I'm reading Advanced Programming in the UNIX Environment, 3rd Edition and misunderstanding a section in it (page 145, Section 5.4 Buffering, Chapter 5).
Line buffering comes with two caveats. First, the size of the buffer that the
standard I/O library uses to collect each line is fixed, so I/O might take place if
we fill this buffer before writing a newline. Second, whenever input is
requested through the standard I/O library from either (a) an unbuffered stream or (b) a line-buffered stream (that requires data to be requested from the kernel),
all line-buffered output streams are flushed. The reason for the qualifier on (b)
is that the requested data may already be in the buffer, which doesn’t require
data to be read from the kernel. Obviously, any input from an unbuffered
stream, item (a), requires data to be obtained from the kernel.
I can't get the bold lines. My English isn't good. So, could you clarify it for me? Maybe in an easier way. Thanks.
The point behind the machinations described is to ensure that prompts appear before the system goes into a mode where it is waiting for input.
If an input stream is unbuffered, every time the standard I/O library needs data, it has to go to the kernel for some information. (That's the last sentence.) That's because the standard I/O library does not buffer any data, so when it needs more data, it has to read from the kernel. (I think that even an unbuffered stream might buffer one character of data, because it would need to read up to a space character, for example, to detect when it has reached the end of a %s format string; it has to put back (ungetc()) the extra character it read so that the next time it needs a character, there is the character it put back. But it never needs more than the one character of buffering.)
If an input stream is line buffered, there may already be some data in its input buffer, in which case it may not need to go to the kernel for more data. In that case, it might not flush anything. This can occur if the scanf() format requested "%s" and you typed hello world; it would read the whole line, but the first scan would stop after hello, and the next scanf() would not need to go to the kernel for the world word because it is already in the buffer.
However, if there isn't any data in the buffer, it has to ask the kernel to read the data, and it ensures that any line-buffered output streams are flushed so that if you write:
printf("Enter name: ");
if (scanf("%63s", name) != 1)
…handle error or EOF…
then the prompt (Enter name:) appears. However, if you'd previously typed hello world and previously read just hello, then the prompt wouldn't necessarily appear because the world was already waiting in the (line buffered) input stream.
This may explain the point.
Let's imagine that you have a pipe in your program and you use it for communication between different parts of your program (single thread program writing and reading from this single pipe).
If you write to the writing end of the pipe, say the letter 'A', and then call the read operation to read from the reading end of the pipe. You would expect that the letter 'A' is read. However, read operation is a system call to the kernel. To be able to return the letter 'A' it must be written to the kernel first. This means that the writing of 'A' must be flushed, otherwise it would stay in your local writing buffer and your program would be locked forever.
In consequence, before calling a read operation all write buffers are flushed. This is what the section (b) says.
The size of the buffer that the standard I/O library is using to collect each line is fixed.
with the help of the fgets function we are getting the line continuously, during that time it will read the content with the specified buffer size or up to newline.
Second, whenever input is requested through the standard I/O library, it can use an unbuffered stream or line-buffered stream.
unbuffered stream - It will not buffer the character, flush the character regularly.
line-buffered - It will store the character into the buffer and then flush when the operation is completed.
lets take without using \n we are going to print the content in printf statement, that time it will buffer all the content until we flush or printing with new line. Like that when the operation is completed the stream buffer is flushed internally.
(b) is that the requested data may already be in the buffer, which doesn't require data to be read from the kernel
In line oriented stream the requested buffer may already in the buffer because the data can be buffered, so we can't required data to read from the kernel once again.
(a) requires data to be obtained from the kernel.
Any input from unbuffered stream item, a data to be get from the kernel due to the unbuffered stream can't store anything in the buffer.
Related
I am going through "C PRIMER PLUS" and there is this topic about "OUTPUT FLUSHING".
Now it says:
printf() statements sends output to an intermediate storage called buffer.
Every now and then, the material in the buffer is sent to the screen. The
standard C rules for when output is sent from the buffer to the screen are
clear:
It is sent when the buffer gets full.
When a newline character is encountered.
When there is impending input.
(Sending the output from the buffer to the screen or file is called flushing
the buffer.)
