I am reading about stream and buffer.
I got that if stream is line buffered then the accumulated characters of buffer get transfered in the from of block whenever the newline character encounter.
and if stream is unbuffered then character are intended to appear from the source or appear at the destination as soon as possible without getting store in buffer.
but if stream is fully buffered then the accumulated characters of buffer get transfered in the form of block whenever the buffer get totally filled.
Now I am unable to understand that in which condition the buffer get totally filled.
The C standard does not explicitly specify the details of stream buffering, but generally a stream has a buffer of fixed size. This is simply an array of bytes that is used for holding data for the stream.
Quite simply, the buffer is totally filled when the number of bytes that have been written to the stream since the last time the buffer was flushed equals the number of bytes in the buffer.
When <stdio.h> is included, it defines a macro BUFSIZ that provides the size of the buffer used by setbuf. You can print it with printf("%d\n", BUFSIZ);. Presumably that is the default size of a buffer for a stream, although the C standard does not explicitly say this. (It says that is the size of the buffer used by the setbuf function, which allows you to provide your own memory for the buffer.)
You can also use the newer setvbuf to request a different size for the buffer. Both setbuf and setvbuf must be used only just after a stream has been opened and before any other operation.
Related
How do input functions like scanf, getc, etc. work? When a program is invoked and execution reaches scanf it stops waiting for input. Does it start reading from the input buffer till it reads code for enter key or the function implementation relies on specific system call or interrupt mechanism? I mean how the OS is involved here?
I searched online and read a few books but none explains that very well.
Basically, the way the stdio functions work is that they maintain a buffer (a char array of some size, generally with a 'head' and 'tail' pointer tracking what part of the buffer is valid/in use). They operate on data in the buffer (input functions read from the buffer, output functions write to the buffer) and call lower-level OS functions to fill or flush the buffer when it is empty or full or when switching a FILE between input and output.
So in the case of getc, if the buffer is empty, it will call an OS function to get some data into the buffer, then will return the first character in the buffer (advancing the pointer, so subsequent getc calls will return subsequent characters from the buffer). In the case of scanf it matches data in the buffer to the format string, consuming it (advancing the pointer) if it matches. If there's not enough data it the buffer, it will call the OS function to get more.
Similarly, when printf is called, data will be written to the buffer and the OS function to actually write the data will not be called until the buffer is flushed. You can call setvbuf to control the output bufferring somewhat -- you can set it to "unbuffered" (which doesn't actually eliminate the buffer, just causes the buffer to be flushed after every call) or "line buffered" (which flushes the buffer whenever a \n (newline) character is written.
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.
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;
}
Today I learned that stdout is line buffered when it's set to terminal and buffered in different cases. So, in normal situation, if I use printf() without the terminating '\n' it will be printed on the screen only when the buffer will be full. How to get a size of this buffer, how big is this?
The actual size is defined by the individual implementation; the standard doesn't mandate a minimum size (based on what I've been able to find, anyway). Don't have a clue on how you'd determine the size of the buffer.
Edit
Chapter and verse:
7.19.3 Files
...
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.
Emphasis added.
"Implementation-defined" is not a euphemism for "I don't know", it's simply a statement that the language standard explicitly leaves it up to the implementation to define the behavior.
And having said that, there is a non-programmatic way to find out; consult the documentation for your compiler. "Implementation-defined" also means that the implementation must document the behavior:
3.4.1
1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made
2 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit
when a signed integer is shifted right.
The Linux when a pipe is created for default pipe size 64K is used.
In /proc/sys/fs/pipe-max-size the maximum pipe size exists.
For the default 1048576 is typical.
For glibc's default file buffer; 65536 bytes seems reasonable.
However, ascertained by grep from the glibc source tree:
libio/libio.h:#define _IO_BUFSIZ _G_BUFSIZ
sysdeps/generic/_G_config.h:#define _G_BUFSIZ 8192
sysdeps/unix/sysv/linux/_G_config.h:#define _G_BUFSIZ 8192
By that the original question might or might not be answered.
For a minute's effort the best guess is 8 kilobytes.
For mere line buffering 8K is adequate.
However, for more than line buffered output
as compared with 64K; 8K is not efficient.
Because for the default pipe size 64K is used and
if a larger pipe size is not expected and
if a larger pipe size is not explicitly set
then for a stdio buffer 64K is recommended.
If performance is required
then meager 8K buffers do not suffice.
By fcntl(pipefd,F_SETPIPE_SZ,1048576)
a pipe's size can be increased.
By setvbuf (stdout,buffer,_IOFBF,1048576)
a stdio provided file buffer can be replaced.
If a pipe is not used
then pipe size is irrelevant.
However, if between two processes data is piped
then by increasing pipe size a performance boon could become.
Otherwise
by the smallest buffer or
by the smallest pipe
a bottleneck is created.
If reading also
then by a larger buffer
by stdio fewer read function invocations might be required.
By the word "might" an important consideration is suggested.
As by provided
by a single write function invocation
by a single read function invocation
as much data can be read.
By a read function invocation
a return with fewer bytes than requested can be expected.
By an additional read function invocation
additional bytes may be gained.
For writing a data line; by stdio overkill is provided.
However, by stdio line buffered output is possible.
