Basic set of functions to encode/decode line breaks in a string. Useful when trying to store program outputs in a text/plain line-based serialization format.

encode-crlf.c

/* This is a quite simple set of functions that turns multi-line strings in
   single line strings with the line breaking characters backslash escaped.
   
The encode_crfl does the following transformation.

transform : character -> string
transform '\n' = "\\n"
transform '\r' = "\\r"
transform '\\' = "\\\\"
transform c = c

   The mirror transformation is provided by decode_crlf and is smart enough
   to identify invalid codifications (in which case it returns NULL).
*/
       
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include <errno.h>

static size_t
compute_encoded_length(const char* input) {
	size_t original_length = strlen(input);
	size_t i;
	size_t result = 0;

	for (i = 0; i < original_length; i++) {
		char c = input[i];
		result += (c == '\\' || c == '\r' || c == '\n') ? 2 : 1;
	}

	return result;
}


char*
encode_crlf(const char* input) {
	size_t original_length = strlen(input);
	size_t encoded_length = compute_encoded_length(input);
	size_t i;
	size_t j;
	char* result;

	result = malloc(sizeof(char) * (encoded_length+1));
	if (result == NULL) {
		return NULL;
	}

	for (i = 0, j = 0; i < original_length; i++, j++) {
		char c = input[i];
		
		if (c == '\\' || c == '\r' || c == '\n') {
			result[j++] = '\\';
			result[j] = (c == '\\') ? '\\'
			          : (c == '\r') ? 'r'
			          : 'n'; /* can only be \n */
		}
		else {
			result[j] = c;
		}
	}
	result[j] = '\0';
	return result;
}


char*
decode_crlf(const char* input) {
	char* result;
	size_t length = strlen(input);
	size_t i;
	size_t j;

	result = malloc(sizeof(char) * (length + 1));
	if (result == NULL) {
		return NULL;
	}

	for (i = 0, j = 0; i < length; i++, j++) {
		if (input[i] == '\\') {
			char c = input[i+1];
			if (c != '\\' && c != 'n' && c != 'r') {
				/* Invalid codification. Free up memory and set errno */
				free(result);
				errno = EDOM;
				return NULL;
			}
			result[j] = (c == '\\') ? '\\'
			          : (c == 'r') ? '\r'
			          : '\n'; /* Can only be n */
			i++;
		}
		else {
			result[j] = input[i];
		}
	}
	result[j] = '\0';

	return result;
}