In C, stdout and stderr both print to the console window by default. Is there any difference between stderr and stdout other than the level of buffering?
One of the differences between stdout and stderr is the level of buffering. In §7.21.3 Files ¶7, the C11 standard says:
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.
Typically, that means that standard output is line buffered (so the data is flushed when a newline is printed, or when the buffer is full), whereas standard error is either line buffered or unbuffered. These characteristics can be changed, of course.
The purpose of the standard error stream is to separate error messages from regular output. This is important in contexts such as shell scripts, where the standard output might be sent to a pipe or to a file. That redirection leaves standard error still going to a different place — usually the terminal. You can capture the standard output separately from the standard error too, at least if the shell is sufficiently powerful.
program > file
program | filter
program 2> error.log | filter
program > file 2> error.log
program 2> error.log
The first two leave the error messages visible on the terminal. The last three capture the error messages in the file error.log — sending the standard output to the filter program, the file or to the terminal window respectively.
By separating the error messages from the standard output, the programs down the pipeline (filter in my example) don't have to interpret the error messages from program, which makes them much, much simpler.
If running a program from the console, or via a batch file, or via a shortcut, use > or 1> to redirect stdout to a file, and use 2> to redirect stderr to a file. For example if reading from stdin, and writing to both stdout and stderr:
myprog <input.txt >output.txt 2>error.txt
or if you want stdout to a file, but stderr to display on the screen, use:
myprog <input.txt >output.txt
stderr is - well - the place for errors. It is really useful. Like make - if a program's return code is not 0 (it completed with errors), it will print stderr. stderr can not be differentiated from stdout by the user, but it's still the recommended place for errors. However, if your program will be run by any automatization program, shell script etc... it can be used to track errors, and always use it.
Related
I was tasked with creating a test program in C that reads the contents of the standard input and then prints them.
But I have a little doubt: what is exactly standard input?
Is it what I type in the keyboard? Is it a file I have to read?
Both of them?
And the same goes for standard output: is it the console? a file?
The C standard (e.g. C99 or C11) defines what should be expected from the standard <stdio.h> header (after having suitably #include-d it). See stdio(3) man page.
Then you have the stdin and stdout and stderr file handles (pointers to some FILE which is an abstract data type).
The fact that stdin is related to some device (e.g. a keyboard) is implementation specific.
You could (but that would be unethical and/or inefficient) implement the C standard with e.g. a room of human slaves (that is unethical, if you use paid workers that would be just inefficient), instead of using a computer. Often, computers gives your some implementation of the C standard thru the help of some operating system.
You may want to know, inside your C program, if stdin is a "keyboard" or redirected from some "file". Unfortunately, AFAIK, there is no C99-standard way to know that.
As you mention, stdin, stdout and stderr should be available in your program at startup (i.e. after entering main ....). Hence, unless you fclose the stdin stream, you can read it (with getchar, scanf, getline, fgets, fscanf ... and friends) without any prior care (so you don't need to fopen it yourself).
On Linux or most Posix systems, you might use as an approximation isatty(STDIN_FILENO) - see isatty(3) for more - to test if stdin "is" the "keyboard" (by testing if it is some tty). See also this & that.
Yes, standard input (stdin) is input exepected from the keyboard. So, could be in the form of user input from a basic program or from a command line argument. Standard output (stdout) is the output of the code, usually to the terminal window. You could output your code almost anywhere, i.e. to a file, to a textbox, browser, but the standard is the stdout which is the terminal.
Hope that helps.
Normally, the standard input is the keyboard and the standard output the screen. However, you can redirect this in the command line using the "<" and ">" symbols. A command line like
dir /s > "Tree.txt"
will change the standard output for the dir command to be the specified file. So all output goes to that file. The called application or command itself doesn't normally even notice the difference.
stdin is file descriptor 0, you can get a file to stdin by:
cat file |yourprog
#or
yourprog <file
likewise for stdout (file descriptor 1)
yourprog | someotherprog #pipe your stdout to the stdin of another program
yourprog > somefile #save stdout to a file
yourprog >> somefile #append stdout to a file
and stderr (fd 2)
yourprog 2> errlogfile
if you have a program that takes a file but doesn't handle stdin, you can use the above formats by doing this (assuming -f if the input file argument)
myprog -f /dev/stdin
//and a horrible example of how not to read from stdin and write to stdout
char buf[4096];
while(write(1,buf,read(0,buf,4096)));
standard output (or stdout) refers to the standardized streams of data that are produced by command line programs (i.e., all-text mode programs) in Linux and other Unix-like operating systems.
