libc/stdio/scanf.3

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.Dd September 5, 2023
.Dt SCANF 3
.Os
.Sh NAME
.Nm scanf ,
.Nm fscanf ,
.Nm sscanf ,
.Nm vscanf ,
.Nm vsscanf ,
.Nm vfscanf
.Nd input format conversion
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn scanf "const char * restrict format" ...
.Ft int
.Fn fscanf "FILE * restrict stream" "const char * restrict format" ...
.Ft int
.Fn sscanf "const char * restrict str" "const char * restrict format" ...
.In stdarg.h
.Ft int
.Fn vscanf "const char * restrict format" "va_list ap"
.Ft int
.Fn vsscanf "const char * restrict str" "const char * restrict format" "va_list ap"
.Ft int
.Fn vfscanf "FILE * restrict stream" "const char * restrict format" "va_list ap"
.Sh DESCRIPTION
The
.Fn scanf
family of functions scans input according to a
.Fa format
as described below.
This format may contain
.Em conversion specifiers ;
the results from such conversions, if any,
are stored through the
.Em pointer
arguments.
The
.Fn scanf
function
reads input from the standard input stream
.Dv stdin ,
.Fn fscanf
reads input from the stream pointer
.Fa stream ,
and
.Fn sscanf
reads its input from the character string pointed to by
.Fa str .
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.
Each successive
.Em pointer
argument must correspond properly with
each successive conversion specifier
(but see the
.Cm *
conversion below).
All conversions are introduced by the
.Cm %
(percent sign) character.
The
.Fa format
string
may also contain other characters.
White space (such as blanks, tabs, or newlines) in the
.Fa format
string match any amount of white space, 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 ".Cm l No (ell)"
.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 hh
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt char
(rather than
.Vt int ) .
.It Cm h
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt "short int"
(rather than
.Vt int ) .
.It Cm l No (ell)
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt "long int"
(rather than
.Vt int ) ,
that the conversion will be one of
.Cm a , e , f ,
or
.Cm g
and the next pointer is a pointer to
.Vt double
(rather than
.Vt float ) ,
or that the conversion will be one of
.Cm c ,
.Cm s
or
.Cm \&[
and the next pointer is a pointer to an array of
.Vt wchar_t
(rather than
.Vt char ) .
.It Cm ll No (ell ell)
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt "long long int"
(rather than
.Vt int ) .
.It Cm L
Indicates that the conversion will be one of
.Cm a , e , f ,
or
.Cm g
and the next pointer is a pointer to
.Vt "long double" .
.It Cm j
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt intmax_t
(rather than
.Vt int ) .
.It Cm t
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt ptrdiff_t
(rather than
.Vt int ) .
.It Cm w Ns Ar N
.Po
where
.Ar N
is 8, 16, 32, or 64
.Pc
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt intN_t
(rather than
.Vt int ) .
.It Cm wf Ns Ar N
.Po
where
.Ar N
is 8, 16, 32, or 64
.Pc
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt int_fastN_t
(rather than
.Vt int ) .
.It Cm z
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt size_t
(rather than
.Vt int ) .
.It Cm q
(deprecated.)
Indicates that the conversion will be one of
.Cm bdioux
or
.Cm n
and the next pointer is a pointer to a
.Vt "long long int"
(rather than
.Vt int ) .
.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 bytes are scanned
in processing the conversion.
In the case of the
.Cm lc ,
.Cm ls
and
.Cm l[
conversions, the field width specifies the maximum number
of multibyte characters that will be scanned.
Before conversion begins,
most conversions skip white space;
this white space is not counted against the field width.
.Pp
The following conversions are available:
.Bl -tag -width XXXX
.It Cm %
Matches a literal
.Ql % .
That is,
.Dq Li %%
in the format string
matches a single input
.Ql %
character.
No conversion is done, and assignment does not occur.
.It Cm b , B
Matches an optionally signed binary integer;
the next pointer must be a pointer to
.Vt "unsigned int" .
.It Cm d
Matches an optionally signed decimal integer;
the next pointer must be a pointer to
.Vt int .
