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

User · Answer

To immediately flush call fflush stdout  or fflush NULL   NULL means flush everything

User · Answer

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

User · Answer

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

User · 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   Edit  From Andy Ross s comment below  you can also disable buffering on stdout by using setbuf   setbuf stdout  NULL     or its secure version setvbuf as explained here  setvbuf stdout  NULL   IONBF  0

User · Answer

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

User · Answer

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  gt 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

User · Answer

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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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

User · Answer

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

User · Answer

Use setbuf stdout  NULL   to disable buffering

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

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