Not reading from stdin properly

Question

I'm trying to mimic the behavior of the unix utility cat, but when I call a command of the form:

cat file1 - file2 - file3

My program will output file1 correctly, then read in from stdin, then when I press EOF, it will print file 2 then file 3, without reading from stdin for the second time.

Why might this be?

    #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ASCII_LENGTH 255
int printfile(FILE *source, int N);

int main(int argc, char *argv[])
{
    int currentarg = 1; //the argument number currently being processed

    FILE *input_file;
    //if there are no arguments, dog reads from standard input
    if(argc == 1 || currentarg == argc)
    {
        input_file = stdin;
        printfile(input_file,0);
    }
    else
    {
        int i;
        for(i = currentarg; i < argc; i++)
        {
            printf("%d      %s\n",i,argv[i]);
            //if file is a single dash, dog reads from standard input
            if(strcmp(argv[i],"-") == 0)
            {
                input_file = stdin;
                printfile(input_file,0);
                fflush(stdin);
                fclose(stdin);
                clearerr(stdin);
            }
            else if ((input_file = fopen(argv[i], "r")) == NULL)
            {
                fprintf(stderr, "%s: %s: No such file or directory\n", argv[0], argv[i]);
                return 1;
            }
            else
            {
                printfile(input_file,0);
                fflush(input_file);
                fclose(input_file);
                clearerr(input_file);
            }
        }
    }
    return 0;
}

int printfile(FILE *source, int N) 
{
    //used to print characters of a file to the screen
    //characters can be shifted by some number N (between 0 and 25 inclusive)
    char c;
    while((c = fgetc(source)) != EOF)
    {
        fputc((c+N)%ASCII_LENGTH,stdout);
    }
    printf("*****    %c     %d",c,c==EOF);
    return 0;
}

I don't suppose you considered looking at the source code for cat — paddy
fopen fails for many reasons other than ENOENT. Check the documentation for strerror and perror. Don't make up reasons: let the system tell you. — William Pursell
You close stdin, no wonder it is closed the next time around ;-) — vonbrand

NPE NPE · Accepted Answer · 2013-01-20T23:12:51

For one thing, you can't expect to be able to read from stdin after you've closed it:

            fclose(stdin);

Not reading from stdin properly

2 Answers