vscanf(3) — Linux manual page


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

NAME         top

       scanf,  fscanf,  sscanf, vscanf, vsscanf, vfscanf - input format con‐

SYNOPSIS         top

       #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         top

       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

       ·      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

       ·      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.

       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 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.

       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

       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

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 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         top

       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         top

       For an explanation of the terms used in this section, see

       │Interface            Attribute     Value          │
       │scanf(), fscanf(),   │ Thread safety │ MT-Safe locale │
       │sscanf(), vscanf(),  │               │                │
       │vsscanf(), vfscanf() │               │                │

CONFORMING TO         top

       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         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 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         top

       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         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 = scanf("%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 scanf() call successfully read a string.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

GNU                              2020-08-13                         SCANF(3)

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