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FMEMOPEN(3P)               POSIX Programmer's Manual              FMEMOPEN(3P)
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  fol-
       lowing,  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  posi-
       tion  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 rela-
       tive  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 posi-
       tion. 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, depend-
       ing 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  buf-
              fer 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 Electri-
       cal 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 .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                         FMEMOPEN(3P)