Why am I getting a segmentation fault?

Hello fellow C programmers. I am trying to write a program for homework and am having trouble when creating the basic foundation for this assignment. When I try to compile I get these errors:

Code:

---

j = 0
student[0]->first_name = SAM
student[0]->last_name =  MALONE
student[0]->month = MAY
student[0]->day = 3
student[0]->year = 1961
j = 1
Segmentation fault: 11

---

First, I need to use a struct, so there is no way around that. My logic is that I need to increase the number in the bracket attached to student so that further down the road in my program I can compare the students (eg. Without the number in the bracket, I can't think of a way to compare them). Can anyone guide me on this seg fault? I tried to debug by printing out `j` and it looks as if it makes one full loop without trouble and then it crashes when it tries to increase `student[0]` to `student[1]`
Any help would be greatly appreciated! :)

Code:

---

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


# define MAX_CHARS_IN_NAME 30


typedef struct
{
    char *first_name;
    char *last_name;
    char month[10];
    int day;
    int year;
} person;


int main()
{
    // Create a file pointer and open the file
    FILE *ifp = fopen("birthday.txt", "r");


    // Test to see if the file opens
    if (!ifp)
    {
        printf("The file is unable to open!\n");
        return -1;
    }


    // Create variables for the number of classes, number of students in the class, birthday month/day/year, number of queried students, queried students
    int i, num_classes, num_students, num_of_queried;
    char queried_student;


    // Read in the number of classes. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_classes);


    // Read in the number of students in the class. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_students);


    // Allocating space for the students in the classroom
    person *student = malloc(sizeof(person));
    student->first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);
    student->last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);


    /*
    // Allocating space for the queried student
    person *queried_student = malloc(sizeof(person));
    queried_student->first_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    queried_student->last_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    */
   
    // Loop through the text file
    for (i=0; i< num_classes; i++)
    {
        int j;


        for (j=0; j< num_students; j++)
        {


            printf("j = %d\n", j);


            // Scan in the student's first name
            fscanf(ifp, "%s", student[j].first_name);
            printf("student[%d]->first_name = %s\n", j, student->first_name);
           
            // Scan in the student's last name
            fscanf(ifp, "%s", student[j].last_name);
            printf("student[%d]->last_name =  %s\n", j, student->last_name);


            // Scan in the student's birth month
            fscanf(ifp, "%s", student[j].month);
            printf("student[%d]->month = %s\n", j, student->month);


            // Scan in the student's birth day
            fscanf(ifp, "%d", &student[j].day);
            printf("student[%d]->day = %d\n", j, student->day);
           
            // Scan in the student's birth year
            fscanf(ifp, "%d", &student[j].year);
            printf("student[%d]->year = %d\n", j, student->year);


            // Scan in the number of queried students
            //fscanf(ifp, "%d", &num_of_queried);
       
            /* int k;


            for (k=0; k< num_of_queried; k++)
            {


            } */


        }
       
       
    }




    // Sort the list in order of birthday, IGNORING THE YEAR!!!


    // print output




    fclose(ifp);
   
    return 0;
}

---

You have made a basic error.

Code:

---

    // Allocating space for the students in the classroom
    person *student = malloc(sizeof(person));
    student->first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);
    student->last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);

    /*
    // Allocating space for the queried student
    person *queried_student = malloc(sizeof(person));
    queried_student->first_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    queried_student->last_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    */

---

The problem is, though the comments say that you have a different intent, the two pieces of code both allocate one struct. So when you follow your plan and increment the number betwixt the brackets, you would be dereferencing student[1] at some point. The variable student[1] does not have any space. This causes the Segmentation fault.

Allocating arrays in C is somewhat difficult. You do rely on malloc to return memory but you can request more than one thing at a time.

Code:

---

person *student = malloc ( sizeof ( person ) * num_students );

---

Assuming the pointer student is not NULL, the pointer is allocated, and you can access up to and including num_students - 1.

The memory should be freed when you are done with it as well. You should call free() for as many pointers as you allocate with malloc() eventually.

Why do you think you can treat student as an array of structs

Code:

---

fscanf(ifp, "%s", student[j].first_name);

---

when you pointed it to an allocation for a single struct?

Code:

---

person *student = malloc(sizeof(person));

---

If you want an array, you need to create an array. Hint: What are you going to do with num_students?

