Join strings by a separator using varargs.

join-strings.c

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

static
size_t compute_joined_length(const char* separator,
                             const char* first,
                             va_list argp) {
	size_t seplen = strlen(separator);
	size_t length = strlen(first);
	const char* next = va_arg(argp, const char*);

	while (next != NULL) {
		length += seplen + strlen(next);
		next = va_arg(argp, const char*);
	}

	return length;
}


char* join_strings(const char* separator, const char* first, ...) {
	size_t joined_length;
	size_t separator_length = strlen(separator);
	char* result = NULL;
	va_list ap;
	
	va_start(ap, first);
	/* There's a one extra character to this length to account for the
	 * null character. */
	joined_length = compute_joined_length(separator, first, ap) + 1;
	va_end(ap);
	
	result = malloc(sizeof(char) * joined_length);
	if (result != NULL) {
		strcpy(result, first);

		va_start(ap, first);
		{
			size_t base_offset = strlen(first);
			const char* next = va_arg(ap, const char*);
			while (next != NULL) {
				strcpy(result + base_offset, separator);
				strcpy(result + base_offset + separator_length, next);
				base_offset += strlen(next) + separator_length;
				next = va_arg(ap, const char*);
			}
		}
		va_end(ap);
		result[joined_length - 1] = '\0';
	}
	return result;
}

At line 35 you have one malloc and there is no free(result) after.
You functions allocs for every call a length of joinded_lenght bytes without freeing the memory.