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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 is the standard error message that is used to print the output on the screen or windows terminal. Stderr is used to print the error on the output screen or window terminal. Stderr is also one of the command output as stdout, which is logged anywhere by default.
In C, to print to STDOUT, you may do this: printf("%sn", your_str); For example, $ cat t.c #include <stdio.
The error message that is delivered via stderr is still sent to the terminal window. We can check the contents of the file to see whether the stdout output went to the file. The output from stdin was redirected to the file as expected. The > redirection symbol works with stdout by default.
The syntax is almost the same as printf. With printf you give the string format and its contents ie:
printf("my %s has %d chars\n", "string format", 30);
With fprintf it is the same, except now you are also specifying the place to print to:
FILE *myFile;
...
fprintf( myFile, "my %s has %d chars\n", "string format", 30);
Or in your case:
fprintf( stderr, "my %s has %d chars\n", "string format", 30);
Some examples of formatted output to stdout and stderr:
printf("%s", "Hello world\n"); // "Hello world" on stdout (using printf)
fprintf(stdout, "%s", "Hello world\n"); // "Hello world" on stdout (using fprintf)
fprintf(stderr, "%s", "Stack overflow!\n"); // Error message on stderr (using fprintf)
#include<stdio.h>
int main ( ) {
printf( "hello " );
fprintf( stderr, "HELP!" );
printf( " world\n" );
return 0;
}
$ ./a.exe
HELP!hello world
$ ./a.exe 2> tmp1
hello world
$ ./a.exe 1> tmp1
HELP!$
stderr is usually unbuffered and stdout usually is. This can lead to odd looking output like this, which suggests code is executing in the wrong order. It isn't, it's just that the stdout buffer has yet to be flushed. Redirected or piped streams would of course not see this interleave as they would normally only see the output of stdout only or stderr only.
Although initially both stdout and stderr come to the console, both are separate and can be individually redirected.
Do you know sprintf? It's basically the same thing with fprintf. The first argument is the destination (the file in the case of fprintf i.e. stderr), the second argument is the format string, and the rest are the arguments as usual.
I also recommend this printf (and family) reference.
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