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   vscanf    ( 3 )

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Примечание (Note)

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.).