Why does printf not flush after the call unless a newline is in the format string?

Question

Why does printf not flush after the call unless a newline is in the format string? Is this POSIX behavior? How might I have printf immediately flush every time?

Maitilde

did you investigated whether this happens with any file or only with terminals? that would sound to be a clever terminal feature not to output uncompleted line from a background program, though i expect it wouldn't apply to the foreground program. – Pancho

Under Cygwin bash I'm seeing this same misbehaviour even if a newline is in the format string. This problem is new to Windows 7; the same source code worked fine on Windows XP. MS cmd.exe flushes as expected. The fix setvbuf(stdout, (char*)NULL, _IONBF, 0) works around the problem, but surely should not have been necessary. I'm using MSVC++ 2008 Express. ~~~ – Shugart

To clarify the title of the question: printf(..) does not do any flushing itself, it's the buffering of stdout that may flush when seeing a newline (if it's line-buffered). It would react the same way to putchar('\n');, so printf(..) is not special in this regard. This is in contrast with cout << endl;, the documentation of which prominently mentions flushing. The documentation of printf doesn't mention flushing at all. – Titania

writing (/flushing) is potentially an expensive operation, it's probably buffered for performance reasons. – Password

@EvgeniSergeev: Is there a consensus that the question has incorrectly diagnosed the problem, and that flushing happens when there is a newline in the output? (putting one in the format string is one way, but not the only way, of getting one in the output). – Eryneryngo

Answer 1

The stdout stream is line buffered by default, so will only display what's in the buffer after it reaches a newline (or when it's told to). You have a few options to print immediately:

• Print to stderrinstead using fprintf (stderr is unbuffered by default):

  fprintf(stderr, "I will be printed immediately");
  

• Flush stdout whenever you need it to using fflush:

  printf("Buffered, will be flushed");
  fflush(stdout); // Will now print everything in the stdout buffer
  

• Disable buffering on stdout by using setbuf:

  setbuf(stdout, NULL);
  

• Or use the more flexible setvbuf:

  setvbuf(stdout, NULL, _IONBF, 0); 
  

Bartholomeo

Or, to disable buffering entirely: setbuf(stdout, NULL); – Smoulder

Also, just wanted to mention that apparently in UNIX a newline will typically only flush the buffer if stdout is a terminal. If the output is being redirected to a file, a newline won't flush. – Malynda

I feel that I should add: I've just been testing this theory, and I am finding that using setlinebuf() on a stream which is not directed to a terminal is flushing at the end of each line. – Bulbul

@bean Well of course it does, that is the whole point of it and it doesn't require a newline with setlinebuf depending on the argument you provide. You can do one of the following: unbuffered, block buffered, and line buffered. Unbuffered will not require a newline and information will appear on the destination file or terminal as soon as it is available. – Cothurnus

"As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device" -- see this question: stackoverflow.com/questions/5229096/… – Scandian

Is there a way to do this persistently in like an IDE option? – Incredible

Just tested, on Mac OS X 10.10.5 (Yosemite) setbuf(stdout, NULL); does NOT have the desired effect. – Truscott

@RuddZwolinski If this is going to be a good canon answer of "why isn't it printing" it seems important to mention the terminal/file distinction as per "Does printf always flush the buffer on encountering a newline?" directly in this highly upvoted answer, vs people needing to read the comments... – Catricecatrina

Also it should explain, why streams are buffered at all in in the first place. That is important to know in order to decide whether to "unbuffer". – Carry

the standard says that stdout is initially fully buffered if the output device can be determined to be a non-interactive one. stackoverflow.com/questions/5229096/… – Dorado

Worth mentioning also exit function from the standard library will flush the buffer. – Unison

Do I need fflush(stdout); if I am using fprintf() to print on a file? – Appellation

"so will only display what's in the buffer after it reaches a newline (or when it's told to)" --> other cases exists: when the buffer is full. – Recipe

"The stdout stream is line buffered by default" C has "As initially opened, ... standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.". As I read, the default for stdout with an interactive device is unbuffered or line buffered. Certainly line buffered is more common default, but not a C specified default. – Recipe

How is setvbuf() more secure than setbuf()? The calls to those two functions you give are strictly equivalent. – Shinto

Answer 2

No, it's not POSIX behaviour, it's ISO C behaviour (well, it is POSIX behaviour but only insofar as they conform to ISO C).

Standard output is line buffered if it can be detected to refer to an interactive device, otherwise it's fully buffered. So there are situations where printf won't flush, even if it gets a newline to send out, such as:

myprog >myfile.txt

This makes sense for efficiency since, if you're interacting with a user, they probably want to see every line. If you're sending the output to a file, it's most likely that there's not a user at the other end (though not impossible, they could be tailing the file). Now you could argue that the user wants to see every character but there are two problems with that.

The first is that it's not very efficient. The second is that the original ANSI C 89 mandate was to primarily codify existing behaviour, rather than invent new behaviour, and those design decisions were made long before ANSI started the process. Even ISO C nowadays treads very carefully when changing existing rules in the standards.

As to how to deal with that, if you fflush (stdout) after every output call that you want to see immediately, that will solve the problem.

