Error in dynamic creation of string array - c

Question

I am trying to read strings from file and insert them into matrix. Every line is one word.
FILE *fp = fopen("zadanie4.txt","r");
if( fp == NULL)
{
perror("Error while opening the file.\n");
exit(EXIT_FAILURE);
}
int symbol, num_of_lines = 0, len_of_string = 0, max_len = 0;
do {
symbol = fgetc(fp);
len_of_string++;
if (symbol == '\n' || feof(fp)) {
num_of_lines++;
if(len_of_string > max_len){
max_len = len_of_string;
}
len_of_string = 0;
}
} while (symbol != EOF);
fclose(fp);
printf("Number of words: %d\n", num_of_lines);
printf("Longest word: %d\n", max_len);
fp = fopen("zadanie4.txt","r");
char (*arr)[num_of_lines] = calloc(num_of_lines, sizeof(char*) * max_len);
int index = 0;
while(fscanf(fp, "%s", arr[index++]) == 1) {
printf("%s\n", arr[index - 1]); //first check to see what is written into array
}
close(fp);
printf("--------------------------\n");
int i;
for(i = 0; i < num_of_lines; i++){
printf("%s\n", arr[i]); //second check
}
I find out size of longest string and allocate memory for number of strings * longest string.
Here is how output looks like if longest word is 5 (+1 for empty '\0'):
Number of words: 6
Longest word: 6
AAAAA
BBBBB
CCCCC
DDDDD
EEEEE
FFFFF
--------------------------
AAAAA
BBBBB
CCCCC
DDDDD
EEEEE
FFFFF
If I add another char to every line:
Number of words: 6
Longest word: 7
AAAAAa
BBBBBb
CCCCCc
DDDDDd
EEEEEe
FFFFFf
--------------------------
AAAAAaBBBBBbCCCCCcDDDDDdEEEEEeFFFFFf
BBBBBbCCCCCcDDDDDdEEEEEeFFFFFf
CCCCCcDDDDDdEEEEEeFFFFFf
DDDDDdEEEEEeFFFFFf
EEEEEeFFFFFf
FFFFFf
Note: Every string is same size in this example, but I want it to work for various sizes.
Can anybody aid me how to properly allocate memory for this array?

Answer 1

Your declaration and allocation of arr are wrong and don't match.
You declare arr as a pointer to an array of num_of_lines characters. Then you allocate max_len number of pointers to character.
You can use variable-length arrays for both the outer and inner arrays:
char arr[num_of_lines][max_len + 1]; // +1 for string terminator
No need for dynamic allocation at all.

Answer 2

I believe you don't really understand how calloc works. You can't assign memory to the members of an array using
char (*arr)[num_of_lines] = calloc(num_of_lines, sizeof(char*) * max_len);
The first parameter of calloc will be the amount of blocks to allocate for an array and the second parameter will be the size of each block in bytes.
Instead you want to define the array like this
char *arr[num_of_lines];
Now you need to loop through each member and allocate memory for it
for(int i = 0; i < num_of_lines; i++) {
arr[i] = malloc(sizeof(char) * max_len);
}
Alternatively you can just define the array as a static array because you know the size of max_len.

Answer 3

You're seeing that behavior because you are over-writing the string terminator (null character == 0) at the end of each string when you add the extra character.
Instead of using one contiguous block of memory, you'll want to keep an array of string pointers, and allocate each string separately:
char *strings[];
strings = calloc(sizeof(char*),NUMBER_OF_STRINGS);
for i = 0; i < NUMBER_OF_STRINGS; i++ {
strings[i] = calloc(sizeof(char), MAX_STRING_LENGTH+1);
}
Then, when you want to add a character to a string, use strcat if the new total is less than the max string length:
strcat(strings[i],"-suffix");
Or, if it is longer, you'll need to reallocate the storage:
strings[i] = realloc(strings[i], MAX_STRING_LEN+EXTRA_BYTES+1);
strcat(strings[i],"-suffix");