# fmemopen

## 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:
    
            *  Zero, if _buf_ is a null pointer
    
            *  The position of the first null byte in the buffer, if one is
               found
    
            *  The value of the _size_ argument, if _buf_ is not a null pointer
               and 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:
    
           **EMFILE**{STREAM_MAX} streams are currently open in the calling
                  process.
    
           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.
    
           **EINVAL** The _size_ argument specifies a buffer size of zero and the
                  implementation does not support this.
    
           **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 version of this standard may mandate specific behavior
           when the _mode_ argument includes **'b'**.
    
           A future version of this standard may require support of zero-
           length buffer streams explicitly.
    


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