sscanf(3) Library Functions Manual sscanf(3)
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 formatted input according to
format as described below. This format may contain conversion specifi-
cations; 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 charac-
ter '%' 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' separa-
tors as defined by the LC_NUMERIC category of the current lo-
cale. (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() allo-
cates 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 subse-
quently 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 hap-
pens first. Most conversions discard initial white space char-
acters (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 conver-
sion to be performed.
The conversion specifications in format are of two forms, either begin-
ning with '%' or beginning with "%n$". The two forms should not be
mixed in the same format string, except that a string containing "%n$"
specifications can include %% and %*. If format contains '%' specifi-
cations, then these correspond in order with successive pointer argu-
ments. In the "%n$" 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 argu-
ment following format.
Conversions
The following type modifier characters can appear in a conversion spec-
ification:
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 un-
signed 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 ini-
tial 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 be-
gin 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 char-
acters (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 de-
fined 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 spe-
cial; when placed between two other characters, it adds all in-
tervening characters to the set. To include a hyphen, make it
the last character before the final close bracket. For in-
stance, [^]0-9-] means the set "everything except close bracket,
zero through nine, and hyphen". The string ends with the ap-
pearance 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 * assign-
ment-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 success-
fully 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
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 attrib-
utes(7).
┌────────────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────┼───────────────┼────────────────┤
│ sscanf(), vsscanf() │ Thread safety │ MT-Safe locale │
└────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
C11, POSIX.1-2008.
HISTORY
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 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 follow-
ing 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 ]8;;https://port70.net/%7Ensz/c/c11/n1570.html#7.21.6.2p10\C11 7.21.6.2/10]8;;\. This is a bug in the ISO C
standard, and not an inherent design issue with the API. However, cur-
rent implementations are not safe from that bug, so it is not recom-
mended to use them. Instead, programs should use functions such as
strtol(3) to parse numeric input. Alternatively, mitigate it by speci-
fying 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 de-
fined 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 archi-
tectures. 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)
Linux man-pages 6.7 2023-12-09 sscanf(3)
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