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About c : Why-does-printf-not-flush-after-the-call-unless-a-newline-is-in-the-format-string

Question Detail

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?

Question Answer

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);

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

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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.
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To immediately flush call fflush(stdout) or fflush(NULL) (NULL means flush everything).
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Note: Microsoft runtime libraries do not support line buffering, so printf(“will print immediately to terminal”):

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setvbuf
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stdout is buffered, so will only output after a newline is printed.

To get immediate output, either:

Print to stderr.
Make stdout unbuffered.

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by default, stdout is line buffered, stderr is none buffered and file is completely buffered.
……………………………………………………
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.
……………………………………………………
Use setbuf(stdout, NULL); to disable buffering.
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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.

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