Understanding scanf() in C (Seg Fault)

Question

I don't understand getting input in C. I have the following code which is producing a segmentation fault:

int main(int argc, char *argv[]){
 while (fgets(buffer, MAX_LEN + 1, input) != NULL) {
  get_command(t, buffer);
 }
return 0;
}

and

static void get_command(Table *t, char *command) {
    COMMAND command_name = 0;
    char *valid_args_str[] = {"aa","bb"};
    int arg_counter = 1;

 scanf("%s", command);

 if (!strncmp(command, "in", 2)) {
  command_name = INSERT;
  while(arg_counter){
   printf("whileloop reached, arg_counter is %d", arg_counter);
   scanf("%s", valid_args_str[arg_counter - 1]);
  }
.
.
.
}

The print statement isn't even being evaluated before i get a segmentation fault. Whats going on?

What is while (fgets(buffer, MAX_LEN + 1, input) != NULL)doing? What is scanf() doing? Can I have one scanf() follow by another? I intent for the arguments to be on the same line.

Thanks!

UPDATE:

Here is updated code. Still same problem. Hopefully this will be a more complete clip:

typedef enum { INSERT = 1, SEARCH, DELETE, RESET, DISPLAY} COMMAND;

static void get_command(Table *t, char *command) {
    COMMAND command_name = 0;
    char valid_args_str[] = {"aa","bb"};
    int arg_counter = 0;
    char other_buffer[MAX_LEN + 1];

    sscanf(command, "%s", other_buffer);

 if (!strncmp(other_buffer, "in", 2)) {
  command_name = INSERT;
  while(++arg_counter){
   printf("whileloop reached, arg_counter - 1 is %d\n", arg_counter - 1);
   scanf("%s", valid_args_str[arg_counter - 1]);
  }
 } else if (!strncmp(other_buffer, "se", 2)) {
.
.
.
execute_comm(t, command_name, valid_args_str);
}

static void execute_comm(Table *t, COMMAND command, char *args[]){
 switch(command){
 case INSERT:
  insert(t, args[0], args[1]);
  break;
.
.
}

and here is my main:

int main(int argc, char *argv[]){

 FILE *input;
 char buffer[MAX_LEN + 1];
 input = stdin;
 if(argc > 2){
  fprintf(stderr, "%s", "Too many arguments");
  perror("Too many arguments");
  exit(EX_OSERR);
 } else if (argc == 2){
  if ((input = fopen(argv[1], "r")) == NULL) {
   fprintf(stderr, "%s", "error opening files");
   perror("error opening file");
   exit(EXIT_FAILURE);
  }
 }

 while (fgets(buffer, MAX_LEN + 1, input) != NULL) {
  get_command(t, buffer);
 }
 fclose(input);
 return 0;
}

What I want to happen is for it to be able to read a file like this:

insert beans lima

delete beans

etc. and from standard input. Some commands have two arguments, some have one, and some none.

Thanks again

Answer 1

I suspect your buffer variable is not allocated. Do you declare as such?

char* buffer;

If so, that is why you are seg faulting.

You need to allocate memory for the buffer.

char buffer[1024];

Answer 2

fgets is reading an entire line (up to newline) from the file in variable FILE* input, and putting that line in buffer.

scanf is reading a sequence of non-whitespace from standard input, and writing that into command, which appears to be the same pointer as buffer.

Something here is almost certainly not what you intended. Maybe you meant

sscanf(command, "%s", other_buffer);