Now, To verify the above statements. I wrote this simple program :
#include<stdio.h>
int main(int argc, char** argv) {
printf("Hello World");
return 0;
}
so, neither the printf() contains a new line, nor it has some impending input(for e.g. a scanf() statement or any other input statement). Then why does it print the contents on the output screen.
Let's suppose first condition validated to true. The buffer got full(Which can't happen at all).
Keeping that in mind, I truncated the statement inside printf() to
printf("Hi");
Still it prints the statement on the console.
So whats the deal here, All of the above conditions are false but still I'm getting the output on screen.
Can you elaborate please. It appears I'm making a mistake in understanding the concept. Any help is highly appreciated.
EDIT: As suggested by a very useful comment, that maybe the execution of exit() function after the end of program is causing all the buffers to flush, resulting in the output on the console. But then if we hold the screen before the execution of exit(). Like this,
#include<stdio.h>
int main(int argc, char** argv) {
printf("Hello World!");
getchar();
return 0;
}
It still outputs on the console.
Output buffering is an optimization technique. Writing data to some devices (hard disks f.e.) is an expensive operation; that's why the buffering appeared. In essence, it avoids writing data byte-by-byte (or char-by-char) and collects it in a buffer in order to write several KiB of data at once.
Being an optimization, output buffering must be transparent to the user (it is transparent even to the program). It must not affect the behaviour of the program; with or without buffering (or with different sizes of the buffer), the program must behave the same. This is what the rules you mentioned are for.
A buffer is just an area in memory where the data to be written is temporarily stored until enough data accumulates to make the actual writing process to the device efficient. Some devices (hard disk etc.) do not even allow writing (or reading) data in small pieces but only in blocks of some fixed size.
The rules of buffer flushing:
It is sent when the buffer gets full.
This is obvious. The buffer is full, its purpose was fulfilled, let's push the data forward to the device. Also, probably there is more data to come from the program, we need to make room for it.
When a newline character is encountered.
There are two types of devices: line-mode and block-mode. This rule applies only to the line-mode devices (the terminal, for example). It doesn't make much sense to flush the buffer on newlines when writing to disk. But it makes a lot of sense to do it when the program is writing to the terminal. In front of the terminal there is the user waiting impatiently for output. Don't let them wait too much.
But why output to terminal needs buffering? Writing on the terminal is not expensive. That's correct, when the terminal is physically located near the processor. Not also when the terminal and the processor are half the globe apart and the user runs the program through a remote connection.
When there is impending input.
It should read "when there is impeding input on the same device" to make it clear.
Reading is also buffered for the same reason as writing: efficiency. The reading code uses its own buffer. It fills the buffer when needed then scanf() and the other input-reading functions get their data from the input buffer.
When an input is about to happen on the same device, the buffer must be flushed (the data actually written to the device) in order to ensure consistency. The program has send some data to the output and now it expects to read back the same data; that's why the data must be flushed to the device in order for the reading code find it there and load it.
But why the buffers are flushed when the application exits?
Err... buffering is transparent, it must not affect the application behaviour. Your application has sent some data to the output. The data must be there (on the output device) when the application quits.
The buffers are also flushed when the associated files are closed, for the same reason. And this is what happens when the application exits: the cleanup code close all the open files (standard input and output are just files from the application point of view), closing forces flushing the buffers.
Part of the specification for exit() in the C standard (POSIX link given) is:
Next, all open streams with unwritten buffered data are flushed, all open streams are closed, …
So, when the program exits, pending output is flushed, regardless of newlines, etc. Similarly, when the file is closed (fclose()), pending output is written:
Any unwritten buffered data for the stream are delivered to the host environment to be written to the file; any unread buffered data are discarded.
And, of course, the fflush() function flushes the output.
The rules quoted in the question are not wholly accurate.
When the buffer is full — this is correct.
When a newline is encountered — this is not correct, though it often applies. If the output device is an 'interactive device', then line buffering is the default. However, if the output device is 'non-interactive' (disk file, a pipe, etc), then the output is not necessarily (or usually) line-buffered.
When there is impending input — this too is not correct, though it is commonly the way it works. Again, it depends on whether the input and output devices are 'interactive'.
