11-22-2005
Actually, fwrite and other stdio routines use a buffer that is private to the process. There is no general way to induce a program to empty its stdio buffers. And sync() never did affect the stdio buffers. On the other hand read() and write() go through the buffer cache or equivalent structure. The data is in core, but it is immediately available to other processes. sync() used to empty the buffer cache. Today sync() might just flush metadata.
As for the original question, why is this real time response needed? What is the consequence of, say, an extra 50 ms delay. How many writes would you expect during an average second? Are there frequent periods several seconds at a time without activity? What OS is in use? Would the 3rd party be willing to unbuffer the data? Can you specify the output file? What happens if the file already exists when the 3rd party program starts?
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LEARN ABOUT OPENDARWIN
tail
TAIL(1) BSD General Commands Manual TAIL(1)
NAME
tail -- display the last part of a file
SYNOPSIS
tail [-F | -f | -r] [-b number | -c number | -n number] [file ...]
DESCRIPTION
The tail utility displays the contents of file or, by default, its standard input, to the standard output.
The display begins at a byte, line or 512-byte block location in the input. Numbers having a leading plus (``+'') sign are relative to the
beginning of the input, for example, ``-c +2'' starts the display at the second byte of the input. Numbers having a leading minus (``-'')
sign or no explicit sign are relative to the end of the input, for example, ``-n 2'' displays the last two lines of the input. The default
starting location is ``-n 10'', or the last 10 lines of the input.
The options are as follows:
-b number
The location is number 512-byte blocks.
-c number
The location is number bytes.
-f The -f option causes tail to not stop when end of file is reached, but rather to wait for additional data to be appended to the
input. The -f option is ignored if the standard input is a pipe, but not if it is a FIFO.
-F The -F option implies the -f option, but tail will also check to see if the file being followed has been renamed or rotated. The
file is closed and reopened when tail detects that the filename being read from has a new inode number. The -F option is ignored if
reading from standard input rather than a file.
-n number
The location is number lines.
-r The -r option causes the input to be displayed in reverse order, by line. Additionally, this option changes the meaning of the -b,
-c and -n options. When the -r option is specified, these options specify the number of bytes, lines or 512-byte blocks to display,
instead of the bytes, lines or blocks from the beginning or end of the input from which to begin the display. The default for the -r
option is to display all of the input.
If more than a single file is specified, each file is preceded by a header consisting of the string ``==> XXX <=='' where ``XXX'' is the name
of the file.
DIAGNOSTICS
The tail utility exits 0 on success, and >0 if an error occurs.
SEE ALSO
cat(1), head(1), sed(1)
STANDARDS
The tail utility is expected to be a superset of the IEEE Std 1003.2-1992 (``POSIX.2'') specification. In particular, the -F, -b and -r
options are extensions to that standard.
The historic command line syntax of tail is supported by this implementation. The only difference between this implementation and historic
versions of tail, once the command line syntax translation has been done, is that the -b, -c and -n options modify the -r option, i.e. ``-r
-c 4'' displays the last 4 characters of the last line of the input, while the historic tail (using the historic syntax ``-4cr'') would
ignore the -c option and display the last 4 lines of the input.
HISTORY
A tail command appeared in PWB UNIX.
BSD
June 6, 1993 BSD