无法从stdin正确读取

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;
}

Answer 1

一方面，您不能期望它在关闭后可以从stdin读取：

            fclose(stdin);

Answer 2

fflush(stdin); is undefined behaviour, as is fflush on all files open only for input. That's sort of like flushing the toilet and expecting the waste to come out of the bowl, because fflush is only defined for files open for output! I would suggest something like for (int c = fgetc(stdin); c >= 0 && c != '\\n'; c = fgetc(stdin)); if you wish to discard the remainder of a line.

Furthermore, fgetc returns int for a reason: Inside the int will be an unsigned char value or EOF . c should be an int, not a char . EOF isn't a character! It's a negative int value. This differentiates it from any possible characters, because successful calls to fgetc will only return a positive integer rather than a negative EOF . fputc expects input in the form of an unsigned char value. char isn't required to be unsigned . Providing your fgetc call is successful and you store the return value into an int , that int should be safe to pass on to fputc .