sscanf
Section: C Library Functions (3)
Updated: 2023-02-05
Index
Return to Main Contents
NAME
sscanf, vsscanf - input string format conversion
LIBRARY
Standard C library
(libc, -lc)
SYNOPSIS
#include <stdio.h>
int sscanf(const char *restrict str,
const char *restrict format, ...);
#include <stdarg.h>
int vsscanf(const char *restrict str,
const char *restrict format, va_list ap);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
vsscanf():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
DESCRIPTION
The
sscanf()
family of functions scans input according to
format
as described below.
This format may contain
conversion specifications;
the results from such conversions, if any,
are stored in the locations pointed to by the
pointer
arguments that follow
format.
Each
pointer
argument must be of a type that is appropriate for the value returned
by the corresponding conversion specification.
If the number of conversion specifications in
format
exceeds the number of
pointer
arguments, the results are undefined.
If the number of
pointer
arguments exceeds the number of conversion specifications, then the excess
pointer
arguments are evaluated, but are otherwise ignored.
sscanf()
These functions
read their input from the string pointed to by
str.
The
vsscanf()
function is analogous to
vsprintf(3).
The
format
string consists of a sequence of
directives
which describe how to process the sequence of input characters.
If processing of a directive fails, no further input is read, and
sscanf()
returns.
A "failure" can be either of the following:
input failure,
meaning that input characters were unavailable, or
matching failure,
meaning that the input was inappropriate (see below).
A directive is one of the following:
- •
-
A sequence of white-space characters (space, tab, newline, etc.; see
isspace(3)).
This directive matches any amount of white space,
including none, in the input.
- •
-
An ordinary character (i.e., one other than white space or '%').
This character must exactly match the next character of input.
- •
-
A conversion specification,
which commences with a '%' (percent) character.
A sequence of characters from the input is converted according to
this specification, and the result is placed in the corresponding
pointer
argument.
If the next item of input does not match the conversion specification,
the conversion fails---this is a
matching failure.
Each
conversion specification
in
format
begins with either the character '%' or the character sequence
"%n$"
(see below for the distinction) followed by:
- •
-
An optional '*' assignment-suppression character:
sscanf()
reads input as directed by the conversion specification,
but discards the input.
No corresponding
pointer
argument is required, and this specification is not
included in the count of successful assignments returned by
scanf().
- •
-
For decimal conversions, an optional quote character (').
This specifies that the input number may include thousands'
separators as defined by the
LC_NUMERIC
category of the current locale.
(See
setlocale(3).)
The quote character may precede or follow the '*'
assignment-suppression character.
- •
-
An optional 'm' character.
This is used with string conversions
(%s,
%c,
%[),
and relieves the caller of the
need to allocate a corresponding buffer to hold the input: instead,
sscanf()
allocates a buffer of sufficient size,
and assigns the address of this buffer to the corresponding
pointer
argument, which should be a pointer to a
char *
variable (this variable does not need to be initialized before the call).
The caller should subsequently
free(3)
this buffer when it is no longer required.
- •
-
An optional decimal integer which specifies the
maximum field width.
Reading of characters stops either when this maximum is reached or
when a nonmatching character is found, whichever happens first.
Most conversions discard initial white space characters (the exceptions
are noted below),
and these discarded characters don't count toward the maximum field width.
String input conversions store a terminating null byte ('\0')
to mark the end of the input;
the maximum field width does not include this terminator.
- •
-
An optional
type modifier character.
For example, the
l
type modifier is used with integer conversions such as
%d
to specify that the corresponding
pointer
argument refers to a
long
rather than a pointer to an
int.
- •
-
A
conversion specifier
that specifies the type of input conversion to be performed.
The conversion specifications in
format
are of two forms, either beginning with '%' or beginning with
"%n$".
The two forms should not be mixed in the same
format
string, except that a string containing
"%n$"
specifications can include
%%
and
%*.
If
format
contains '%'
specifications, then these correspond in order with successive
pointer
arguments.
In the
"%n$"
form (which is specified in POSIX.1-2001, but not C99),
n
is a decimal integer that specifies that the converted input should
be placed in the location referred to by the
n-th
pointer
argument following
format.
Conversions
The following
type modifier characters
can appear in a conversion specification:
- h
-
Indicates that the conversion will be one of
d, i, o, u, x, X, or n
and the next pointer is a pointer to a
short
or
unsigned short
(rather than
int).