In some scenarios line buffered output is essential.
If writing to a proc virtual file system provided file or
if writing to a sys virtual file system provided file
then in a single write buffer
the line feed byte should be included.
If a second write is used
then an unexpected outcome could become.
If read write and stdio are mixed
then caveats exist.
Before
a write function invocation
a fflush function invocation is required.
Because stderr is not buffered;
for stderr the fflush function invocation is not required.
By read fewer than expected bytes might be provided.
By stdio the previous bytes might already be buffered.
Not mixing unistd and stdio I/O is good advise, but often ignored.
Mixing buffered input is unreasonable.
Mixing unbuffered input is possible.
Mixing buffered output is plausible.
By stdio buffered IO convenience is provided.
Without stdio buffered IO is possible.
However, for the code additional bytes are required.
When a sufficient sized buffer is leveraged;
compared with stdio provided output functions;
the write function invocation is not necessarily slower.
However, when a pipe is not involved
then by function mmap superior IO can be provided.
On a pipe by mmap an error is not returned.
However, in the address space the data is not provided.
On a pipe by lseek an error is provided.
Lastly by man 3 setvbuf a good example is provided.
If on the stack the buffer is allocated
then before a return a fclose function invocation
must not be omitted.
The actual question was
"In C, what's the size of stdout buffer?"
By 8192 that much might be answered.
By those who encounter this inquiry
curiosity concerning buffer input/output efficiency might exist.
By some inquiries the goal is implicitly approached.
By a preference for terse replies
the pipe size significance and
the buffer size significance and
mmap is not explicated.
This reply explicates.
here are some pretty interesting answers on a similar question.
on a linux system you can view buffer sizes from different functions, including ulimit.
Also the header files limits.h and pipe.h should contain that kind of info.
You could set it to unbuffered, or just flush it.
This seems to have some decent info when the C runtime typically flushes it for you and some examples. Take a look at this.
What I need to do is use the read function from unistd.h to read a file
line by line. I have this at the moment:
n = read(fd, str, size);
However, this reads to the end of the file, or up to size number of bytes.
Is there a way that I can make it read one line at a time, stopping at a newline?
The lines are all of variable length.
I am allowed only these two header files:
#include <unistd.h>
#include <fcntl.h>
The point of the exercise is to read in a file line by line, and
output each line as it's read in. Basically, to mimic the fgets()
and fputs() functions.
You can read character by character into a buffer and check for the linebreak symbols (\r\n for Windows and \n for Unix systems).
You'll want to create a buffer twice the length of your longest line you'll support, and you'll need to keep track of your buffer state.
Basically, each time you're called for a new line you'll scan from your current buffer position looking for an end-of-line marker. If you find one, good, that's your line. Update your buffer pointers and return.
If you hit your maxlength then you return a truncated line and change your state to discard. Next time you're called you need to discard up to the next end of line, and then enter your normal read state.
If you hit the end of what you've read in, then you need to read in another maxline chars, wrapping to the start of the buffer if you hit the bottom (ie, you may need to make two read calls) and then continue scanning.
All of the above assumes you can set a max line length. If you can't then you have to work with dynamic memory and worry about what happens if a buffer malloc fails. Also, you'll need to always check the results of the read in case you've hit the end of the file while reading into your buffer.
Unfortunately the read function isn't really suitable for this sort of input. Assuming this is some sort of artificial requirement from interview/homework/exercise, you can attempt to simulate line-based input by reading the file in chunks and splitting it on the newline character yourself, maintaining state in some way between calls. You can get away with a static position indicator if you carefully document the function's use.
This is a good question, but allowing only the read function doesn't help! :P
Loop read calls to get a fixed number of bytes, and search the '\n' character, then return a part of the string (untill '\n'), and stores the rest (except '\n') to prepend to the next character file chunk.
Use dynamic memory.
Greater the size of the buffer, less read calls used (which is a system call, so no cheap but nowadays there are preemptive kernels).
...
Or simply fix a maximum line length, and use fgets, if you need to be quick...
If you need to read exactly 1 line (and not overstep) using read(), the only generally-applicable way to do that is by reading 1 byte at a time and looping until you get a newline byte. However, if your file descriptor refers to a terminal and it's in the default (canonical) mode, read will wait for a newline and return less than the requested size as soon as a line is available. It may however return more than one line, if data arrives very quickly, or less than 1 line if your program's buffer or the internal terminal buffer is shorter than the line length.
Unless you really need to avoid overstep (which is sometimes important, if you want another process/program to inherit the file descriptor and be able to pick up reading where you left off), I would suggest using stdio functions or your own buffering system. Using read for line-based or byte-by-byte IO is very painful and hard to get right.
Well, it will read line-by-line from a terminal.
Some choices you have are:
Write a function that uses read when it runs out of data but only returns one line at a time to the caller
Use the function in the library that does exactly that: fgets().
Read only one byte at a time, so you don't go too far.
If you open the file in text mode then Windows "\r\n" will be silently translated to "\n" as the file is read.
If you are on Unix you can use the non-standard1 gcc 'getline()' function.
1 The getline() function is standard in POSIX 2008.
Convert file descriptor to FILE pointer.
FILE* fp = fdopen(fd, "r");
Then you can use getline().