Perl flush file buffer

This is not the same thing as flushing a buffered handle, since queuing up a bunch of buffered writes then doing a single flush has a much lower performance cost than writing to an unbuffered handle. It's painfully complicated, as evidenced by the fact that PostgreSQL has its own tool just to test file syncing methods. If you want to make sure it's really, truly, honestly on the hard drive in permanent storage you must flush it to the file system in your program.

Start abstracting your file-append operation into a sub that opens the file in append mode, writes to it, and closes it. The process observing the file will encounter a closed file that gets modified whenever the function is called. I'm trapped with the same dilemma atm where we have an issue with rotation of the log being written. If you've already output to the filehandle and need to ensure that it gets to the physical file, you need to use the IO::Handle flush and sync methods.

Why must I do this? Note that you need the exclamation marks at every begin of every line that contains Perl script. I had the same problem with the only difference of writing the same file over and over again with new content. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How do I flush a file in Perl? Ask Question. Asked 11 years ago. Active 4 months ago. Viewed 66k times. Peter Mortensen Mihran Hovsepyan Mihran Hovsepyan It works nicely. Add a comment. Active Oldest Votes.

Thank you. But this doesn't help me. It reads only after perl ends its job. Reprinted with permission. When my program terminates abnormally, the output is incomplete! My web server says I didn't send the right headers, but I'm sure I did! I'm trying to send data over the network, but nothing is sent!

When Perl wants to read data from the disk, or to write it to the network, or to read or write data anywhere, Perl has to make a request to the operating system and ask that the data be read or written. Don't confuse this with Perl's system function, which is totally unrelated. Making a system call is a relatively slow operation.

By contrast, a typical computer operation, such as assigning to a variable or adding two numbers, takes a fraction of a microsecond. If Perl made a system call for every read operation, that would be 10, system calls in all one extra to detect end-of-file , and if the file was on the disk, it would have to wait for the disk at least 10, times.

That would be very slow. For efficiency, Perl uses a trick called buffering. The first time you ask to read from the file, Perl has to make a system call.

Since system calls are expensive, it plans ahead. If you tried to read a little bit of text, you will probably want to read the rest later. The blocks on your disk are probably about 8K bytes, and your computer hardware is probably designed to transfer an entire block of data from the disk at once. So instead of asking the system for a little bit of text, Perl actually asks the system for an entire blockful, which hardly takes longer to get than a little bit would have.

Then it stores this block of data in a region of memory that is called a buffer , and gives you back the one line you asked for. The next time you ask for a line, Perl already has the line you want in memory in the 8K buffer. It doesn't have to make another system call; it just gives you the next line out of the buffer. Eventually you read up to the end of the buffer, and then Perl makes another system call to get another bufferful of data.

If lines typically have about 60 characters each, then the 10,line file has about , characters in it. Reading the file line-by-line with buffering only requires 75 system calls and 75 waits for the disk, instead of 10, For writing, Perl uses the same trick. When you write data to a file with print , the data doesn't normally go into the file right away.

Instead, it goes into a buffer. When the buffer is full, Perl writes all the data in the buffer at once. This is called flushing the buffer. But the buffering can sometimes surprise you. Suppose there are 1, items, and the program will have to think about each one for two minutes. The program will take about 35 hours to complete, and you'd like to be able to peek at the log file to see how it is doing.

You start up the program, wait ten minutes, and peek at the log filebut there's nothing there. What happened? Is the program stuck? Is it taking five times as long to think about the items as you thought it would? No, the program is not stuck, or even slow. The problem is that the print s to the log file are being buffered. The program has thought about the first five items, and it wrote the log messages for them, but the writes went into the buffer, and Perl isn't going to make a system call to send the buffer to the disk until the buffer is full.

The buffer is probably 8K bytes, and the log messages are about 28 bytes each, so what's going to happen is that you won't see anything in the log file until about ten hours from now, when the first messages will appear all at once. After that, you won't get any more messages for another ten hours.

That's a problem, because it's not what you wanted. Here you don't really care about the efficiency gain of buffered writes. On the plus side of buffering, you're saving about four seconds over the hour lifetime of the program.

Other programs can still see these changes, they seem "written", but they'll be lost if the OS or machine crashes. Disk-level write-back buffering of writes. The OS thinks these are written to disk, but the disk is actually just storing them in volatile memory on the drive.

If the OS crashes the data won't be lost, but if power fails it might be unless the disk can write it out first.

This is a big problem with cheap consumer SSDs. If you're writing via pipes there's also the pipe buffer to consider. This is not the same thing as flushing a buffered handle, since queuing up a bunch of buffered writes then doing a single flush has a much lower performance cost than writing to an unbuffered handle.

It's painfully complicated, as evidenced by the fact that PostgreSQL has its own tool just to test file syncing methods. If you want to make sure it's really, truly, honestly on the hard drive in permanent storage you must flush it to the file system in your program.

Start abstracting your file-append operation into a sub that opens the file in append mode, writes to it, and closes it. The process observing the file will encounter a closed file that gets modified whenever the function is called. I'm trapped with the same dilemma atm where we have an issue with rotation of the log being written.

If you've already output to the filehandle and need to ensure that it gets to the physical file, you need to use the IO::Handle flush and sync methods. Why must I do this? Note that you need the exclamation marks at every begin of every line that contains Perl script. I had the same problem with the only difference of writing the same file over and over again with new content.

Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How do I flush a file in Perl? Ask Question. Asked 11 years ago. Active 4 months ago. Viewed 66k times. Peter Mortensen Mihran Hovsepyan Mihran Hovsepyan