read system call taking reading from STDIN

read system call taking reading from STDIN - c

When does the Read System call terminates when taking input from STDIN ??

There are quite a few parts to this.
First, let's clarify the distinction between OS-level IO and stdio-level IO. read(2) and write(2) (POSIX IO) are specified by POSIX, and operate using file descriptors (numbers starting from 0); fread(3) and fwrite(3) (stdio IO) are specified by ISO C and operate on file handles, such as STDIN, which on POSIX systems encapsulate file descriptors and add some things (such as output buffering) on top of them.
So, read(2) and write(2) don't do any buffering on their own. The buffering you see on standard input (file descriptor 0, not STDIN, which is one abstraction above that) is done by the terminal (or terminal emulation). Search for canonical mode to disable it.
At the stdio-level, fwrite(3) (and printf(3), fprintf(3), et al.) does output buffering depending on what the output is connected to.
See also:
How to check if a key was pressed in Linux?
Single characters are not printed on the terminal
Does printing to the screen cause a switch to kernel mode and running OS code in Unix?

Related

listen() in socket programming in c [duplicate]

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?

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);

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.

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.

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

stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
Print to stderr.
Make stdout unbuffered.

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

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.

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.

sleep() function from <unistd.h> not executing line by line in C [duplicate]

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?

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);

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.

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

stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
Print to stderr.
Make stdout unbuffered.

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

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.

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.

c pipe wait for message to reach some size then send? [duplicate]

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?

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);

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.

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

stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
Print to stderr.
Make stdout unbuffered.

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

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.

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.

Sublime text not flushing automatically, c code [duplicate]

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?

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);

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.

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

stdout is buffered, so will only output after a newline is printed.
To get immediate output, either:
Print to stderr.
Make stdout unbuffered.

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

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.

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.

Stdin is from keyboard or from a file

Can a program detect if it's stdin is
coming from keyboard input vs. being
redirected from a file? in c ?

Yes, on most systems. On POSIX systems (Linux, Mac OS X, BSD, plus many more), you can use the isatty(3) function on file descriptor 0 (standard input). On Windows, you can use _isatty. For example:
if(isatty(0))
{
// standard input is a terminal device
}
Note that many programs use isatty to alter their output buffering behavior: if standard output is a terminal, the output is line buffered (e.g. it's flushed after every newline gets printed), whereas if standard output is not a terminal, it's fully-buffered (e.g. it's flushed only when the buffer becomes full, the process exits normally, or the stream is explicitly flushed by the application).

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