Convert any number from base X to base X

base-convertor.c

char* base_converter(char* nbasefrom, int basefrom, int baseto) {
	const char* SYMBOLS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	if (basefrom<=0 || basefrom>strlen(SYMBOLS) || baseto<=0 || baseto>strlen(SYMBOLS)) {
		fprintf(stderr, "Base unallowed");
		return NULL;
	}
	int i, nbaseten=0;
	if (basefrom!=10) {
		int sizenbasefrom = strlen(nbasefrom);
		for (i=0; i<sizenbasefrom; i++) {
			int mul, mul_ok=-1;
			for (mul=0; mul<strlen(SYMBOLS); mul++) {
				if (nbasefrom[i]==SYMBOLS[mul]) {
					mul_ok = 1;
					break;
				}
			}
			if (mul>=basefrom) {
				fprintf(stderr, "Symbol unallowed in basefrom");
				return NULL;
			}
			if (mul_ok==-1) {
				fprintf(stderr, "Symbol not found");
				return NULL;
			}
			int exp = (sizenbasefrom-i-1);	
			if (exp==0) nbaseten += mul;
			else nbaseten += mul*pow(basefrom, exp);
		}
	} else nbaseten = atoi(nbasefrom);
	if (baseto!=10) { 
		char* nbaseto = malloc(sizeof(char));
		int size = 0;
		while (nbaseten>0) {
			char* nbaseto_re = realloc(nbaseto, (++size)*sizeof(char));
			if (nbaseto_re==NULL) {
				fprintf(stderr, "Memory error");
				return NULL;
			}
			nbaseto = nbaseto_re;
			int mod = nbaseten%baseto;
			if (mod<0 || mod>=strlen(SYMBOLS)) {
				fprintf(stderr, "Out of bounds error");
				return NULL;
			}
			char symbol = SYMBOLS[mod];
			nbaseto[size-1] = symbol;
			nbaseten = (int)nbaseten/baseto;
		}
		for (i=0; i<size; i++) nbaseto[i] = nbaseto[size-1-i]; 
		return nbaseto;
	} else {
		int nbasetenlen = 0, _nbaseten = nbaseten;
		while (_nbaseten>0) {
			nbasetenlen++;
			_nbaseten/=10;
		}
		char* nbaseten_chr = malloc((1+nbasetenlen)*sizeof(char));
		sprintf(nbaseten_chr, "%d", nbaseten);
		return nbaseten_chr;
	}
	return "0";
}