으아.. Segmentation Fault 에러 ㅜ..ㅜ

과제를 하는도중에 포인터로 열심히 짰는데 함수를 한번호출하면 잘 작동하는데 두번쨰 호출부터 Segmentation fault가 뜨게됩니다.. 왜이런걸까요??

char* add(char* seq1, char* seq2){
Stack* stack;
char* start_seq1 = NULL;
char* start_seq2 = NULL;
char* end_seq1 = NULL;
char* end_seq2 = NULL;
char* result = NULL;
char character_temp;
int value = 0;
int carry = 0;

stack = (Stack*)malloc(sizeof(Stack));
start_seq1 = (char*)malloc(strlen(seq1));
start_seq2 = (char*)malloc(strlen(seq2));
result = (char*)malloc((strlen(seq2)+strlen(seq1))/2);

strcpy(start_seq1, seq1);
strcpy(start_seq2, seq2);

end_seq1 = start_seq1+strlen(seq1)-1;
end_seq2 = start_seq2+strlen(seq2)-1;

while(start_seq1 != end_seq1 && start_seq2 != end_seq2)
{
value = (int)(*end_seq1 - '0');
value += (int)(*end_seq2 - '0');
value += carry;
carry = value/10;
value %= 10;
character_temp = (char)value+'0';
push(stack,(char*)&character_temp);
end_seq1--;
end_seq2--;
}
if(start_seq1 == end_seq1)
{
value = (int)(*end_seq1 - '0');
value += (int)(*end_seq2 - '0');
value += carry;
carry = value/10;
value %= 10;
character_temp = (char)value+'0';
push(stack,(char*)&character_temp);
end_seq1--;
end_seq2--;

while((start_seq2-1) != end_seq2){
value = (int)(*end_seq2 - '0');
value += carry;
carry = value/10;
value %= 10;
character_temp = (char)value+'0';
push(stack,(char*)&character_temp);
end_seq2--;
}
}
else if(start_seq2 == end_seq2)
{
value = (int)(*end_seq1 - '0');
value += (int)(*end_seq2 - '0');
value += carry;
carry = value/10;
value %= 10;
character_temp = (char)value+48;
push(stack,(char*)&character_temp);
end_seq1--;
end_seq2--;

while((start_seq1-1) != end_seq1){
value = (int)(*end_seq1 - '0');
value += carry;
carry = value/10;
value %= 10;
character_temp = (char)value+48;
push(stack,(char*)&character_temp);
end_seq1--;
}
}
if(carry != 0){
character_temp = (char)carry+'0';
push(stack,(char*)&character_temp);
}

while(stack->count != 0)
{
strcat(result,pop(stack));
}
return result;
}

의식의 흐름대로 코드를짜서 현재는 좀 지저분합니다 ㅠㅠ