The output buffering mode can be modified by calling setvbuf()
to set no buffering, line buffering or full buffering.
The standard says (§7.21.3):
¶3 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.
…
¶7 At program startup, three text streams are predefined and need not be opened explicitly — standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). 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.
Also, §5.1.2.3 Program execution says:
The input and output dynamics of interactive devices shall take place as specified in 7.21.3. The intent of these requirements is that unbuffered or line-buffered output appear as soon as possible, to ensure that prompting messages actually appear prior to a program waiting for input.
The strange behavior of printf, buffering can be explained with below simple C code. please read through entire thing execute and understand as the below is not obvious (bit tricky)
#include <stdio.h>
int main()
{
int a=0,b=0,c=0;
printf ("Enter two numbers");
while (1)
{
sleep (1000);
}
scanf("%d%d",&b,&c);
a=b+c;
printf("The sum is %d",a);
return 1;
}
EXPERIMENT #1:
Action: Compile and Run above code
Observations:
The expected output is
Enter two numbers
But this output is not seen
EXPERIMENT #2:
Action: Move Scanf statement above while loop.
#include <stdio.h>
int main()
{
int a=0,b=0,c=0;
printf ("Enter two numbers");
scanf("%d%d",&b,&c);
while (1)
{
sleep (1000);
}
a=b+c;
printf("The sum is %d",a);
return 1;
}
Observations: Now the output is printed (reason below in the end)(just by scanf position change)
EXPERIMENT #3:
Action: Now add \n to print statement as below
#include <stdio.h>
int main()
{
int a=0,b=0,c=0;
printf ("Enter two numbers\n");
while (1)
{
sleep (1000);
}
scanf("%d%d",&b,&c);
a=b+c;
printf("The sum is %d",a);
return 1;
}
Observation: The output Enter two numbers is seen (after adding \n)
EXPERIMENT #4:
Action: Now remove \n from the printf line, comment out while loop, scanf line, addition line, printf line for printing result
#include <stdio.h>
int main()
{
int a=0,b=0,c=0;
printf ("Enter two numbers");
// while (1)
// {
// sleep (1000);
// }
// scanf("%d%d",&b,&c);
// a=b+c;
// printf("The sum is %d",a);
return 1;
}
Observations: The line "Enter two numbers" is printed to screen.
ANSWER:
The reason behind the strange behavior is described in Richard Stevens book.
PRINTF PRINTS TO SCREEN WHEN
The job of printf is to write output to stdout buffer. kernel flushes output buffers when
kernel need to read something in from input buffer. (EXPERIMENT #2)
when it encounters newline (since stdout is by default set to
linebuffered)(EXPERIMENT #3)
after program exits (all output buffers are flushed) (EXPERIMENT #4)
By default stdout set to line buffering
so printf will not print as the line did not end.
if it is no buffered, all lines are output as is.
Full buffered then, only when buffer is full it is flushed.
Well I been doing a lot of searching on google and on here about how to flush the input stream properly. All I hear is mixed arguments about how fflush() is undefined for the input stream, and some say just do it that way, and others just say don't do it, I haven't had much luck on finding a clear efficient/proper way of doing so, that the majority of people agree on.. I am quite new at programming so I don't know all the syntax/tricks of the language yet, so my question which way is the most efficient/proper solution to clearing the C input stream??
Use the getchar() twice before I try to receive more input?
Just use the fflush() function on the input? or
This is how I thought I should do it.
void clearInputBuf(void);
void clearInputBuf(void)
{
int garbageCollector;
while ((garbageCollector = getchar()) != '\n' && garbageCollector != EOF)
{}
}
So, whenever I need to read a new scanf(), or use getchar() to pause the program I just call the clearInputBuf.. So what would be the best way out of the three solutions or is there a even better option?
All I hear is mixed arguments about how fflush() is undefined for the input stream
That's correct. Don't use fflush() to flush an input stream.
(A) will work for simple cases where you leave single character in the input stream (such as scanf() leaving a newline).
(B) Don't use. It's defined on some platforms. But don't rely on what standard calls undefined behaviour.
(C) Clearly the best out of the 3 options as it can "flush" any number of characters in the input stream.
