scanf

SCANF(3)                    Linux Programmer's Manual                   SCANF(3)



NAME
       scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conversion

SYNOPSIS
       #include <stdio.h>

       int scanf(const char *restrict format, ...);
       int fscanf(FILE *restrict stream,
                  const char *restrict format, ...);
       int sscanf(const char *restrict str,
                  const char *restrict format, ...);

       #include <stdarg.h>

       int vscanf(const char *restrict format, va_list ap);
       int vfscanf(FILE *restrict stream,
                  const char *restrict format, va_list ap);
       int vsscanf(const char *restrict str,
                  const char *restrict format, va_list ap);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       vscanf(), vsscanf(), vfscanf():
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

DESCRIPTION
       The scanf() 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.

       The scanf() function reads input from the standard input stream stdin,
       fscanf() reads input from the stream pointer stream, and sscanf() reads
       its input from the character string pointed to by str.

       The vfscanf() function is analogous to vfprintf(3) and reads input from
       the stream pointer stream using a variable argument list of pointers (see
       stdarg(3).  The vscanf() function scans a variable argument list from the
       standard input and the vsscanf() function scans it from a string; these
       are analogous to the vprintf(3) and vsprintf(3) functions respectively.

       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 scanf() 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: scanf() 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, scanf() 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).  Specifying two l characters is equivalent to L.  If used
              with %c or %s, the corresponding parameter is considered as a
              pointer to a wide character or wide-character string respectively.

       L      Indicates that the conversion will be either e, f, or g and the
              next pointer is a pointer to long double or 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.  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
       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.  EOF is
       also returned if a read error occurs, in which case the error indicator
       for the stream (see ferror(3)) is set, and errno is set to indicate the
       error.

ERRORS
       EAGAIN The file descriptor underlying stream is marked nonblocking, and
              the read operation would block.

       EBADF  The file descriptor underlying stream is invalid, or not open for
              reading.

       EILSEQ Input byte sequence does not form a valid character.

       EINTR  The read operation was interrupted by a signal; see signal(7).

       EINVAL Not enough arguments; or format is NULL.

       ENOMEM Out of memory.

       ERANGE The result of an integer conversion would exceed the size that can
              be stored in the corresponding integer type.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────────────────┬───────────────┬────────────────┐
       │Interface                              Attribute     Value          │
       ├───────────────────────────────────────┼───────────────┼────────────────┤
       │scanf(), fscanf(), sscanf(), vscanf(), │ Thread safety │ MT-Safe locale │
       │vsscanf(), vfscanf()                   │               │                │
       └───────────────────────────────────────┴───────────────┴────────────────┘

CONFORMING TO
       The functions fscanf(), scanf(), and sscanf() conform to C89 and C99 and
       POSIX.1-2001.  These standards do not specify the ERANGE error.

       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 scanf() allocate a buffer for an input string, with a
       pointer to that buffer being returned in *buf:

           char *buf;
           scanf("%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 starting with version 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
       All functions are fully C89 conformant, but provide the additional
       specifiers q and a as well as an additional behavior of the L and l
       specifiers.  The latter may be considered to be a bug, as it changes the
       behavior of specifiers defined in C89.

       Some combinations of the type modifiers and conversion specifiers defined
       by ANSI C 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
       ANSI C 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 = scanf("%m[a-z]", &p);
           if (n == 1) {
               printf("read: %s\n", p);
               free(p);
           } else if (errno != 0) {
               perror("scanf");
           } else {
               fprintf(stderr, "No matching characters\n");
           }

       As shown in the above example, it is necessary to call free(3) only if
       the scanf() call successfully read a string.

SEE ALSO
       getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)

COLOPHON
       This page is part of release 5.11 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



GNU                                2021-03-22                           SCANF(3)