Inputing array of strings inside a function - c

Question

I am trying to input to this array of strings with myFgets, but it is telling me that I have an access violation error when I do so.
What am I doing wrong here?
#include <stdio.h>
#include <string.h>
void myFgets(char str[], int size)
{
fgets(str, size, stdin);
str[strcspn(str, "\n")] = 0;
}
void enterFriends(char** friendsArr)
{
int i = 0;
for (i = 0; i < 3; i++)
{
printf("enter friend %d: ", i + 1);
myFgets(friendsArr[i], 10);
}
}
int main(void)
{
char friendsArr[3][10] = { ' ' };
int i = 0;
enterFriends(friendsArr);
for (i = 0; i < 3; i++)
{
puts(friendsArr[i]);
}
return 0;
}

Answer 1

Continuing from the comment, your access violation is because friendsArr is type char (*)[10] which is not the same as char **. (the first is a pointer-to-array, the second is a pointer-to-pointer) Type controls pointer arithmetic. (e.g. the offset between p and p + 1). Your 3 x 10 2D array is an array of 1D arrays.
Since an array is converted to a pointer to its first element1, friendsArr is converted to a pointer to the first array (e.g. a 10-character array), which is why the formal type on access is char (*)[10] (a pointer to array of 10-characters). So when you access the next pointer, the offset is to the beginning of the next 1D array (offset of 10-characters/bytes).
When you attempt to pass the array as char **, the offset is no longer 10-character, but instead sizeof (a_pointer). So the offset will be either 4-bytes on x86 of 8-bytes on x86_64. So what happens is you read the first string into the first 1D array, advance 8-bytes (or 4-bytes) and then attempt to read the next string into the same 1D array beginning at the 8th (or 4th) element of the array corrupting the input.
Which brings up the next critical point you are missing. You must ALWAYS check the return of every input function used to determine whether the input succeeded or failed. It is 100% valid for the user to press ctrl + d (or ctrl + z on windows) to generate a manual EOF to cancel input. By checking the return of fgets() you can properly handle that case. Further, any function you write that takes input must have a meaningful return type that can communicate success or failure of the input back to the caller.
In myFgets(), returning a pointer to the string on success (or NULL on failure) would tell enterFriends() if that input succeeded or failed. Since enterFriends() also relies on that return, it too needs a meaningful return type to communicate how many inputs succeeded. Which brings up an additional point, you cannot simply loop a fixed number of times in enterFriends(), but instead need to condition your loop on a successful read of the number of friends less than your maximum. If the user cancels input, you still want to be able to return the number that were successfully entered.
(note: you can adjust the test to require that all inputs are made, re-prompting for the canceled input -- up to you)
Don't use MagicNumbers in your code (e.g. 3, 10), instead:
#define NFRIENDS 3 /* if you need a constant, #define one (or more) */
#define MAXC 1024
(note: Rule: Don't Skimp on Buffer Size, 10 is way to short for user input. Unless you are on an embedded system with limited physical memory, then a 256-byte to 2048-byte buffer is fine. For example, GCC defines BUFSIZ as 8192, VS defines it as 512)
This also plays into whether your code is mixing VLAs (Variable Length Arrays) as parameters with your declarations of Plain-Old arrays in main(). While the difference is largely semantics in this case, be aware that compiler support for VLAs is optional beginning with C11.
So putting that altogether, you can rewrite your code slightly, tweaking the function parameters and returns to avoid VLAs and provided minimal necessary returns to communicate the success or failure of input and how many friends were read as follows:
#include <stdio.h>
#include <string.h>
#define NFRIENDS 3 /* if you need a constant, #define one (or more) */
#define MAXC 1024
/* every function that takes input must provide a meaningful return
* that can indicate success or failure to the caller.
* returns pointer to string on success, NULL otherwise.
*/
char *myFgets (char *str, int size)
{
if (!fgets (str, size, stdin)) { /* validate EVERY input */
puts ("(user canceled input)");
return NULL;
}
str[strcspn(str, "\n")] = 0; /* good job using strcspn() */
return str;
}
/* do not use MagicNumbers, pass a parameter for number of friends */
int enterFriends (char (*friendsArr)[MAXC], size_t size, int nfriends)
{
int i = 0;
while (i < nfriends) { /* can't use a fixed number */
printf ("enter friend %d: ", i + 1);
if (!myFgets (friendsArr[i], size)) /* if user cancels, break loop */
break;
i += 1; /* only increment after good read */
}
return i; /* return number of friends read */
}
int main(void)
{
char friendsArr[NFRIENDS][MAXC] = {""}; /* initialize all elements zero */
int i = 0,
friends = 0;
friends = enterFriends (friendsArr, MAXC, NFRIENDS);
for (i = 0; i < friends; i++) {
puts (friendsArr[i]);
}
}
Example Use/Output
$ ./bin/friendsarray
enter friend 1: Mickey Mouse
enter friend 2: Minnie Mouse
enter friend 3: Goofy (the dog)
Mickey Mouse
Minnie Mouse
Goofy (the dog)
or intentionally canceling the last input:
$ ./bin/friendsarray
enter friend 1: Mickey Mouse
enter friend 2: Minnie Mouse
enter friend 3: (user canceled input)
Mickey Mouse
Minnie Mouse
which properly preserves the two good inputs before the cancellation occurred.
Look things over and let me know if you have further questions.
Footnotes:
1. C11 Standard - 6.3.2.1 Other Operands - Lvalues, arrays, and function designators(p3)

Answer 2

The way that you are passing a 2D array in a function is not correct.
Simply pass in the array size along with it.
Eg:
void enterFriends(int m,int n,char friendsArr[m][n]){
}
// while calling that function
enterFriends(3,10,friendsArr);
Reference link: https://www.techiedelight.com/pass-2d-array-function-parameter/
Full Code:
#include <stdio.h>
#include <string.h>
void myFgets(char str[], int size)
{
fgets(str, size, stdin);
str[strcspn(str, "\n")] = 0;
}
void enterFriends(int m,int n,char friendsArr[m][n])
{
int i = 0;
for (i = 0; i < 3; i++)
{
printf("enter friend %d: ", i + 1);
myFgets(friendsArr[i], 10);
}
}
int main(void)
{
char friendsArr[3][10] = { ' ' };
int i = 0;
enterFriends(3,10,friendsArr);
for (i = 0; i < 3; i++)
{
puts(friendsArr[i]);
}
return 0;
}

Answer 3

void myFgets(char str[], int size)
{
fgets(str, size, stdin);
str[strcspn(str, "\n")] = 0;
}
void enterFriends(size_t size, char (*friendsArr)[size])
{
int i = 0;
for (i = 0; i < 3; i++)
{
printf("enter friend %d: ", i + 1);
myFgets(friendsArr[i], 10);
}
}
int main(void)
{
char friendsArr[3][10] = { ' ' };
int i = 0;
enterFriends(10, friendsArr);
for (i = 0; i < 3; i++)
{
puts(friendsArr[i]);
}
}