fmemopen(3p) — Linux manual page


FMEMOPEN(3P)              POSIX Programmer's Manual             FMEMOPEN(3P)

PROLOG         top

       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         top

       fmemopen — open a memory buffer stream

SYNOPSIS         top

       #include <stdio.h>

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

DESCRIPTION         top

       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

       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

       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         top

       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         top

       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         top

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

           static char buffer[] = "foobar";

           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);

               return (0);

       This program produces the following output:

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



RATIONALE         top

       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.


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

SEE ALSO         top

       fdopen(3p), fopen(3p), freopen(3p), fseek(3p), malloc(3p),

       The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group                 2013                        FMEMOPEN(3P)

Pages that refer to this page: stdio.h(0p)fclose(3p)fdopen(3p)fflush(3p)fopen(3p)freopen(3p)open_memstream(3p)open_wmemstream(3p)