Question

I am trying read stdout of my own program into 2 arrays like this

#include<stdio.h>

int main ()
{
    char arr[100]={0};
    char arr2[100]={0};

    printf("Hello world\n"); // This writes to stdout

    fgets( arr, 80, stdout );

    fseek ( stdout, 0, SEEK_SET );

    fgets ( arr2, 80, stdout );
    printf ("First array is %s\n", arr );
    printf ("Second array is %s\n", arr2 );

    return 0;

}

The output is not what I expect. That is both the arrays are empty instead of containing Hello World as I expected.

I went through this post which suggests dealing with pipes to accomplish what I want but doesn’t tell me why my above code doesn’t work?

EDIT: Though it would be nice to know alternatives to make the above work as it should, I am more curious on the problems involved in reading stdout of the same program

Answer 1

Not every file is seekable, readable or writeable. Stdout is usually a kind that can’t be read back.

Most likely, stdout will be a pipe. In that case, your program holds the writable end, and someone else holds the readable end. The pipe implementation just transfers data and does not keep it; once it has been read at the other end, there is no way to get it back.

If you want a file that can be read back, create a regular temporary file, or your own pipe, and use fprintf/fscanf instead of printf/scanf. Alternatively, do freopen on stdout to reassign it to another file/pipe, then printf will operate on that new file.