Difficulty with array - seg fault after just 2 assignments - c

Question

it's my first post so I apologize in advance if I've posted the code format wrong.
I've been trying to work out where I'm going wrong here for awhile now and haven't been able to find an answer. I keep getting a segmentation fault after two Lines of a text file have been scanned into my arrays. Text file follows the pattern of : City1 City2 distance.
I feel like it has something to do with the memory but I can't understand why.
#include <stdio.h>
#include <stdlib.h>
#include <string.h> //Chosen to use this library to break text file down.
#include "list.h" //Using file created in lab 3 earlier this year.
#define DYNAMIC_RESIZE 0 ///Might not be needed...
#define Max_Lines 40
#define LINE_SIZE 150
int main()
{
FILE *Distances_File = fopen("Distances.txt", "r");
char *City1[Max_Lines];
char *City2[Max_Lines];
int *Distances[Max_Lines];
City1[Max_Lines] = malloc(sizeof(Max_Lines));
City2[Max_Lines] = malloc(sizeof(Max_Lines));
Distances[Max_Lines] = malloc(sizeof(Max_Lines));
char File_Line[LINE_SIZE];
int Line_Count = 0;
if (!Distances_File) {
printf("File could not open");
return 1;
}
if ( Distances_File != NULL )
{
///Intro to the program.
printf("This is a program that will calculate the shortest distance between selected \ncities.");
printf("\nSo wish me luck :(\n \n");
while(fgets(File_Line, sizeof(File_Line), Distances_File))
{
//printf("%s", File_Line);
sscanf(File_Line, "%s%s%s", City1[Line_Count], City2[Line_Count], Distances[Line_Count]);
///Still issue of Distances being a char.
printf("%s\n%s\n%s\n\n", City1[Line_Count], City2[Line_Count], Distances[Line_Count]);
Line_Count++;
printf("%d", Line_Count);
}
}
}

Answer 1

This statement
char *City1[Max_Lines];
declare an array of char pointers and the size of the array is Max_Lines.
And here you are allocating memory to an invalid index of the array:
City1[Max_Lines] = malloc(sizeof(Max_Lines));
Max_Lines value is 40, so the valid index of the array of size Max_Lines will be 0-39.
You need to allocate memory to all the pointers of array City1 and City2 before using them.
May you write a function to perform this allocation, like this:
void allocate_city_mem(char *arr[], size_t sz) {
for(size_t i = 0; i < sz; i++) {
arr[i] = malloc(LINE_SIZE);
if (NULL == arr[i])
exit(EXIT_FAILURE);
}
}
In doing so, you need to make sure to free the dynamically allocated memory once you have done with it. You can do:
void free_city_mem(char *arr[], size_t sz) {
for(size_t i = 0; i < sz; i++) {
free(arr[i]);
arr[i] = NULL;
}
}
In your program, I can see that the Max_Lines and LINE_SIZE are small values, so as an alternative you can do this:
char City1[Max_Lines][LINE_SIZE];
With this, you don't need to take care of any allocation/deallocation of memory.
Also, no need to take the array of integer pointer for storing distance. Distances could be an array of integers.
In your code, you are calling fopen() and checking the return value of it (if (!Distances_File) {....) somewhere below in the code after calling malloc. As a good programming practice, you should immediately check the return value of library function in such cases because if they fail, there is no point in proceeding further.
Also, when using scanf family functions, make sure that format specifier corresponding to the parameter passed should be correct.
Collectively all above points, the main() will be something like this:
int main() {
FILE *Distances_File = fopen("Distances.txt", "r");
if (!Distances_File) {
printf("File could not open");
return 1;
}
char City1[Max_Lines][LINE_SIZE];
char City2[Max_Lines][LINE_SIZE];
int Distances[Max_Lines];
char File_Line[LINE_SIZE];
int Line_Count = 0;
//Intro to the program.
printf("This is a program that will calculate the shortest distance between selected \ncities.");
printf("\nSo wish me luck :(\n \n");
while(fgets(File_Line, sizeof(File_Line), Distances_File)) {
printf("Line number : %d\n", Line_Count+1);
//printf("%s", File_Line);
sscanf(File_Line, "%s%s%d", City1[Line_Count], City2[Line_Count], &Distances[Line_Count]);
///Still issue of Distances being a char.
printf("%s\n%s\n%d\n\n", City1[Line_Count], City2[Line_Count], Distances[Line_Count]);
Line_Count++;
}
return 0;
}

Answer 2

You have undefined behavior accessing City1[Max_Lines] where the size of the array is Max_Lines. You should change it instead allocate memory and make those pointers point to it. So you will do something like this:-
#define MAXLEN 100
...
for(size_y i = 0; i < Max_Lines; i++){
City1[i] = malloc(MAXLEN);
/* check malloc return value */
}
And also here you can simply do this too,
char City1[Max_Lines][MAXLEN];
Because here if you want to get MAXLEN byte buffer then instead of dynamic allocation you can do this. The only problem is that if Max_lines and MAXLEN is larger then there is a possibilty of constrained by stack size. Then dynamic allocation will be a rescue.
Also you should check the return value fopen. There are cases when fopen fails. You need to handle those cases separately.

Answer 3

try below variant:
#include <stdio.h>
#include <stdlib.h>
#include <string.h> //Chosen to use this library to break text file down.
//#include "list.h" //Using file created in lab 3 earlier this year.
#define DYNAMIC_RESIZE 0 ///Might not be needed...
#define Max_Lines 40
#define LINE_SIZE 150
typedef struct {
char data[LINE_SIZE];
} LineBuffer;
int main()
{
FILE *Distances_File = fopen("Distances.txt", "r");
LineBuffer *City1;
LineBuffer *City2;
int *Distances;
char File_Line[LINE_SIZE];
int Line_Count = 0;
City1 = (LineBuffer *)malloc(sizeof(LineBuffer)*Max_Lines);
City2 = (LineBuffer *)malloc(sizeof(LineBuffer)*Max_Lines);
Distances = (int *)malloc(Max_Lines);
if (!Distances_File) {
printf("File could not open");
return 1;
}
if ( Distances_File != NULL ) {
///Intro to the program.
printf("This is a program that will calculate the shortest distance between selected \ncities.");
printf("\nSo wish me luck :(\n \n");
while(fgets(File_Line, sizeof(File_Line), Distances_File) && Line_Count < Max_Lines)
{
//printf("%s", File_Line);
sscanf(File_Line, "%s%s%d", City1[Line_Count], City2[Line_Count], Distances[Line_Count]);
///Still issue of Distances being a char.
printf("%s\n%s\n%d\n\n", City1[Line_Count], City2[Line_Count], Distances[Line_Count]);
Line_Count++;
printf("%d", Line_Count);
}
}
free(City1);
free(City2);
free(Distances);
}
edit
sizeof(int) should be 4 bytes so malloc(sizeof(Max_Lines)) will allocate 4 bytes