OPEN_MEMSTREAM(3)         (2020-06-09)          OPEN_MEMSTREAM(3)

          open_memstream, open_wmemstream -  open a dynamic memory
          buffer stream

          #include <stdio.h>

          FILE *open_memstream(char **ptr, size_t *sizeloc);

          #include <wchar.h>

          FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);

     Feature Test Macro Requirements for glibc (see

          open_memstream(), open_wmemstream():
              Since glibc 2.10:
                  _POSIX_C_SOURCE >= 200809L
              Before glibc 2.10:

          The open_memstream() function opens a stream for writing  to
          a  memory  buffer.   The  function dynamically allocates the
          buffer, and the buffer automatically grows as needed.   Ini-
          tially,  the  buffer  has a size of zero.  After closing the
          stream, the caller should free(3) this buffer.

          The locations pointed to by ptr  and  sizeloc  are  used  to
          report,  respectively,  the current location and the size of
          the buffer.  The locations referred to by these pointers are
          updated each time the stream is flushed (fflush(3)) and when
          the stream is closed (fclose(3)).  These values remain valid
          only as long as the caller performs no further output on the
          stream.  If further output is  performed,  then  the  stream
          must again be flushed before trying to access these values.

          A null byte is maintained at the end of  the  buffer.   This
          byte is not included in the size value stored at sizeloc.

          The stream maintains the notion of a current position, which
          is  initially  zero  (the  start of the buffer).  Each write
          operation  implicitly  adjusts  the  buffer  position.   The
          stream's  buffer  position  can  be  explicitly changed with
          fseek(3) or fseeko(3).  Moving the buffer position past  the
          end  of the data already written fills the intervening space
          with null characters.

          The open_wmemstream() is similar  to  open_memstream(),  but

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     OPEN_MEMSTREAM(3)         (2020-06-09)          OPEN_MEMSTREAM(3)

          operates on wide characters instead of bytes.

          Upon    successful    completion,    open_memstream()    and
          open_wmemstream() return a FILE pointer.  Otherwise, NULL is
          returned and errno is set to indicate the error.

          open_memstream()  was  already  available  in  glibc  1.0.x.
          open_wmemstream() is available since glibc 2.4.

          For an explanation of the terms used in  this  section,  see
          attributes(7).     allbox;    lb    lb    lb    l    l    l.
          Interface Attribute Value T{ open_memstream(),
          open_wmemstream T}   Thread safety  MT-Safe

          POSIX.1-2008.   These  functions  are   not   specified   in
          POSIX.1-2001, and are not widely available on other systems.

          There is no file descriptor associated with the file  stream
          returned  by these functions (i.e., fileno(3) will return an
          error if called on the returned stream).

          In glibc before version 2.7,  seeking  past  the  end  of  a
          stream  created  by  open_memstream()  does  not enlarge the
          buffer; instead the fseek(3) call fails, returning -1.

          See fmemopen(3).

          fmemopen(3), fopen(3), setbuf(3)

          This page is part of release 5.10  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

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