Memory allocation failure on last loop iteration

Question

So... in the past I've been told that my questions aren't good... I believe mostly because I haven't isolated out problematic code well enough. I'll do my best to ask a pointed, concise, and to the point question in this post. I'm certainly open to suggestions about how my question could be asked better. Thanks.

I'm working on a small project in C that will serve as a prototype for a larger, buggy program that I've been working on for some time. I'm trying to work out the details in a smaller program first. I have two structs:

struct list
{
    char ownerName[20];
    int ownerAge;
    char sex;
}owner;

and

struct list2
{
    char petName[20];
    char owner[20];
    char animal[4];
    char breed[50];
    char color[20];
}pets;

The program is supposed to fgets ownerName from user input and compare it to ".owner" in the pets struct. The ownerName and petName elements should then be copied into an array, and the name of the owner and his/her pets will be printed in a list. While I'm aware I don't need the owner struct to accomplish this, I'm using it to model the other program I'm writing.

I'm using

if (strcmp(pets[i].owner, name) == 0) 

to compare the struct elements and seem to have this part down.

The variable j counts the number of records that meet this criteria, and the variable l = j + 1. I call the array using:

char *petsList[l];

The size of the array is dictated by l (j + 1) because I need j elements for the petNames + 1 element for the owner name.

I've also created a pointer to the petsList array via the following:

char *(*ptr)[l] = &petsList

The owner name is added to the array via the following command:

(*ptr)[0] = (char *)malloc(sizeof(name));
strcpy ( (*ptr)[0], name);

The petNames are added to the the array petsList using a for loop. I've initialized i = 1 to prevent petsList[0] from being overwritten and am trying to write petNames to the array via the following loop:

 i = 1;

        for (k=0; k < PETS; k++)
        {
            if (strcmp(pets[k].owner, name) == 0)
            {
                (*ptr)[i] = (char *)malloc(sizeof(pets[k].petName));
                if (!*(ptr)[i])
                {
                    puts("\nMemory Allocation Error");
                    exit (1);
                }
                strcpy( (*ptr)[i], pets[k].petName);
                i++;
             }
         }

Let's say for a given input of name, I get three pets that match. The loop iterates the first two times just fine, but then on the third iteration of the loop, I get a memory allocation error. This happens on the last iteration of the loop consistently. For example, if I have 2 pets associated with the ownerName, the list will run the first iteration fine and fail on the second; if I have 4 pets associated with the ownerName, the loop will run fine the first 3 times and fail on the fourth, so it appears that the final iteration of the loop consistently fails. I've tried changing the code a number of times, but am now at a loss for how I can move forward with this program. Any help is greatly appreciated.

Thanks.

Answer 1

This is a little weird. Maybe:

if(!*(ptr)[i])

should be

if(!(*ptr)[i])

?

Answer 2

Don`t cast the malloc return value.

Since I can`t really check with a minimal example.

char *(*ptr)[l] = &petsList

why do you make such a complex construct? I am not even sure what it is supposed to accomplish. do you want all pets in the first index and the next index to contain the owner? this could be accomplished just with the petslist

what structure do you really need at the end?

is it something like:

array:

0 = owner
1 = pet 1
2 = pet 2

or something like

0,0 = owner       1,0 = owner 2        etc.
0,1 = pet 1       1,1 = pet 3
0,2 = pet 2       1,2 = pet 4

Ok here is a working example of what you want to do. you can easily extend it to do the second data arrangement. If you have any questions feel free to ask

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define PETAMOUNT   40

struct list2
{
    char petName[20];
    char owner[20];
    char animal[4];
    char breed[50];
    char color[20];
};

int main() {
    struct list2 *pets;             // list of all pets
    char name[128];                 // contain name of the owner, get from stdin

    unsigned int i;                 // i and j are both counter variable
    unsigned int j;

    fgets(name, 128, stdin);        // get string from stdin
    name[strlen(name) - 1] = '\0';  // remove newline

    pets = malloc(PETAMOUNT * sizeof(struct list2));    // allocate memory for the list of all pets
    if (pets == NULL) {
        printf("malloc err\n");
        exit(1);
    }

    for (i = 0; i < PETAMOUNT; i++) {           // initialize some pets and some owners
        strcpy(pets[i].petName, "petname ");
        strcpy(pets[i].owner, "owner ");
        pets[i].petName[7] = i + '0';           // there are PETAMOUNT of petnames. petname0, petname1 etc
        pets[i].owner[5] = (i / 4) + '0';       // there are PETAMOUNT / 4 owners. owner0 has petname0 to petname3, owner1 has petname4 to 7 etc
    }


    char ***petslist;                       // a list of list of strings or 3d char array
    petslist = malloc(sizeof(char **));     // allocate pointer to contain a double array
    petslist[0] = malloc(sizeof(char *));   // allocate a pointer to contain the name of the owner
    if (petslist[0] == NULL) {
        printf("malloc err\n");
        exit(1);
    }
    petslist[0][0] = malloc(strlen(name) + 1); // allocate memory to contain the owner
    if (petslist[0][0] == NULL) {
        printf("malloc err\n");
        exit(1);
    }
    strcpy(petslist[0][0], name);             // copy owner into the first index

    for (i = 0, j = 1; i < PETAMOUNT; i++) {      // go through all pets 
        if (strcmp(pets[i].owner, name) == 0) {   // if the owner of the current pet is the same as the inputted owner  
            petslist[0] = realloc(petslist[0], (j + 1) * sizeof(char *));   // allocate pointer for the next pet
            petslist[0][j] = malloc(strlen(pets[i].petName) + 1);           // allocate memory to contain the chars of the pet
            if (petslist[0][j] == NULL) {
                printf("malloc err\n");
                exit(1);
            }
            strcpy(petslist[0][j], pets[i].petName);    // copy the petname into the array
            j++;
        }
    }

    puts("petslist:");              // print it all out
    for (i = 0; i < j; i++) {
        printf("|%s|\n", petslist[0][i]);
    }

    exit(0);

    }

currently I always write to the [0][0] but if you realloc you can make room for more after that