混淆stdin，stdout和stderr的工作方式

Question

I have below program,

void Print()
{
    printf("\nCall from Print\n");
}

int main()
{
    FILE * pFile;
    char mystring [100];

    pFile = freopen ("myfile.txt" , "r", stdin);
    if (pFile == NULL)
    {
        perror ("Error opening file");
    }
    else 
    {
        if ( fgets (mystring , 100 , pFile) != NULL )
        {
            freopen("myfile.txt" , "a", stdout);
            Print();
            printf("Here it is\n");
            //puts (mystring);
        }
        fclose(stdout);
        fclose (pFile);

    }
    printf("Hello World\n");
    return 0;
}

Now when I am execting the program I can not see the output in console window. All the outputs are redirected into myfile.txt file. I want the output should come in both console and myfile.txt too.

After all that why printf("Hello World\\n") is not getting printed in console. How to make it printed in console also?

My am working in windows-7, visual studio-2010

Answer 1

The easiest way would be to be more explicit about it, by manually printing to both stdout (the original, to get output to the console) and to your file.

By re-opening stdout to point at the file, you remove the connection to the console window, which is why no output appears.

You could also only use stdout , and use an external tool such as tee to duplicate the output into a file.

Answer 2

the

freopen("myfile.txt" , "a", stdout);

will make your stdout output to the file myfile.txt This function will redirect the output from console to the file myfile.txt

Even if you use fclose(stdout); , this will not back the output of stdout to the console it will only close the myfile.txt

refer to the following link for more details Strange behaviour when redirecting stdout in C

in order to get the output in both the console and in the file, You have tokeep the stdout untouch do not reopen it with freaopen() and do not close it. and You have to print your message twice in the file and in the stdout

Answer 3

I have also faced such requirements once.

I have made one macro for such printing log. That print macro expands in one function which print that message in my log file and stdout or stderr depending upon some control variable setting.