.It Cm i
Matches an optionally signed integer;
the next pointer must be a pointer to
.Vt int .
The integer is read
in base 2 if it begins with
.Ql 0b
or
.Ql 0B ,
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
.Vt "unsigned int" .
.It Cm u
Matches an optionally signed decimal integer;
the next pointer must be a pointer to
.Vt "unsigned int" .
.It Cm x , X
Matches an optionally signed hexadecimal integer;
the next pointer must be a pointer to
.Vt "unsigned int" .
.It Cm a , A , e , E , f , F , g , G
Matches a floating-point number in the style of
.Xr strtod 3 .
The next pointer must be a pointer to
.Vt float
(unless
.Cm l
or
.Cm L
is specified.)
.It Cm s
Matches a sequence of non-white-space characters;
the next pointer must be a pointer to
.Vt char ,
and the array must be large enough to accept all the sequence and the
terminating
.Dv NUL
character.
The input string stops at white space
or at the maximum field width, whichever occurs first.
.Pp
If an
.Cm l
qualifier is present, the next pointer must be a pointer to
.Vt wchar_t ,
into which the input will be placed after conversion by
.Xr mbrtowc 3 .
.It Cm S
The same as
.Cm ls .
.It Cm c
Matches a sequence of
.Em width
count
characters (default 1);
the next pointer must be a pointer to
.Vt char ,
and there must be enough room for all the characters
(no terminating
.Dv NUL
is added).
The usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in the format.
.Pp
If an
.Cm l
qualifier is present, the next pointer must be a pointer to
.Vt wchar_t ,
into which the input will be placed after conversion by
.Xr mbrtowc 3 .
.It Cm C
The same as
.Cm lc .
.It Cm \&[
Matches a nonempty sequence of characters from the specified set
of accepted characters;
the next pointer must be a pointer to
.Vt char ,
and there must be enough room for all the characters in the string,
plus a terminating
.Dv NUL
character.
The usual skip of leading white space 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
.Em 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
.Dq "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.
.Pp
If an
.Cm l
qualifier is present, the next pointer must be a pointer to
.Vt wchar_t ,
into which the input will be placed after conversion by
.Xr mbrtowc 3 .
.It Cm p
Matches a pointer value (as printed by
.Ql %p
in
.Xr printf 3 ) ;
the next pointer must be a pointer to
.Vt 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
.Vt int .
This is
.Em not
a conversion, although it can be suppressed with the
.Cm *
flag.
.El
.Pp
The decimal point
character is defined in the program's locale (category
.Dv LC_NUMERIC ) .
.Pp
For backwards compatibility, a
.Dq 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 mbrtowc 3 ,
.Xr printf 3 ,
.Xr strtod 3 ,
.Xr strtol 3 ,
.Xr strtoul 3 ,
.Xr wscanf 3
.Sh STANDARDS
The functions
.Fn fscanf ,
.Fn scanf ,
.Fn sscanf ,
.Fn vfscanf ,
.Fn vscanf
and
.Fn vsscanf
conform to
.St -isoC-99 .
.Sh HISTORY
The functions
.Fn scanf ,
.Fn fscanf ,
and
.Fn sscanf
first appeared in
.At v7 ,
and
.Fn vscanf ,
.Fn vsscanf ,
and
.Fn vfscanf
in
.Bx 4.3 Reno .
.Sh BUGS
Earlier implementations of
.Nm
treated
.Cm \&%D , \&%E , \&%F , \&%O
and
.Cm \&%X
as their lowercase equivalents with an
.Cm l
modifier.
In addition,
.Nm
treated an unknown conversion character as
.Cm \&%d
or
.Cm \&%D ,
depending on its case.
This functionality has been removed.
.Pp
Numerical strings are truncated to 512 characters; for example,
.Cm %f
and
.Cm %d
are implicitly
.Cm %512f
and
.Cm %512d .
.Pp
The
.Cm %n$
modifiers for positional arguments are not implemented.
.Pp
The
.Nm
family of functions do not correctly handle multibyte characters in the
.Fa format
argument.