Runtime Error with Printing Double Pointer Char Array

Question

I am working on creating a system for entering student names and scores into arrays and printing the same information to the screen, unfortunately I keep getting the weird output.

I stepped through my program using a debugger and it showed that everything runs smooth until I get to the function that prints the student's information, there the double pointer char arrays have messed up values.

Here's an image of what I see when I run the program. (http://s28.postimg.org/nv29feawt/Error.png)

Note: Although I am aware that there are better and easier ways of doing this I am required to complete this assignment using dynamically allocated memory and arrays.

int main(void)
{
    char **firstNames;
    char **lastNames;
    float *scores;

    int recordsLength;
    printf("Please indicate the number of records you want to enter: ");
    scanf("%d", &recordsLength);
    printf("\n\n");

    firstNames = (char **)malloc(recordsLength * sizeof(char *));
    lastNames = (char **)malloc(recordsLength * sizeof(char *));
    scores = (float *)malloc(recordsLength * sizeof(float));

    int i = 0;
    while(i < recordsLength)
    {
        createNewEntry(i, firstNames, lastNames, scores);
        i++;
    }

    printEntry(0, firstNames, lastNames, scores);
    free(firstNames);
    free(lastNames);
    free(scores);
    return 0;
}

void clearScreen()
{
#ifdef _WIN32 
    system("cls");
#elif _unix_ 
    system("clear");
#endif
}

void printEntry(int entryID, char *firstNames[], char *lastNames[], float scores[])
{
    clearScreen();
    printf("|-------------------------------------------------------------------------|\n");
    printf("|                           Student Entry                                 |\n");
    printf("|-------------------------------------------------------------------------|\n|\n");
    printf("|   First Name: %s   Last Name: %s   Score: %.1f\n|\n|\n|\n", firstNames[entryID], lastNames[entryID], scores[entryID]);
    printf("|-------------------------------------------------------------------------|\n");
    printf("|                                                                         |\n");
    printf("|-------------------------------------------------------------------------|\n\n");
}

void createNewEntry(int index, char *firstNames[], char *lastNames[], float scores[])
{
    printf("Please input the records of the new student.\n\n\n");
    char first[20];
    char last[20];
    float score = 100.0f;

    printf("Please enter the student's first name: ");
    scanf("%s", &first);
    printf("\n\n");

    printf("Please enter the student's last name: ");
    scanf("%s", &last);
    printf("\n\n");

    printf("Please enter the student's score: ");
    scanf("%f", &score);
    printf("\n\n");

    firstNames[index] = (char *)malloc((strlen(first)) * sizeof(char));
    firstNames[index] = first;

    lastNames[index] = (char *)malloc((strlen(last)) * sizeof(char));
    lastNames[index] = last;
    printf("first name: %s", firstNames[index]);
    printf("last name: %s", lastNames[index]);
    scores[index] = score;
}

firstNames[index] = first; and lastNames[index] = last; look suspect. What you're doing is copying the pointer, not the array's contents. — bwinata
– bwinata, Commented Nov 14, 2014 at 6:05
I hope you have include stdio.h in your original assignment. Moreover, it would be better if you cut down your code and post a shorter version of code (minimal code that demonstrates your issue. For example a version with only firstNames and no lastNames. — Mohit Jain
– Mohit Jain, Commented Nov 14, 2014 at 6:05
you need strcpy from char first[20]; to firstNames[index] and so on, because wheen your returned from createNewEntry() your "firstNames[index]" becomes invalid — Ivan Ivanovich
– Ivan Ivanovich, Commented Nov 14, 2014 at 6:08
wheen your do this: lastNames[index] = (char *)malloc((strlen(last)) * sizeof(char)); lastNames[index] = last; you simply remove your pointer, that kept in the previous row — Ivan Ivanovich
– Ivan Ivanovich, Commented Nov 14, 2014 at 6:11

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

firstNames[index] = (char *)malloc((strlen(first)) * sizeof(char));
firstNames[index] = first;  /* You are missing your allocated memory block and assigning local */

Above lines are incorrect. You can not assign c-strings with assignation = operator. You should use strcpy for that.

You are assigning local array to firstnames, which has no life after the function ends. This invokes an undefined behavior. (You are seeing the strange characters, but it can be even worse).

Should be Re-written as (Similar for last name also)

firstNames[index] = malloc((strlen(first) + 1) * sizeof(char)); /* +1 for \0 */
if(firstNames[index] == NULL) {
  /* Malloc failed, error handling */
  ...
}
/* else */
strcpy(firstNames[index], first);  /* Use strcpy to copy contents */

Live example here

Before freeing firstNames and lastNames, you should free all the member of firstNames and lastNames in a loop.

edited Jun 20, 2020 at 9:12

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answered Nov 14, 2014 at 6:00

Mohit Jain

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1 Comment

Jonathan Leffler Over a year ago

Or you can use firstNames[index] = strdup(first); — if necessary, writing strdup(). It is a POSIX function but not a Standard C function. Also, the code should check that malloc() succeeds before using the allocated memory.

Jonathan Leffler · Accepted Answer · 2015-03-21 06:14:03Z

0

I agree with the answer of Mohit Jain, adding to that you can even use sprintf.

edited Mar 21, 2015 at 6:14

Jonathan Leffler

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answered Nov 14, 2014 at 6:09

Achyuta Aich

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1 Comment

Jonathan Leffler Over a year ago

While you could use sprintf() — or even snprintf() — it is not clear why you would want to do so. You could use memcpy() or memmove() instead of strcpy() too, if you're careful to capture the length found by strlen() — that might be better.

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