Segmentation Fault 11 when assigning struct variable - c

Question

I am trying to initialize some variables in my struct, but I am getting a seg fault when assigning my front variable to equal zero. Specifically newBuff->front = 0;
typedef struct buffer{
pthread_mutex_t lock;
pthread_cond_t shout;
int front;
int rear;
char bytes[1024];
} buffer;
int main(int argc, char const *argv[]) {
FILE *file = fopen(argv[1], "r");
if (argc != 2){
printf("You must enter in a file name\n");
}
printf("%lu\n", sizeof(file));
int shmid;
char path[] = "~";
key_t key = ftok(path, 7);
shmid = shmget(key, SIZE, 0666 | IPC_CREAT | IPC_EXCL); //shared memory creation
buffer* newBuff = (buffer*) shmat(shmid, 0, 0);
newBuff->front = 0;

Answer 1

You're not checking the value of newBuff returned by shmat() to ensure that it is not invalid, e.g. (void*) -1 (per http://man7.org/linux/man-pages/man2/shmop.2.html). You also need to check the return value of shmget() to ensure that it succeeded in the first place.
Almost certainly, newBuff is -1, and trying to dereference that gives you a segfault.

Answer 2

several things I can see:
Wrong control of the arguments. You are checking them, but are you exiting your program? ;)
You are not checking the result when you invoke functions as shmat. Review the manual (man shmat).
Said the above, I can not see your whole code, but this is my recommendation:
typedef struct buffer{
pthread_mutex_t lock;
pthread_cond_t shout;
int front;
int rear;
char bytes[1024];
} buffer;
int main(int argc, char const *argv[]) {
int shmid = -1;
FILE *file = NULL;
if (argc != 2){
printf("You must enter in a file dumbass\n");
// And you must terminate here your program!
return 1;
}
file = fopen(argv[1], "r");
// Another check that you are not making and can raise a SIGVSEG
if (file == NULL) {
printf("The file '%s' can not be opened\n", argv[1]);
return 1;
}
printf("File size: %lu\n", sizeof(file));
char path[] = "~";
key_t key = ftok(path, 7);
// Another check
if (key == -1) {
fclose(f);
printf("The path '%s' does not exist or cannot be accessed\n", path);
return 1;
}
shmid = shmget(key, SIZE, 0666 | IPC_CREAT | IPC_EXCL);
// One more check
if (shmid == -1) {
fclose(f);
printf("An error happened getting shared memory identifier\n");
return 1;
}
buffer* newBuff = (buffer*)shmat(shmid, 0, 0);
// And finally! Another potential source that could raise a SIGVSEG
if (buffer == NULL) {
fclose(f);
printf("An error happened getting the shared memory area\n");
return 1;
}
newBuff->front = 0;
Please! Check every return of the functions! You can not imagine how many real problems happen because such returns are not checked properly because of bad practices.