But if you read lines (such as using fgets()), you'll probably have much less need to clear input streams.
It depends on what you think of as "flushing an input stream".
For output streams, the flush operation makes sure that all data that were written to the stream but were being kept buffered in memory have been flushed to the underlying filesystem file. That's a very well defined operation.
For input streams, there is no well defined operation of what flushing the stream should do. I would say that it does not make any sense.
Some implementations of the C standard library redefine the meaning of "flush" for input streams to mean "clear the rest of the current line which has not been read yet". But that's entirely arbitrary, and other implementations choose to do nothing instead.
As of C11 this disparity has been corrected, and the standard now explicitly states that the fflush() function does not work with input streams, precisely because it makes no sense, and we do not want each runtime library vendor to go implementing it in whatever way they feel like.
So, please by all means do go ahead and implement your clearInputBuf() function the way you did, but do not think of it as "flushing the input stream". There is no such thing.
It turns out to be platform dependent.
The fflush() cannot have an input stream as a parameter because according to the c standard, IT'S UNDEFINED BEHAVIOR since the behavior is not defined anywhere.
On Windows, there is a defined behavior for fflush() and it does what you need it to do.
On Linux, there is fpurge(3) which does what you want it to do too.
The best way is to simply read all characters in a loop until
A newline character is found.
EOF is returned from getchar().
like your clearInputBuf() function.
Note that flushing an output stream means writing all unwritten data to the stream, the data that is still in a buffer waiting to be flushed. But reading all the unread bytes from a stream, does not have the same meaning.
That's why it doesn't make sense to fflush() an input stream. On the other hand fpurge() is designed specifically for this, and it's name is a better choice because you want to clear the input stream and start fresh. The problem is, it's not a standard function.
Reading fpurge(3) should clarify why fflush(stdin) is undefined behavior, and why an implementation like the one on Windows doesn't make sense because it makes fflush() behave differently with different inputs. That's like making c compliant with PHP.
The problem is more subtile than it looks:
On systems with elaborate terminal devices, such as unix and OS/X, input from the terminal is buffered at 2 separate levels: the system terminal uses a buffer to handle line editing, from just correcting input with backspace to full line editing with cursor and control keys. This is called cooked mode. A full line of input is buffered in the system until the enter key is typed or the end-of-file key combination is entered.
The FILE functions perform their own buffering, which is line buffered by default for streams associated with a terminal. The buffer size in set to BUFSIZ by default and bytes are requested from the system when the buffered contents have been consumed. For most requests, a full line will be read from the system into the stream buffer, but in some cases such as when the buffer is full, only part of the line will have been read from the system when scanf() returns. This is why discarding the contents of the stream buffer might not always suffice.
Flushing the input buffer may mean different things:
discarding extra input, including the newline character, that have been entered by the user in response to input requests such as getchar(), fgets() or scanf(). The need for flushing this input is especially obvious in the case of scanf() because most format lines will not cause the newline to be consumed.
discarding any pending input and waiting for the user to hit a key.
You can implement a fluch function portably for the first case:
int flush_stream(FILE *fp) {
int c;
while ((c = getc(fp)) != EOF && c != '\n')
continue;
return c;
}
And this is exactly what your clearInputBuf() function does for stdin.
For the second case, there is no portable solution, and system specific methods are non trivial.
I had started learning C programming, so I'm a beginner, while learning about standard streams of text, I came up with the lines "stdout" stream is buffered while "stderr" stream is not buffered, but I am not able to make sense with this lines.
I already have read about "buffer" on this forum and I like candy analogy, but I am not able to figure out what is meant when one says: "This stream is buffered and the other one is not." What is the effect?
What is the difference?
Update: Does it affect the speed of processing?
Buffer is a block of memory which belongs to a stream and is used to hold stream data temporarily. When the first I/O operation occurs on a file, malloc is called and a buffer is obtained. Characters that are written to a stream are normally accumulated in the buffer (before being transmitted to the file in chunks), instead of appearing as soon as they are output by the application program. Similarly, streams retrieve input from the host environment in blocks rather than on a character-by-character basis. This is done to increase efficiency, as file and console I/O is slow in comparison to memory operations.
