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 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.
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 (^). 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 |
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.
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.).
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.
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.
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)