sscanf(3) - Linux manual page (original) (raw)
sscanf(3) Library Functions Manual sscanf(3)
NAME top
sscanf, vsscanf - input string format conversionLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 >= 200112LDESCRIPTION top
The **sscanf**() family of functions scans formatted 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)](../man3/vsprintf.3.html).
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)](../man3/isspace.3.html)). 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 **sscanf**().
• 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)](../man3/setlocale.3.html).) 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)](../man3/free.3.html)
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 _intmaxt_
or a _uintmaxt_. 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 _ptrdifft_.
This modifier was introduced in C99.
**z** As for **h**, but the next pointer is a pointer to a _sizet_.
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** Matches an optionally signed decimal integer; the next
pointer must be a pointer to _int_.
**i** 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** Matches an unsigned octal integer; the next pointer must be
a pointer to _unsigned int_.
**u** Matches an unsigned decimal integer; the next pointer must
be a pointer to _unsigned int_.
**x** 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** Equivalent to **x**.
**f** Matches an optionally signed floating-point number; the
next pointer must be a pointer to _float_.
**e** Equivalent to **f**.
**g** Equivalent to **f**.
**E** Equivalent to **f**.
**a** (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 (**^**). 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, **[^]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)](../man3/printf.3.html));
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 top
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.ERRORS top
**EILSEQ** Input byte sequence does not form a valid character.
**EINVAL** Not enough arguments; or _format_ is NULL.
**ENOMEM** Out of memory.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌───────────────────────────────┬───────────────┬────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├───────────────────────────────┼───────────────┼────────────────┤
│ **sscanf**(), **vsscanf**() │ Thread safety │ MT-Safe locale │
└───────────────────────────────┴───────────────┴────────────────┘STANDARDS top
C11, POSIX.1-2008.HISTORY top
C89, 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**(1) documentation of _GNU libc_
_(glibc-1.08)_ for a more concise description.NOTES top
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 -DISOC99SOURCE_ (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 top
Numeric conversion specifiers Use of the numeric conversion specifiers produces undefined behavior for invalid input. See C11 7.21.6.2/10 ⟨https://port70.net/%7Ensz/c/c11/n1570.html#7.21.6.2p10⟩. 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. Alternatively, mitigate it by specifying a maximum field width.
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
systems. 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 top
To use the dynamic allocation conversion specifier, specify **m** as a
length modifier (thus **%ms** or **%m[**_range_**]**). The caller must [free(3)](../man3/free.3.html)
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)](../man3/free.3.html)
only if the **sscanf**() call successfully read a string.SEE ALSO top
[getc(3)](../man3/getc.3.html), [printf(3)](../man3/printf.3.html), [setlocale(3)](../man3/setlocale.3.html), [strtod(3)](../man3/strtod.3.html), [strtol(3)](../man3/strtol.3.html), [strtoul(3)](../man3/strtoul.3.html)COLOPHON top
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Pages that refer to this page:curs_scanw(3x), pmextractvalue(3), scanf(3), stdio(3)