GCC provides three types of buffering - unbuffered, block buffered, and line buffered. Unbuffered means that characters appear on the destination file as soon as written (for an output stream), or input is read from a file on a character-by-character basis instead of reading in blocks (for input streams). Block buffered means that characters are saved up in the buffer and written or read as a block. Line buffered means that characters are saved up only till a newline is written into or read from the buffer.
stdin and stdout are block buffered if and only if they can be determined not to refer to an interactive device else they are line buffered (this is true of any stream). stderr is always unbuffered by default.
The standard library provides functions to alter the default behaviour of streams. You can use fflush to force the data out of the output stream buffer (fflush is undefined for input streams). You can make the stream unbuffered using the setbuf function.
Buffering is collecting up many elements before writing them, or reading many elements at once before processing them. Lots of information out there on the Internet, for example, this
and other SO questions like this
EDIT in response to the question update: And yes, it's done for performance reasons. Writing and reading from disks etc will in any case write or read a 'block' of some sort for most devices, and there's a fair overhead in doing so. So batching these operations up can make for a dramatic performance difference
A program writing to buffered output can perform the output in the time it takes to write to the buffer which is typically very fast, independent of the speed of the output device which may be slow.
With buffered output the information is queues and a separate process deals with the output rendering.
With unbuffered output, the data is written directly to the output device, so runs at the speed on the device. This is important for error output because if the output were buffered it would be possible for the process to fail before the buffered output has made it to the display - so the program might terminate with no diagnostic output.
int main()
{
printf("Hello"); // doesn't display anything on the screen
printf("\n"); // hello is display on the screen
return 0;
}
All characters(candidate of printing) are buffered until a new line is received? Correct?
Q1 - Why does it wait before printing on terminal until a newline char?
Q2 - Where are the characters of first printf (i.e. "Hello") buffered?
Q3 - What is the flow of printing printf()->puts()->putchar() -> now where? driver? Does the driver has a control to wait until \n?
Q4 - What is the role stdout that is attached to a process?
Looking for a in-depth picture. Feel free to edit the question, if something doesn't makes sense.
printf is not writing directly to the screen, instead it writes to the output stream, which is by default buffered. The reason for this is, that there may not even be a screen attached and the output can go to a file as well. For performance reasons, it is better for a system if access to disc is buffered and then executed in one step with appropriately sized chunks, rather than writing every time.
You can even change the size of the buffer and set it to 0, which means that all output goes directly to the target, which may be usefull for logging purposes.
setbuf(stdout, NULL);
The buffer is flushed either when it is full, or if certain criterions are fullfilled, like printing a newline. So when you would execute the printf in a loop, you would notice that it will write out in chunks unless you have a newline inbetween.
I'll start with some definitions and then go on to answer your questions.
File: It is an ordered sequence of bytes. It can be a disk file, a stream of bytes generated by a program (such as a pipeline), a TCP/IP socket, a stream of bytes received from or sent to a peripheral device (such as the keyboard or the display) etc. The latter two are interactive files. Files are typically the principal means by which a program communicates with its environment.
Stream: It is a representation of flow of data from one place to another, e.g., from disk to memory, memory to disk, one program to another etc. A stream is a source of data where data can be put into (write) or taken data out of (read). Thus, it's an interface for writing data into or reading data from a file which can be any type as stated above. Before you can perform any operation on a file, the file must be opened. Opening a file associates it with a stream. Streams are represented by FILE data type defined in stdio.h header. A FILE object (it's a structure) holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information. FILE objects are allocated and managed internally by the input/output library functions and you should not try to create your own objects of FILE type, the library does it for us. The programs should deal only with pointers to these objects (FILE *) rather than the objects themselves.
Buffer: Buffer is a block of memory which belongs to a stream and is used to hold stream data temporarily. When the first I/O operation occurs on a file, malloc is called and a buffer is obtained. Characters that are written to a stream are normally accumulated in the buffer (before being transmitted to the file in chunks), instead of appearing as soon as they are output by the application program. Similarly, streams retrieve input from the host environment in blocks rather than on a character-by-character basis. This is done to increase efficiency, as file and console I/O is slow in comparison to memory operations.
