xtail

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <errno.h>
#include <limits.h>
#include <stdbool.h>

#define BLOCKSIZE 1048576

void seekt(FILE *input, int lines);
void unseekt(FILE *input, int lines);
void btail(FILE *input, long bytes);
void readprint(FILE *input);
long validate_input(const char *arg);

int main(int argc, char **argv){
    FILE *input = stdin;
    int lines = 10;
    long bytes = 0;
    bool verbose = false;

    while ((argc > 1) && (argv[1][0] == '-')){
        switch(argv[1][1]){
            case 'n':
                if(!argv[1][2]){
                    if(!argv[2]){
                        fprintf(stderr, "Option 'n' requires an argument\n");
                        exit(EXIT_FAILURE);
                    } else {
                        lines = validate_input(argv[2]);
                        argc--;
                        argv++;
                    }
                }
                else
                    lines = validate_input(argv[1]+2);
                break;
            case 'h':
                fprintf(stdout, "A helpful text\n");
                return 0;
            case 'v':
                verbose = true;
                break;
            case 'c':
                if(!argv[1][2]){
                    if(!argv[2]){
                        fprintf(stderr, "Option 'c' requires an argument\n");
                        exit(EXIT_FAILURE);
                    } else {
                        bytes = validate_input(argv[2]);
                        argc--;
                        argv++;
                    }
                }
                else
                    bytes = validate_input(argv[1]+2);
                break;
            default:
                fprintf(stderr,"Invalid option\n");
                exit(EXIT_FAILURE);
        }
        argc--;
        argv++;
    }

    if(!argv[1]){
        if(fseek(stdin, 0L, SEEK_SET) == 0){
            // stdin, seekable
            seekt(input, lines);
        }
        else{
            // stdin, unseekable
            unseekt(input, lines);
        }
    } else {
        for(int i = 1; i < argc; i++){
            input = fopen(argv[i], "rb");
            if (input == NULL){
                fprintf(stderr,"Could not open file \"%s\" for reading.\n", argv[i]);
                exit(EXIT_FAILURE);
            }
            if(verbose)
                printf("==> %s <==\n", argv[i]);
            if(bytes > 0)
                btail(input, bytes);
            else{
                size_t ret = 0;
                if((ret = fseek(input, 0L, SEEK_SET)) == 0){
                    // file, seekable
                    seekt(input, lines);
                }
                else{
                    // file, unseekable
                    unseekt(input, lines);
                }
            }
        }
    }
    return 0;
}

void unseekt(FILE *input, int lines){
    char buffer[BLOCKSIZE];
    size_t read, wrote, range;
    long target = 0;
    int lcount = 0;

    read = fread(buffer, sizeof(char), BLOCKSIZE, input);
    if(fread(buffer, 1, 1, input) != 0){
        fprintf(stderr, "Read error\n");
        exit(EXIT_FAILURE);
    }

    if(*(buffer+read-1) == '\n')
        lcount--;

    for(target = read; target > 0; target--){
        if(buffer[target] == '\n')
            lcount++;
        if(lcount == lines){
            target++;
            break;
        }
    }

    range = read-target;
    wrote = fwrite(buffer+target, sizeof(char), range, stdout);

    if(fflush(stdout) != 0 || wrote < range){
        fprintf(stderr, "Write error\n");
        exit(EXIT_FAILURE);
    }
}

void seekt(FILE *input, int lines){
    char c;
    int linenum = 0;
    long bytecount = 0;
    long fsize = 0;

    fseek(input, 0L, SEEK_END);
    fsize = ftell(input);
    fseek(input, -1L, SEEK_END);
    if((c = fgetc(input)) == '\n'){
        linenum--;
    } else ungetc(c, input);

    while(-bytecount < fsize){
        fseek(input, --bytecount, SEEK_END);
        c = fgetc(input);
        if (c == '\n')
            linenum++;
        else
            ungetc(c, input);
        if(linenum == lines)
            break;
    }

    readprint(input);
    fclose(input);
}

void btail(FILE *input, long bytes){
    long fsize;

    fseek(input, 0L, SEEK_END);
    fsize = ftell(input);

    if(bytes > fsize)
        bytes = fsize;

    fseek(input, -bytes, SEEK_END);
    readprint(input);
    fclose(input);
}

void readprint(FILE *input){
    char buffer[BLOCKSIZE];
    size_t read, wrote;

    read = fread(buffer, sizeof(char), BLOCKSIZE, input);
    if(fread(buffer, 1, 1, input) != 0){
        fprintf(stderr, "Read error\n");
        exit(EXIT_FAILURE);
    }

    wrote = fwrite(buffer, sizeof(char), read, stdout);
    if(fflush(stdout) != 0 || wrote < read){
        fprintf(stderr, "Write error\n");
        exit(EXIT_FAILURE);
    }
}

long validate_input(const char *arg){
    char *endptr = NULL;
    long num = 0;
    errno = 0;

    num = strtol(arg, &endptr, 10);
    if (arg == endptr){ // input is not a number
        fprintf(stderr, "Invalid number of lines or bytes: \'%s\'\n", arg);
        exit(1);
    }
    else if(errno == ERANGE && num == LONG_MAX){
        fprintf(stderr, "Number of lines or bytes \'%s\' is too large\n", arg);
        exit(1);
    }
    else if (errno == 0 && arg && *endptr != 0){ // more chars remain after num
        fprintf(stderr, "Invalid number of lines or bytes: \'%s\'\n", arg);
        exit(1);
    }
    else if (errno != 0 && num == 0){
        fprintf(stderr, "Unknown error\n");
        exit(1);
    }
    else if (num < 0)
        return -num;

    return num;
}