a global variable cannot be assigned twice - c

Question

I have faced some problem when doing TDD.
I designed this function:
String *subString;
String *getWordAndUpdate(String *line, char *delimiter) {
String *word = malloc(sizeof(String));
int i;
stringLeftTrim(line);
word->length = 0;
word->startIndex = 0;
for(i = line->startIndex; i < line->length; i++) {
if(line->rawString[i] != ' ') {
word->rawString[i] = line->rawString[i];
line->startIndex++;
word->length++;
} else {
break;
}
}
line->length = line->length - i;
return word;
}
With my String struct:
typedef struct {
char *rawString;
int startIndex;
int length;
} String;
When I call this getWordAndUpdate function the first time it works fine, but when i call second time it says bad memory access.
extern String *subString;
void test_getWordAndUpdate_should_get_the_first_word_from_a_line_of_instruction() {
String oneLineString = {"movwf 0x10", 0, 10};
subString = getWordAndUpdate(&oneLineString, " ,;"); // works fine
TEST_ASSERT_EQUAL('m', subString->rawString[0]);
TEST_ASSERT_EQUAL('o', subString->rawString[1]);
TEST_ASSERT_EQUAL('v', subString->rawString[2]);
TEST_ASSERT_EQUAL('w', subString->rawString[3]);
TEST_ASSERT_EQUAL('f', subString->rawString[4]);
TEST_ASSERT_NOT_EQUAL(' ', subString->rawString[5]);
TEST_ASSERT_NOT_EQUAL('0', subString->rawString[6]);
TEST_ASSERT_NOT_EQUAL('x', subString->rawString[7]);
TEST_ASSERT_NOT_EQUAL('1', subString->rawString[8]);
TEST_ASSERT_NOT_EQUAL('0', subString->rawString[9]);
TEST_ASSERT_EQUAL(5, oneLineString.startIndex);
TEST_ASSERT_EQUAL(5, oneLineString.length);
TEST_ASSERT_EQUAL(0, subString->startIndex);
TEST_ASSERT_EQUAL(5, subString->length);
subString = getWordAndUpdate(&oneLineString, " ,;"); // bad memory access
}

Answer 1

You function getWordAndUpdate modify your object oneLineString in theses lines of code:
line->length--;
line->startIndex++;
So, when you call it a second times, your object in not consistent.
Try to make a copy of oneLineString or find an algo that not modify your parameter.
Maybe, you can use this kind of function:
String *subString;
String *getWordAndUpdate(String *line, char *delimiter) {
String *word = malloc(sizeof(String));
int i;
for (i = 0; line->rawString[i] != ' '; ++i)
word->rawString[i] = line->rawString[i];
word->length = i;
return word;
}
And you don't take care yet of your delimiter parameter.