Runtime error in pointer to structure - c

Question

I am doing unit testing using google C++ testing tools. I am a novice C programmer as well as tester.
The following is my code snippet :
TEST_F (def_fix, SR_M_T) {
I_M_S * arg;
RM__S_F_R_T* message_data;
char mT[MAX_M_T_LENGTH] = "data";
strcpy(message_data->mT,mT);
message_data->rUID[0] = '\0';
message_data->uT[0] = '\0';
unsigned char buffer[sizeof(RM__S_F_R_T)+2];
memcpy(&buffer[2],message_data,sizeof(RM__S_F_R_T));
buffer[0] = 1; //This assignment is not important for this test
buffer[1] = 2; //This assignment is not important for this test
arg->payload_data = buffer;
handleSR(arg);
}
What the above does is creating an incoming message. This incoming message arg is to be used as parameter to call a function handleSR (void *).
def_fix is a default fixture class which contains SetUp() and TearDown() and nothing fancy in them.
RM__S_F_R_T is a structure which contains and 3 strings.
I_M_S is a structure containing a pointer to RM__S_F_R_T.
When I google test this, it gives a segmentation error. However when I change:
RM__S_FR_R_ * message_data; to RM__S_FR_R_ message_data;and use . operator instead of -> operator, things tend to work. Can anyone explain?

Answer 1

These two statements:
RM__S_F_R_T* message_data;
strcpy(message_data->mT,mT);
are the culprit. You never initialise the RM__S_F_R_T (who designed this thing?!) pointer. When not using it as a pointer, you create an instance on the stack that you can use.
To fix either don't use a pointer (you already found that out), or allocate memory:
message_data = malloc(sizeof(RM__S_F_R_T));
don't forget to free(message_data) when you are done with the instance, or you'll leak memory.

Answer 2

It's because you declare message_data to be a pointer, but don't point it at anything. So then it will point to a random memory location, causing undefined behavior when you dereference it.
Either declare it as not a pointer, or allocate memory for it.

Answer 3

You have not provided memory to RM__S_F_R_T*.Use message_data after
message_data = (RM__S_F_R_T*)malloc(sizeof(RM__S_F_R_T));

Answer 4

message_data doesn't point to anything. You need to pass an initialised RM__S_F_R_T struct pointer to your function as an argument or initialise it inside the function itself.
Further, I presume message_data->mT is a char pointer. It's not pointing to anything currently.
You can assign space for it using malloc, or if it's a char array then make sure it's big enough to hold mT into it (provided its struct is already initialised).
The above applies to all of message_data's members.
As the other posters have already mentioned, don't forget to free() anything you have malloc()-ed.

Answer 5

Initialize I_M_S * arg and RM__S_F_R_T* message_data:
I_M_S * arg = malloc(sizeof(I_M_S));
RM__S_F_R_T* message_data = malloc(sizeof(RM__S_F_R_T));
Also, make sure you don't use these allocated pointers after the scope of function TEST_F() because you assign arg->payload_data = buffer where scope of localized variable buffer ends after the function TEST_F().
And so, free these pointers before you leave TEST_F():
free(arg);
free(message_data);