- hh
-
As for
h,
but the next pointer is a pointer to a
signed char
or
unsigned char.
- j
-
As for
h,
but the next pointer is a pointer to an
intmax_t
or a
uintmax_t.
This modifier was introduced in C99.
- l
-
Indicates either that the conversion will be one of
d, i, o, u, x, X, or n
and the next pointer is a pointer to a
long
or
unsigned long
(rather than
int),
or that the conversion will be one of
e, f, or g
and the next pointer is a pointer to
double
(rather than
float).
If used with
%c
or
%s,
the corresponding parameter is considered
as a pointer to a wide character or wide-character string respectively.
- ll
-
(ell-ell)
Indicates that the conversion will be one of
b,
d,
i,
o,
u,
x,
X,
or
n
and the next pointer is a pointer to a
long long
or
unsigned long long
(rather than
int).
- L
-
Indicates that the conversion will be either
e, f, or g
and the next pointer is a pointer to
long double
or
(as a GNU extension)
the conversion will be
d, i, o, u, or x
and the next pointer is a pointer to
long long.
- q
-
equivalent to
L.
This specifier does not exist in ANSI C.
- t
-
As for
h,
but the next pointer is a pointer to a
ptrdiff_t.
This modifier was introduced in C99.
- z
-
As for
h,
but the next pointer is a pointer to a
size_t.
This modifier was introduced in C99.
The following
conversion specifiers
are available:
- %
-
Matches a literal '%'.
That is,
%%
in the format string matches a
single input '%' character.
No conversion is done (but initial white space characters are discarded),
and assignment does not occur.
- d
-
Deprecated.
Matches an optionally signed decimal integer;
the next pointer must be a pointer to
int.
- i
-
Deprecated.
Matches an optionally signed integer; the next pointer must be a pointer to
int.
The integer is read in base 16 if it begins with
0x
or
0X,
in base 8 if it begins with
0,
and in base 10 otherwise.
Only characters that correspond to the base are used.
- o
-
Deprecated.
Matches an unsigned octal integer; the next pointer must be a pointer to
unsigned int.
- u
-
Deprecated.
Matches an unsigned decimal integer; the next pointer must be a
pointer to
unsigned int.
- x
-
Deprecated.
Matches an unsigned hexadecimal integer
(that may optionally begin with a prefix of
0x
or
0X,
which is discarded); the next pointer must
be a pointer to
unsigned int.
- X
-
Deprecated.
Equivalent to
x.
- f
-
Deprecated.
Matches an optionally signed floating-point number; the next pointer must
be a pointer to
float.
- e
-
Deprecated.
Equivalent to
f.
- g
-
Deprecated.
Equivalent to
f.
- E
-
Deprecated.
Equivalent to
f.
- a
-
Deprecated.
(C99) Equivalent to
f.
- s
-
Matches a sequence of non-white-space characters;
the next pointer must be a pointer to the initial element of a
character array that is long enough to hold the input sequence and
the terminating null byte ('\0'), which is added automatically.
The input string stops at white space or at the maximum field
width, whichever occurs first.
- c
-
Matches a sequence of characters whose length is specified by the
maximum field width
(default 1); the next pointer must be a pointer to
char,
and there must be enough room for all the characters
(no terminating null byte is added).
The usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in the format.
- [
-
Matches a nonempty sequence of characters from the specified set of
accepted characters; the next pointer must be a pointer to
char,
and there must be enough room for all the characters in the string, plus a
terminating null byte.
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
[
character and a close bracket
]
character.
The set
excludes
those characters if the first character after the open bracket is a
circumflex
(ha).
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
-
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,
[ha]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.
- p
-
Matches a pointer value (as printed by
%p
in
printf(3));
the next pointer must be a pointer to a pointer to
void.
- 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
int,
or variant whose size matches the (optionally)
supplied integer length modifier.
This is
not
a conversion and does
not
increase the count returned by the function.
The assignment can be suppressed with the
*
assignment-suppression character, but the effect on the
return value is undefined.
Therefore
%*n
conversions should not be used.
RETURN VALUE
On success, these functions return the number of input items
successfully matched and assigned;
this can be fewer than provided for,
or even zero, in the event of an early matching failure.
The value
EOF
is returned if the end of input is reached before either the first
successful conversion or a matching failure occurs.
