packs an "array" of strings into a flattened string "array" with supporting offset array to get each string.

flat_string.c

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

/**
 * returns a string that contains multiple strings.
 * 'offsets' is used to store the offset of each string.
 * 'len' is the number of strings and offsets written.
 */
char *new_flat_string(size_t **offsets, size_t *len, ...) {
	va_list args; va_start(args, len);
	
	/// first run: collect total length.
	size_t total_len = 0;
	size_t count = 0;
	for(;;) {
		char const *sub = va_arg(args, char const*);
		if( sub==NULL ) {
			break;
		}
		
		total_len += (strlen(sub) + 1);
		++count;
	}
	*len = count;
	va_end(args);
	
	/// next, allocate our buffers and then fill in 'buf'.
	char *buf = calloc(total_len, sizeof *buf);
	if( buf==NULL ) {
		return NULL;
	}
	
	*offsets = calloc(*len, sizeof **offsets);
	if( *offsets==NULL ) {
		free(buf);
		return NULL;
	}
	
	va_list reads; va_start(reads, len);
	char *iter = buf;
	size_t offs = 0;
	for( size_t i=0; i < *len; i++ ) {
		char const *sub = va_arg(reads, char const*);
		if( sub==NULL ) {
			break;
		}
		strcat(iter, sub);
		(*offsets)[i] = offs;
		size_t const n = strlen(sub) + 1;
		offs += n;
		iter += n;
	}
	va_end(reads);
	return buf;
}

Example Usage:

int main() {
    size_t *idxs = NULL;
    size_t len = 0;
    char *strs = new_flat_string(&idxs, &len, "first", "second", "third", "1", "keks", NULL);
    for( size_t i=0; i < len; i++ ) {
        printf("%zu : '%s'\n", i, &strs[idxs[i]]);
    }
    free(idxs); idxs = NULL;
    free(strs); strs = NULL;
}