The C library provides three predefined text streams (FILE *) open and available for use at program start-up. These are stdin (the standard input stream, which is the normal source of input for the program), stdout (the standard output stream, which is used for normal output from the program), and stderr (the standard error stream, which is used for error messages and diagnostics issued by the program). Whether these streams are buffered or unbuffered is implementation-defined and not required by the standard.
GCC provides three types of buffering - unbuffered, block buffered, and line buffered. Unbuffered means that characters appear on the destination file as soon as written (for an output stream), or input is read from a file on a character-by-character basis instead of reading in blocks (for input streams). Block buffered means that characters are saved up in the buffer and written or read as a block. Line buffered means that characters are saved up only till a newline is written into or read from the buffer.
stdin and stdout are block buffered if and only if they can be determined not to refer to an interactive device else they are line buffered (this is true of any stream). stderr is always unbuffered by default.
The standard library provides functions to alter the default behaviour of streams. You can use fflush to force the data out of the output stream buffer (fflush is undefined for input streams). You can make the stream unbuffered using the setbuf function.
Now, let's come to your questions.
Unmarked question: Yes, becausestdout normally refers to a display terminal unless you have output redirection using > operator.
Q1: It waits because stdout is newline buffered when it refers to a terminal.
Q2: The characters are buffered, well, in the buffer allocated to the stdout stream.
Q3: Flow of the printing is: memory --> stdout buffer --> display terminal. There are kernel buffers as well controlled by the OS which the data pass through before appearing on the terminal.
Q4: stdout refers to the standard output stream which is usually a terminal.
Finally, here's a sample code to experiment things before I finish my answer.
#include <stdio.h>
#include <limits.h>
int main(void) {
// setbuf(stdout, NULL); // make stdout unbuffered
printf("Hello, World!"); // no newline
// printf("Hello, World!"); // with a newline
// only for demonstrating that stdout is line buffered
for(size_t i = 0; i < UINT_MAX; i++)
; // null statement
printf("\n"); // flush the buffer
return 0;
}
Yes, by default, standard output is line buffered when it's connected to a terminal. The buffer is managed by the operating system, normally you don't have to worry about it.
You can change this behavior using setbuf() or setvbuf(), for example, to change it to no buffer:
setbuf(stdout, NULL);
All the functions of printf, puts, putchar outputs to the standard output, so they use the same buffer.
If you wish, you can flush out the characters before the new line by calling
fflush(stdout);
This can be handy if you're slowly printing something like a progress bar where each character gets printed without a newline.
int main()
{
printf("Hello"); // Doesn't display anything on the screen
fflush(stdout); // Now, hello appears on the screen
printf("\n"); // The new line gets printed
return 0;
}
Could anyone clarify on the types of buffers used by a program?
For eg:
I have a C program that reads from a stdin to stdout.
What are the buffers involved here? I'm aware that there are 2.
One provided by the kernel on which a user don't have any control.
One provided with standard streams namely stdout, stdin and stderr. Each having a separate buffer.
Is my understanding correct?
Thanks,
John
If you are working on linux/unix then you could more easily understand that there are three streams namely
1.STDIN: FILE DESCRIPTOR VALUE 0 (IN unix)
2.STDOUT :FILE DESCRIPTOR VALUE 1
3.STDERR :FILE DESCRIPTOR VALUE 2
By default these streams correspond to keyboard and monitor.In unix we can change these streams to read input from file instead of keyboard.To display output on a file rather than monitor using close(),dup() system calls.Yes there are 3 buffers involved.To clear the contents of input buffer in c we use fflush() function.
If you want to know more about handling these streams in UNIX then let me Know.
The kernel (or other underlying system) could have any number of layers of buffering, depending on what device is being read from and the details of the kernel implementation; in some systems there is no buffering at that level, with the data being read directly into the userspace buffer.
The stdio library allocates a buffer for stdin; the size is implementation-dependent but you can control the size and even use your own buffer with setvbuf. It also allows you to control whether I/O is fully buffered (as much data is read into the buffer as is available), line buffered (data is is only read until a newline is encountered), or unbuffered. The default is line buffering if the system can determine that the input is a terminal, else fully buffered.
The story is similar for stdout. stderr is by default unbuffered.