Alternatively, you can use setvbuf before operating on stdout, to set it to unbuffered and you won't have to worry about adding all those fflush lines to your code:

setvbuf (stdout, NULL, _IONBF, BUFSIZ);

Just keep in mind that may affect performance quite a bit if you are sending the output to a file. Also keep in mind that support for this is implementation-defined, not guaranteed by the standard.

ISO C99 section 7.19.3/3 is the relevant bit:

When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block.

When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled.

When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.

Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment.

Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions.

Chesser

I just came across a scenario where even there is a '\n', printf() doesn't flush. It was overcome by adding a fflush(stdout), as you mentioned here. But I am wondering the reason why '\n' failed to flush the buffer in printf(). – Chymotrypsin

@QiangXu, standard output is line buffered only in the case where it can be definitively determined to refer to an interactive device. So, for example, if you redirect output with myprog >/tmp/tmpfile, that is fully buffered rather than line buffered. From memory, the determination as to whether your standard output is interactive is left to the implementation. – Chesser

furthermore on Windows calling setvbuf(...., _IOLBF) will not work as _IOLBF is the same as _IOFBF there: msdn.microsoft.com/en-us/library/86cebhfs.aspx – Cacilie

Answer 3

It's probably like that because of efficiency and because if you have multiple programs writing to a single TTY, this way you don't get characters on a line interlaced. So if program A and B are outputting, you'll usually get:

program A output
program B output
program B output
program A output
program B output

This stinks, but it's better than

proprogrgraam m AB  ououtputputt
prproogrgram amB A  ououtputtput
program B output

Note that it isn't even guaranteed to flush on a newline, so you should flush explicitly if flushing matters to you.

Algo

Funny, I think this is the only answer which actually answers the "why?" - applying an appropriate amount of guessing. The others explain that it is buffered (which does seem less necessary towards an OP who demonstrates awareness of that by using the term "flushing") and how to avoid/control it. Admittedly there is enough detail there to provide helpful insights in answers. But only this one discusses why and has that angle of answering all to itself. Cheers. – Hildahildagard

Note the usually in this answer - there are plenty of scenarios (colors, long lines, stderr) where the output will be interlaced. – Fungi

Answer 4

To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).

Bathsheeb

Keep in mind fflush(NULL); is usually a very bad idea. It will kill performance if you have many files open, especially in a multi-threaded environment where you'll fight with everything for locks. – Diffusive

Answer 5

stdout is buffered, so will only output after a newline is printed.

To get immediate output, either:

1. Print to stderr.
2. Make stdout unbuffered.

Mystify

Or fflush(stdout). – Platte

"so will only output after a newline is printed." Not only this but at least 4 other cases. buffer full, write to stderr (this answer mentions later), fflush(stdout), fflush(NULL). – Recipe

"stdout is buffered" is not true, as hinted at by bullet point 2. By default, stdout is block buffered when it is a regular file and line buffered when it is a tty. Perhaps simply add "by default" to the phrase "stdout is buffered". – Cetology

Answer 6

Note: Microsoft runtime libraries do not support line buffering, so printf("will print immediately to terminal"):

https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/setvbuf

Gem

Worse than printf going immediately to the terminal in the "normal" case is the fact that printf and fprintf get more coarsely buffered even in cases where their output is put to immediate use. Unless MS has fixed things, that makes it impossible for one program to capture stderr and stdout from another and identify in what sequence things were sent to each. – Mauri

no, it doesn't print that immediately to the terminal unless no buffering was set. By default full buffering is used – Neptunian

Answer 7

by default, stdout is line buffered, stderr is none buffered and file is completely buffered.

Taunyataupe

Answer 8

You can fprintf to stderr, which is unbuffered, instead. Or you can flush stdout when you want to. Or you can set stdout to unbuffered.

Cockcrow

Answer 9

Use setbuf(stdout, NULL); to disable buffering.

Hitch

Answer 10

There are generally 2 levels of buffering-

1. Kernel buffer Cache (makes read/write faster)

2. Buffering in I/O library (reduces no. of system calls)

Let's take example of fprintf and write().

When you call fprintf(), it doesn't wirte directly to the file. It first goes to stdio buffer in the program's memory. From there it is written to the kernel buffer cache by using write system call. So one way to skip I/O buffer is directly using write(). Other ways are by using setbuff(stream,NULL). This sets the buffering mode to no buffering and data is directly written to kernel buffer. To forcefully make the data to be shifted to kernel buffer, we can use "\n", which in case of default buffering mode of 'line buffering', will flush I/O buffer. Or we can use fflush(FILE *stream).

Now we are in kernel buffer. Kernel(/OS) wants to minimise disk access time and hence it reads/writes only blocks of disk. So when a read() is issued, which is a system call and can be invoked directly or through fscanf(), kernel reads the disk block from disk and stores it in a buffer. After that data is copied from here to user space.

Similarly that fprintf() data recieved from I/O buffer is written to the disk by the kernel. This makes read() write() faster.

Now to force the kernel to initiate a write(), after which data transfer is controlled by hardware controllers, there are also some ways. We can use O_SYNC or similar flags during write calls. Or we could use other functions like fsync(),fdatasync(),sync() to make the kernel initiate writes as soon as data is available in the kernel buffer.

Coryden