gists/c-programming/io/freadln.c

/*
 * freadln.c
 *
 * The `freadln` function reads a line from STDIN into a string, allocating
 * memory for it.
 *
 * Author: Intel A80486DX2-66
 * License: Creative Commons Zero 1.0 Universal
 *
 * TODO: Figure out potential problems
 * TODO: Add 'flushing' of STDIN (while there are characters, read them) before
 * the input reading loop to avoid input queues
 * [optional] TODO: Handle EOF in a special way to indicate EOF
 */

#include "freadln.h"

int freadln(FILE* f, char** output, size_t* length_out) {
	/*
	 * The length of STDIN line is counted without any terminating characters.
	 *
	 * return value:
	 * 	freadln_OK: no errors, the length of STDIN line has been stored in
	 * 	`length_out`
	 * 	freadln_ERROR: an error occurred (see errno)
	 * 	>= 0: length of read line
	 */

	if (output == NULL)
		return freadln_ERROR;

	// NOTE: if the file is STDIN, flush STDOUT before waiting for input to
	// make sure a prompt is displayed
	fflush(stdout);

	freadln_length_type length = 0; // initial length

	*output = malloc((length + 1) * sizeof(char));
	if (*output == NULL)
		return freadln_ERROR;

	int character;
	while ((character = fgetc(f)) != EOF
	       /* stop on a newline character: */ && character != '\n') {
		(*output)[length] = (char) character;

		// integer overflow and integer limit check, to keep array boundaries
		if ((freadln_length_type) (length + 2) <= (freadln_length_type) length)
		{
			errno = ERANGE;
			freadln_success_epilogue;
		} else
			length++;

		char* temp = realloc(*output, (length + 1) * sizeof(char));

		// If the function fails to allocate new memory, return the string that
		// has already been accumulated.
		if (temp == NULL) {
			// keep errno;
			freadln_success_epilogue;
		}

		*output = temp;
	}

	errno = 0;
	freadln_success_epilogue;
}

#ifdef TEST
int main(void) {
	// stdin test
	printf("Type something> ");
	char* line;
	if (finreadln(&line, NULL) != freadln_OK) {
		perror("freadln");
		exit(EXIT_FAILURE);
	}
	printf("Input string: '%s'\n", line);

	// file test
#define TEST_FILE "freadln_test.txt"
	FILE* f = fopen(TEST_FILE, "w");
	if (f == NULL) {
		perror("fopen");
		exit(EXIT_FAILURE);
	}
	fprintf(f, "Hello, world!\nAnother line\n\n");
	FILE* new_f = freopen(TEST_FILE, "r", f);
	if (new_f == NULL) {
		perror("freopen");
		exit(EXIT_FAILURE);
	}
	f = new_f;
	for (int i = 0; i < 4; i++) {
		size_t line_length;
		if (freadln(f, &line, &line_length) != freadln_OK) {
			perror("freadln");
			exit(EXIT_FAILURE);
		} else if (feof(f)) {
			printf("File: EOF, breaking the loop\n");
			break;
		}
		printf("File, line #%d: '%s' (%zu characters)\n", i + 1, line,
		       line_length);
	}
	fclose(f);

	return 0;
}
#endif