EOF
is also returned if a read error occurs,
in which case the error indicator for the stream (see
ferror(3))
is set, and
errno
is set to indicate the error.
ERRORS
- EILSEQ
-
Input byte sequence does not form a valid character.
- EINVAL
-
Not enough arguments; or
format
is NULL.
- ENOMEM
-
Out of memory.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
Interface | Attribute | Value
|
sscanf(),
vsscanf()
| Thread safety | MT-Safe locale
|
STANDARDS
These functions conform to C99 and POSIX.1-2001.
The
q
specifier is the 4.4BSD notation for
long long,
while
ll
or the usage of
L
in integer conversions is the GNU notation.
The Linux version of these functions is based on the
GNU
libio
library.
Take a look at the
info
documentation of
GNU
libc (glibc-1.08)
for a more concise description.
NOTES
The 'a' assignment-allocation modifier
Originally, the GNU C library supported dynamic allocation for string inputs
(as a nonstandard extension) via the
a
character.
(This feature is present at least as far back as glibc 2.0.)
Thus, one could write the following to have
sscanf()
allocate a buffer for a string,
with a pointer to that buffer being returned in
*buf:
char *buf;
sscanf(str, "%as", &buf);
The use of the letter
a
for this purpose was problematic, since
a
is also specified by the ISO C standard as a synonym for
f
(floating-point input).
POSIX.1-2008 instead specifies the
m
modifier for assignment allocation (as documented in DESCRIPTION, above).
Note that the
a
modifier is not available if the program is compiled with
gcc~-std=c99
or
gcc~-D_ISOC99_SOURCE
(unless
_GNU_SOURCE
is also specified), in which case the
a
is interpreted as a specifier for floating-point numbers (see above).
Support for the
m
modifier was added to glibc 2.7,
and new programs should use that modifier instead of
a.
As well as being standardized by POSIX, the
m
modifier has the following further advantages over
the use of
a:
- •
-
It may also be applied to
%c
conversion specifiers (e.g.,
%3mc).
- •
-
It avoids ambiguity with respect to the
%a
floating-point conversion specifier (and is unaffected by
gcc~-std=c99
etc.).
BUGS
Numeric conversion specifiers
Use of the numeric conversion specifiers produces Undefined Behavior
for invalid input.
See
C11 7.21.6.2/10
This is a bug in the ISO C standard,
and not an inherent design issue with the API.
However,
current implementations are not safe from that bug,
so it is not recommended to use them.
Instead,
programs should use functions such as
strtol(3)
to parse numeric input.
This manual page deprecates use of the numeric conversion specifiers
until they are fixed by ISO C.
Nonstandard modifiers
These functions are fully C99 conformant, but provide the
additional modifiers
q
and
a
as well as an additional behavior of the
L
and
ll
modifiers.
The latter may be considered to be a bug, as it changes the
behavior of modifiers defined in C99.
Some combinations of the type modifiers and conversion
specifiers defined by C99 do not make sense
(e.g.,
%Ld).
While they may have a well-defined behavior on Linux, this need not
to be so on other architectures.
Therefore it usually is better to use
modifiers that are not defined by C99 at all, that is, use
q
instead of
L
in combination with
d, i, o, u, x, and X
conversions or
ll.
The usage of
q
is not the same as on 4.4BSD,
as it may be used in float conversions equivalently to
L.
EXAMPLES
To use the dynamic allocation conversion specifier, specify
m
as a length modifier (thus
%ms
or
%m[range]).
The caller must
free(3)
the returned string, as in the following example:
char *p;
int n;
errno = 0;
n = sscanf(str, "%m[a-z]", &p);
if (n == 1) {
printf("read: %s\n", p);
free(p);
} else if (errno != 0) {
perror("sscanf");
} else {
fprintf(stderr, "No matching characters\n");
}
As shown in the above example, it is necessary to call
free(3)
only if the
sscanf()
call successfully read a string.
SEE ALSO
getc(3),
printf(3),
setlocale(3),
strtod(3),
strtol(3),
strtoul(3)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- Conversions
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- STANDARDS
-
- NOTES
-
- The 'a' assignment-allocation modifier
-
- BUGS
-
- Numeric conversion specifiers
-
- Nonstandard modifiers
-
- EXAMPLES
-
- SEE ALSO
-
This document was created by
man2html,
using the manual pages.
Time: 12:14:53 GMT, April 27, 2024