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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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sscanf(3) Library Functions Manual sscanf(3)
sscanf, vsscanf - input string format conversion
Standard C library (libc, -lc)
#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
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).
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 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).) 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 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));
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.
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.
EILSEQ Input byte sequence does not form a valid character.
EINVAL Not enough arguments; or format is NULL.
ENOMEM Out of memory.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼────────────────┤
│ sscanf(), vsscanf() │ Thread safety │ MT-Safe locale │
└───────────────────────────────┴───────────────┴────────────────┘
C11, POSIX.1-2008.
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.
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.).
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.
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.
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
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Linux man-pages 6.15 2025-05-17 sscanf(3)
Pages that refer to this page: curs_scanw(3x), scanf(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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