Confused how stdin, stdout and stderr work

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

score 3 · Answer 1 · answered Apr 04 '13 at 10:32

3

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.

answered Apr 04 '13 at 10:32

unwind

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:- This can be done with `boost` and `unix`. But how to do with Visual Studio 2010? – Rasmi Ranjan Nayak Apr 04 '13 at 10:57

score 3 · Accepted Answer · edited May 23 '17 at 12:23

3

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

edited May 23 '17 at 12:23

Community

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1

answered Apr 04 '13 at 10:32

MOHAMED

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you have already changed the stdout output and also you have close it. – MOHAMED Apr 04 '13 at 10:37
@MOHAMMED: My question is why `printf("Hello World\n")` is not getting printed in console – Rasmi Ranjan Nayak Apr 04 '13 at 10:38
@RasmiRanjanNayak And my answer is still because you have already changed the stdout output and also you have close then the stdout with fclose(stdout); – MOHAMED Apr 04 '13 at 10:40
:- Thanks for the update. Just wanted to know how can I resolve this problem. How can I print the output in both console and file too – Rasmi Ranjan Nayak Apr 04 '13 at 10:50
@RasmiRanjanNayak The answer now update for the print the output in both console and file too – MOHAMED Apr 04 '13 at 10:54

score 0 · Answer 3 · answered Apr 04 '13 at 13:27

0

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.

answered Apr 04 '13 at 13:27

Jeegar Patel

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:- Could you pls expand your comment bit more – Rasmi Ranjan Nayak Apr 04 '13 at 13:57

Confused how stdin, stdout and stderr work

3 Answers3