CODEWITHSUNDEEP
cstdout
Pavan Manjunath
Pavan Manjunath

Reputation: 28525

Not able to read from stdout

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

Upvotes: 2

Views: 2925

Answers (3)

Juan Francisco M&#237;guez
Juan Francisco M&#237;guez

Reputation: 1

I suggest you to use gotoxy, its a very simple command where you place coordinates of the position of the stdout.

COORD coord={0,0};
void gotoxy(int x,int y){
   coord.X=x;coord.Y=y;
   SetConsoleCursorPosition(GetStdHandle(STD_OUTPUT_HANDLE),coord);
}

Upvotes: 0

hamstergene
hamstergene

Reputation: 24439

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.

Upvotes: 5

polslinux
polslinux

Reputation: 1779

This is the correct code:

#include<stdio.h>

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

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

fgets( arr, 80, stdin );

fgets ( arr2, 80, stdin );
printf("\n");
for(i=0; i<80; i++){
    printf ("%c", arr[i]);
}
for(j=0; j<80; j++){
    printf ("%c", arr2[j]);
}

return 0;
}

1) fgets has to read from stdin and not from stdout :)
2) you cannot print all the array with printf array, you must iterate into it with a for cicle

Upvotes: 0

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