Print the given string and all of its shifted variations

caesarshiftall.c

/*
 * caesarshiftall.c
 *
 * Print the given string and all of its shifted variations.
 * Print a newline after each.
 * Return 0 on success.
 * Return non-zero on failure. See ERROR_ defines.
 *
 * Example compilation:
 * gcc caesarshiftall.c -o caesarshiftall -std=c89 -Wall -Wextra -Wconversion
 *
 * Info on Caesar shift: https://en.wikipedia.org/wiki/Caesar_cipher
 *
 * License:
 *
 * BSD Zero Clause License
 *
 * Copyright (C) 2021 by Costava
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <stdio.h>
#include <stdlib.h>

/* Wrong number of args given to program */
#define ERROR_NUM_ARGS 1
/* fputc errored */
#define ERROR_FPUTC 2
/* fputs errored */
#define ERROR_FPUTS 3

/* fputc but exit with code ERROR_FPUTC if error. */
void checked_fputc(const char ch, FILE *const stream) {
	if (fputc(ch, stream) == EOF) {
		exit(ERROR_FPUTC);
	}
}

/* fputs but exit with code ERROR_FPUTS if error. */
void checked_fputs(const char *str, FILE *stream) {
	if (fputs(str, stream) == EOF) {
		exit(ERROR_FPUTS);
	}
}

/*
 * If the given character is an uppercase or lowercase letter,
 *  shift it forward in the alphabet by 1.
 * Z becomes A. z becomes a.
 * Else return the given character.
 */
char shift(const char c) {
	if (c >= 'A' && c <= 'Z') {
		return (char)('A' + ((c - 'A' + 1) % 26));
	}
	else if (c >= 'a' && c <= 'z') {
		return (char)('a' + ((c - 'a' + 1) % 26));
	}

	return c;
}

int main(int argc, char *argv[]) {
	if (argc != 2) {
		checked_fputs("USAGE: ./caesarshiftall <string>\n", stderr);
		return ERROR_NUM_ARGS;
	}

	char *const target = argv[1];

	/* Print given string */
	checked_fputs(target, stdout);
	checked_fputc('\n', stdout);

	/* Print all shifts of given string */
	int i;
	for (i = 0; i < 25; i += 1) {
		/* Shift each letter */
		char *c = target;
		while (*c != '\0') {
			*c = shift(*c);
			c += 1;
		}

		checked_fputs(target, stdout);
		checked_fputc('\n', stdout);
	}

	return 0;
}