or even zero, in the event of a matching failure.
Zero indicates that, while there was input available, no conversions were
assigned; typically this is due to an invalid input character,
such as an alphabetic character for a
.Ql %d
conversion.
The value
.Dv EOF
is returned if an input failure occurs before any conversion such as an
end-of-file occurs.
If an error or end-of-file occurs after conversion has begun,
the number of conversions which were successfully completed is returned.
.Sh SEE ALSO
.Xr getc 3 ,
.Xr printf 3 ,
.Xr strtod 3 ,
.Xr strtol 3 ,
.Xr strtoul 3
.Sh STANDARDS
The functions
.Fn fscanf ,
.Fn scanf ,
and
.Fn sscanf
conform to
.St -ansiC .
.Sh HISTORY
The functions
.Fn vscanf ,
.Fn vsscanf ,
and
.Fn vfscanf
first appeared in
.Bx 4.3 Reno .
.Sh BUGS
All of the backwards compatibility formats will be removed in the future.
.Pp
Numerical strings are truncated to 512 characters; for example,
.Cm %f
and
.Cm %d
are implicitly
.Cm %512f
and
.Cm %512d .