I was tasked with creating a test program in C that reads the contents of the standard input and then prints them.
But I have a little doubt: what is exactly standard input?
Is it what I type in the keyboard? Is it a file I have to read?
Both of them?
And the same goes for standard output: is it the console? a file?
The C standard (e.g. C99 or C11) defines what should be expected from the standard <stdio.h> header (after having suitably #include-d it). See stdio(3) man page.
Then you have the stdin and stdout and stderr file handles (pointers to some FILE which is an abstract data type).
The fact that stdin is related to some device (e.g. a keyboard) is implementation specific.
You could (but that would be unethical and/or inefficient) implement the C standard with e.g. a room of human slaves (that is unethical, if you use paid workers that would be just inefficient), instead of using a computer. Often, computers gives your some implementation of the C standard thru the help of some operating system.
You may want to know, inside your C program, if stdin is a "keyboard" or redirected from some "file". Unfortunately, AFAIK, there is no C99-standard way to know that.
As you mention, stdin, stdout and stderr should be available in your program at startup (i.e. after entering main ....). Hence, unless you fclose the stdin stream, you can read it (with getchar, scanf, getline, fgets, fscanf ... and friends) without any prior care (so you don't need to fopen it yourself).
On Linux or most Posix systems, you might use as an approximation isatty(STDIN_FILENO) - see isatty(3) for more - to test if stdin "is" the "keyboard" (by testing if it is some tty). See also this & that.
Yes, standard input (stdin) is input exepected from the keyboard. So, could be in the form of user input from a basic program or from a command line argument. Standard output (stdout) is the output of the code, usually to the terminal window. You could output your code almost anywhere, i.e. to a file, to a textbox, browser, but the standard is the stdout which is the terminal.
Hope that helps.
Normally, the standard input is the keyboard and the standard output the screen. However, you can redirect this in the command line using the "<" and ">" symbols. A command line like
dir /s > "Tree.txt"
will change the standard output for the dir command to be the specified file. So all output goes to that file. The called application or command itself doesn't normally even notice the difference.
stdin is file descriptor 0, you can get a file to stdin by:
cat file |yourprog
#or
yourprog <file
likewise for stdout (file descriptor 1)
yourprog | someotherprog #pipe your stdout to the stdin of another program
yourprog > somefile #save stdout to a file
yourprog >> somefile #append stdout to a file
and stderr (fd 2)
yourprog 2> errlogfile
if you have a program that takes a file but doesn't handle stdin, you can use the above formats by doing this (assuming -f if the input file argument)
myprog -f /dev/stdin
//and a horrible example of how not to read from stdin and write to stdout
char buf[4096];
while(write(1,buf,read(0,buf,4096)));
standard output (or stdout) refers to the standardized streams of data that are produced by command line programs (i.e., all-text mode programs) in Linux and other Unix-like operating systems.
Why should we use stderr when printing a custom error message from printf works fine?
For example why use stderr at all when I can just write a statement like this:
printf("error! you didn't... blah blah blah");
It is good practice to redirect all error messages to stderr, while directing regular output to stdout. It is beneficial to do this because anything written to stderr is not buffered, i.e., it is immediately written to the screen so that the user can be warned immediately.
stderr stands for standard error stream.
In console programming, it is the console -- the screen. It is essentially the same as stdout.
The general practice is to redirect all error messages to stderr and all regular output to stdout.
The reason is that it is possible to redirect standard output to a file instead of the screen. So If you perform a dir > dirlist.txt command on the command prompt, the directory listing goes into the text file instead of the screen. If you code to redirect error messages to stderr, the errors will always go to the screen instead of the file so that the user could be warned immediately instead of ending up seeing unexpected results in the file.
Using printf() will display the error message to the console, which will be stored in the stdout buffer, but using stderr, is different.
stderr can be used as an argument for any function that takes an argument of type FILE* expecting an output stream, like fputs or fprintf.
Although in many cases, both stdout and stderr are associated with the same output device (like the console), applications may differentiate between what is sent to stdout vs stderr for situations when one of them is redirected. For example, it is a common practice to redirect the regular output of a console program (stdout) to a file, while expecting the error messages to keep appearing in the console.
