cat.c

#include <stdio.h>

int main(int argc, char *argv[]){
    FILE *fp;
    void filecopy(FILE *, FILE *);
    char *prog = argv[0];
    
    if(argc == 1)
        filecopy(stdin, stdout);
    else
        while (--argc > 0)
            if ((fp = fopen(*++argv, "r")) == NULL){
                fprintf(stderr, "%s: can't open %s \n", prog, *argv);
                exit(1);
            } else {
                filecopy(fp, stdout);
                fclose(fp);
            }
    if(ferror(stdout)){
        fprintf(stderr, "%s: error writing stdout \n", prog);
        exit(2);
    }
    exit(0);
}

void filecopy(FILE *ifp, FILE *ofp){
    int c;

    while((c = getc(ifp)) != EOF)
        putc(c, ofp);
}