sscanf(3) — Linux manual page


sscanf(3)               Library Functions Manual               sscanf(3)

NAME         top

       sscanf, vsscanf - input string format conversion

LIBRARY         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

           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

DESCRIPTION         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).

       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

       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

       •      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

       The following type modifier characters can appear in a conversion

       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

       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

       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

       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

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

       The value EOF is returned if the end of input is reached before
       either the first successful conversion or a matching failure

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
       │ 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 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,

       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

       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.,

       •  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 
       ⟨⟩.  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

   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         top

       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);
           } else if (errno != 0) {
           } 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         top

       getc(3), printf(3), setlocale(3), strtod(3), strtol(3),

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                      sscanf(3)

Pages that refer to this page: curs_scanw(3x)pmextractvalue(3)scanf(3)stdio(3)