It is also possible to redirect stderr to another destination from within a program using the freopen function.
Also, stderr is never fully buffered on startup. It is library-dependent whether the stream is line-buffered or not-buffered by default (see setvbuf).
It is a good practice.
Lets say you use linux. If so, you can run your program following way:
./program >out 2>errout
'out' file will contain only STDOUT.
'errout' file will contain only STDERR
So if your output is hundreds of lines long, it is easier to look for few errors in 'errout' rather than look through tons of non-error lines combined with error lines.
If you redirect your program's output into a file you still would want to see the errors on screen.
In addition to stderr not being buffered, it is also nice to split output into errors vs. normal output to allow other programs to use of your program more easily. That way, the calling program can redirect standard or error output selectively depending on what it needs to know. This same facility can be used manually through Unix shells - here's one way I use that sometimes:
% find / -iname hello.txt
find: /.DocumentRevisions-V100: Permission denied
find: /.fseventsd: Permission denied
find: /.MobileBackups: Permission denied
find: /.Spotlight-V100: Permission denied
find: /.Trashes: Permission denied
^C
% find / -iname hello.txt 2>/dev/null <-- filter "Permission denied" errors
int main(void)
{
char buf[] = "standard err, output.\n";
printf("standard output.\n");
if (write(STDERR_FILENO,buf, 22) != 22)
printf("write err!\n");
exit(0);
}
Compile using:
gcc -Wall text.c
Then running in the shell:
./a.out > outfile 2 >& 1
Result：outfile´s content are：
standard err, output.
standard output.
./a.out 2 >& 1 >outfile
Result：
This first prints to the terminal: standard err, output.
and the content of outfile are: standard output.
Questions:
I want to ask the difference between 2 >& fd and 2 > file.
Are they all equal to the function dup()?
Another question: why are the contents of outfile:
standard err, output.
standard output.
I expected the content of outfile to be:
standard output.
standard err, output
Actually, in bash, >& is quite similar to dup2. That is, the file descriptor to which it is applied will refer to the same file as the descriptor to the right. So:
$ ./a.out > outfile 2>& 1
It will redirect stdout(1) to the file outfile and, after that, will dup2 stderr(2) to refer to the same file as stdout(1). That is, both stdout and stderr are being redirected to the file.
$ ./a.out 2>& 1 >outfile
It will redirect stderr(2) to refer to the same file as stdout(1), that is, the console, and after that, will redirect stdout(1) to refer to the file outfile. That is, stderr will output to the console and stdout to the file.
And that's exactly what you are getting.
Paradigm Mixing
While there are reasons to do all of these things deliberately, as a learning experience it is probably going to be confusing to mix operations over what I might call "domain boundaries".
Buffered vs non-buffered I/O
The printf() is buffered, the write() is a direct system call. The write happens immediately no matter what, the printf will be (usually) buffered line-by-line when the output is a terminal and block-by-block when the output is a real file. In the file-output case (redirection) your actual printf output will happen only when you return from main() or in some other fashion call exit(3), unless you printf a whole bunch of stuff.
Historic csh redirection vs bash redirection
The now-forgotten (but typically still in a default install) csh that Bill Joy wrote at UCB while a grad student had a few nice features that have been imported into kitchen-sink shells that OR-together every shell feature ever thought of. Yes, I'm talking about bash here. So, in csh, the way to redirect both standard output and standard error was simply to say cmd >& file which was really more civilized that the bag-of-tools approach that the "official" Bourne shell provided. But the Bourne syntax had its good points elsewhere and in any case survived as the dominant paradigm.
But the bash "native" redirection features are somewhat complex and I wouldn't try to summarize them in a SO answer, although others seem to have made a good start. In any case you are using real bash redirection in one test and the legacy-csh syntax that bash also supports in another, and with a program that itself mixes paradigms. The main issue from the shell's point of view is that the order of redirection is quite important in the bash-style syntax while the csh-style syntax simply specifies the end result.
There are several loosely related issues here.
Style comment: I recommend using 2>&1 without spaces. I wasn't even aware that the spaced-out version works (I suspect it didn't in Bourne shell in the mid-80s) and the compressed version is the orthodox way of writing it.
The file-descriptor I/O redirection notations are not all available in the C shell and derivatives; they are avialable in Bourne shell and its derivatives (Korn shell, POSIX shell, Bash, ...).
