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

       #include <stdarg.h>

       int vscanf(const char *format, va_list ap);
       int vsscanf(const char *str, const char *format, va_list ap);
       int vfscanf(FILE *stream, const char *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 int
              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 int or
              unsigned short int (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 int or
              unsigned long int (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.

       D      Equivalent to ld; this exists only for backward compatibility.
              (Note: thus only in libc4.  In libc5 and glibc the %D is
              silently ignored, causing old programs to fail mysteriously.)

       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; 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(),   │ Thread safety │ MT-Safe locale │
       │sscanf(), vscanf(),  │               │                │
       │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.

EXAMPLE
       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 5.03 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                               2019-03-06                          SCANF(3)