PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

       fmemopen — open a memory buffer stream

SYNOPSIS

       #include <stdio.h>

       FILE *fmemopen(void *restrict buf, size_t size,
           const char *restrict mode);

DESCRIPTION

       The fmemopen() function shall associate the buffer given by the buf and size arguments with a stream. The
       buf argument shall be either a null pointer or point to a buffer that is at least size bytes long.

       The mode argument points to a string. If the string is one of the following, the stream shall  be  opened
       in the indicated mode. Otherwise, the behavior is undefined.

       r       Open the stream for reading.

       w       Open the stream for writing.

       a       Append; open the stream for writing at the first null byte.

       r+      Open the stream for update (reading and writing).

       w+      Open the stream for update (reading and writing). Truncate the buffer contents.

       a+      Append;  open  the  stream for update (reading and writing); the initial position is at the first
               null byte.

       Implementations shall accept all mode strings allowed by fopen(), but the use of the character 'b'  shall
       produce  implementation-defined  results,  where  the resulting FILE * need not behave the same as if 'b'
       were omitted.

       If a null pointer is specified as the buf argument, fmemopen() shall allocate size bytes of memory as  if
       by a call to malloc().  This buffer shall be automatically freed when the stream is closed.  Because this
       feature is only useful when the stream is opened for updating (because there is no way to get  a  pointer
       to the buffer) the fmemopen() call may fail if the mode argument does not include a '+'.

       The  stream  shall  maintain  a  current  position in the buffer. This position shall be initially set to
       either the beginning of the buffer (for r and w modes) or to the first null byte in  the  buffer  (for  a
       modes).  If no null byte is found in append mode, the initial position shall be set to one byte after the
       end of the buffer.

       If buf is a null pointer, the initial position shall always be set to the beginning of the buffer.

       The stream shall also maintain the size of the current buffer contents; use of fseek() or fseeko() on the
       stream  with  SEEK_END  shall seek relative to this size. For modes r and r+ the size shall be set to the
       value given by the size argument. For modes w and w+ the initial size shall be zero and for modes  a  and
       a+ the initial size shall be either the position of the first null byte in the buffer or the value of the
       size argument if no null byte is found.

       A read operation on the stream shall not advance the current buffer position beyond  the  current  buffer
       size.  Reaching  the  buffer  size  in a read operation shall count as ``end-of-file''. Null bytes in the
       buffer shall have no special meaning for reads. The read operation shall  start  at  the  current  buffer
       position of the stream.

       A write operation shall start either at the current position of the stream (if mode has not specified 'a'
       as the first character) or at the current size of the stream (if mode had 'a' as the first character). If
       the  current  position at the end of the write is larger than the current buffer size, the current buffer
       size shall be set to the current position. A write operation on the stream shall not advance the  current
       buffer size beyond the size given in the size argument.

       When a stream open for writing is flushed or closed, a null byte shall be written at the current position
       or at the end of the buffer, depending on the size of the contents.  If  a  stream  open  for  update  is
       flushed  or  closed and the last write has advanced the current buffer size, a null byte shall be written
       at the end of the buffer if it fits.

       An attempt to seek a memory buffer stream to a negative position or to a position larger than the  buffer
       size given in the size argument shall fail.

RETURN VALUE

       Upon  successful  completion,  fmemopen()  shall  return  a pointer to the object controlling the stream.
       Otherwise, a null pointer shall be returned, and errno shall be set to indicate the error.

ERRORS

       The fmemopen() function shall fail if:

       EINVAL The size argument specifies a buffer size of zero.

       The fmemopen() function may fail if:

       EINVAL The value of the mode argument is not valid.

       EINVAL The buf argument is a null pointer and the mode argument does not include a '+' character.

       ENOMEM The buf argument is a null pointer and the allocation of a buffer of length size has failed.

       EMFILE {FOPEN_MAX} streams are currently open in the calling process.

       The following sections are informative.

EXAMPLES

           #include <stdio.h>
           #include <string.h>

           static char buffer[] = "foobar";

           int
           main (void)
           {
               int ch;
               FILE *stream;

               stream = fmemopen(buffer, strlen (buffer), "r");
               if (stream == NULL)
                   /* handle error */;

               while ((ch = fgetc(stream)) != EOF)
                   printf("Got %c\n", ch);

               fclose(stream);
               return (0);
           }

       This program produces the following output:

           Got f
           Got o
           Got o
           Got b
           Got a
           Got r

APPLICATION USAGE

       None.

RATIONALE

       This interface has been introduced to eliminate many of the errors encountered  in  the  construction  of
       strings, notably overflowing of strings. This interface prevents overflow.

FUTURE DIRECTIONS

       A future revision of this standard may mandate specific behavior when the mode argument includes 'b'.

SEE ALSO

       fdopen(), fopen(), freopen(), fseek(), malloc(), open_memstream()

       The Base Definitions volume of POSIX.1‐2008, <stdio.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications  Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In  the  event
       of  any  discrepancy between this version and the original IEEE and The Open Group Standard, the original
       IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
       http://www.unix.org/online.html .

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       https://www.kernel.org/doc/man-pages/reporting_bugs.html .