The difference between >file or 2>file and 2>&1 is what the shell has to do. The first two arrange for output written to a file descriptor (1 in the first case, aka standard output; 2 in the second case, aka standard error) to go to the named file. This means that anything written by the program to standard output goes to file instead. The third notation arranges for 2 (standard error) to go to the same file descriptor as 1 (standard output); anything written to standard error goes to the same file as standard output. It is trivially implemented using dup2(). However, the standard error stream in the program will have its own buffer and the standard output stream in the program will have its own buffer, so the interleaving of the output is not completely determinate if the output goes to a file.
You run the command two different ways, and (not surprisingly) get two different results.
./a.out > outfile 2>&1
I/O redirections are processed left to right. The first one sends standard output to outfile. The second sends standard error to the same place as standard output, so it goes to outfile too.
./a.out 2>&1 >outfile
The first redirection sends standard error to the place where standard output is going, which is currently the terminal. The second redirection then sends standard output to the file (but leaves standard error going to the terminal).
The program uses the printf() function and the write() system call. When the printf() function is used, it buffers its output. If the output is going to a terminal, then it is normally 'line buffered', so output appears when a newline is added to the buffer. However, when the output is going to a file, it is 'fully buffered' and output does not appear until the file stream is flushed or closed or the buffer fills. Note that stderr is not fully buffered, so output written to it appears immediately.
If you run your program without any I/O redirection, you will see:
standard output.
standard err, output
By contrast, the write() system call immediately transfers data to the output file descriptor. In the example, you write to standard error, and what you write will appear immediately. The same would have happened if you had used fprintf(stderr, ...). However, suppose you modified the program to write to STDOUT_FILENO; then when the output is to a file, the output would appear in the order:
standard err, output
standard output.
because the write() is unbuffered while the printf() is buffered.
The 2>&1 part makes the shell do something like that:
dup2(1, 2);
This makes fd 2 a "copy" of fd 1.
The 2> file is interpreted as
fd = open(file, ...);
dup2(fd, 2);
which opens a file and puts the filedescriptor into slot 2.
I am trying to redirect the output of a c program to file, even when it generates some errors because of problems with the input data. I can send the output but the error messages to a file.
Does somebody know how to do it?
From within C source code, you can redirect outputs using freopen():
General outputs:
freopen("myfile.txt", "w", stdout);
Errors:
freopen("myfile_err.txt", "w", stderr);
(This answer applies to bash shell, and similar flavors. You didn't specify your environment and this sort of question needs that detail.)
I assume you know about basic redirection with ">". To also capture STDERR in addition to STDOUT, use the following syntax:
command > file-name 2>&1
For some more background on standard streams and numbers:
http://en.wikipedia.org/wiki/Standard_streams#Standard_input_.28stdin.29
This depends on what you mean and what platform you are using. Very often you can accomplish this from the command line, which has been covered in another answer. If you use this method to accomplish this you should be aware that FILE * stderr is typically written immediately (unbuffered) while FILE * stdout may be buffered (usually line buffered) so you could end up with some of your error messages appearing to have been printed earlier than some other messages, but actually the other messages are just being printed late.
From within a C program you can also do something similar within the stdio system using freopen, which will effect the FILE *, so you could make fprintf(stderr, "fungus"); print to something besides what stderr normally would print to.
But if you want to know how to make a program redirect the actual file descriptors under a unix like system you need to learn about the dup and dup2 system calls. They allow you to duplicate a file descriptor.
int fd = open("some_file", O_WRONLY);
dup2(2,fd);
close(fd);
This code will make "some_file" the new stderr at the OS level. The dup2 call will close and replace file descriptor 2 (stderr, which is usually used by FILE * stderr but not necessarily if you call freopen(x,y,stderr) since that may make FILE *stderr use a different file descriptor).
This is how shell programs redirect input and output of programs. The open all of the files that the new program will need, fork, then the child uses dup2 to set up the files descriptors for the new program, then it closes any files that the new program won't need (usually just leaving 0, 1, and 2 open), and then uses one of the exec functions to become the program that the shell was told to run. (some of this isn't entirely accurate because some shells may rely on close on exe flags)
Using a simple linux command you can save the output into the file. here is a simple linux terminal command.
ls > file.txt
The output of this command will be stored into the file.
same as you can store the output of the program like this suppose, object file name is a, run the following command to save output in a file:
./a > file.txt