I multiplied the person struct by num_students but am still getting the seg fault :(

Quote:

---

Originally Posted by C0__0D

I multiplied the person struct by num_students but am still getting the seg fault :(

---

And what about "first_name" and "last_name"? Every element of "student" is a struct which contains two char pointers, thus you need to allocate memory for each char pointer too.

Bye, Andreas

I took everyones advice and attempted to malloc enough space for the number of students:

Code:

---





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


# define MAX_CHARS_IN_NAME 30


typedef struct
{
    char *first_name;
    char *last_name;
    char month[10];
    int day;
    int year;
} person;


int main()
{
    // Create a file pointer and open the file
    FILE *ifp = fopen("birthday.txt", "r");


    // Test to see if the file opens
    if (!ifp)
    {
        printf("The file is unable to open!\n");
        return -1;
    }


    // Create variables for the number of classes, number of students in the class, birthday month/day/year, number of queried students, queried students
    int i, num_classes, num_students, num_of_queried;
    char queried_student;


    // Read in the number of classes. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_classes);


    // Read in the number of students in the class. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_students);


    // Allocating space for the students in the classroom
    person *student = malloc(sizeof(person)*num_students);
    student->first_name = malloc(sizeof((char) * MAX_CHARS_IN_NAME)*num_students);
    student->last_name = malloc(sizeof((char) * MAX_CHARS_IN_NAME)*num_students);


    /*
    // Allocating space for the queried student
    person *queried_student = malloc(sizeof(person));
    queried_student->first_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    queried_student->last_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    */
   
    // Loop through the text file
    for (i=0; i< num_classes; i++)
    {
        int j;


        for (j=0; j< num_students; j++)
        {


            printf("j = %d\n", j);


            // Scan in the student's first name
            fscanf(ifp, "%s", student[j].first_name);
            printf("student[%d]->first_name = %s\n", j, student->first_name);
           
            // Scan in the student's last name
            fscanf(ifp, "%s", student[j].last_name);
            printf("student[%d]->last_name =  %s\n", j, student->last_name);


            // Scan in the student's birth month
            fscanf(ifp, "%s", student[j].month);
            printf("student[%d]->month = %s\n", j, student->month);


            // Scan in the student's birth day
            fscanf(ifp, "%d", &student[j].day);
            printf("student[%d]->day = %d\n", j, student->day);
           
            // Scan in the student's birth year
            fscanf(ifp, "%d", &student[j].year);
            printf("student[%d]->year = %d\n", j, student->year);


            // Scan in the number of queried students
            //fscanf(ifp, "%d", &num_of_queried);
       
            /* int k;


            for (k=0; k< num_of_queried; k++)
            {


            } */


        }
       
       
    }




    // Sort the list in order of birthday, IGNORING THE YEAR!!!


    // print output




    fclose(ifp);
   
    return 0;
}

---

But now I am getting an error that says:

Code:

---

birthday.c: In function ‘main’:
birthday.c:48: error: invalid type argument of ‘unary *’
birthday.c:49: error: invalid type argument of ‘unary *’

---

Also, I am not the best when thinking about what variables need to be dynamically allocated or not, in my structs should I have everything as a pointer? Or is first name and last name only as pointers correct?

Quote:

---

Originally Posted by C0__0D

Also, I am not the best when thinking about what variables need to be dynamically allocated or not, in my structs should I have everything as a pointer? Or is first name and last name only as pointers correct?

---

Then do what we all do - get out a pencil and paper and diagram your data objects so can see what you have and/or what you want to have.

You don't "need" pointers, you could just as viably have fixed-sized arrays, but dynamic allocation allows you to not waste space. Having to accommodate a name like "Srinivasaraghavan" when most names are more like "Smith", it might make sense to use dynamic allocation. (Of course, always follow your assignment spec).

Code:

---

// Allocating space for the students in the classroom
person *student = malloc(sizeof(person)*num_students);
student->first_name = malloc(sizeof((char) * MAX_CHARS_IN_NAME)*num_students);
student->last_name = malloc(sizeof((char) * MAX_CHARS_IN_NAME)*num_students);

---

At the first line above you allocate memory for num_students person structures, i.e. you can use the memory as an array of num_students person structures.
Inside each structure you have two char pointers for which you have to allocate memory too. All in all you need 2 * num_students malloc() calls to allocate memory for all char pointers. Does this sound like using a for-loop will be convenient?

If you don't understand the above I think you should stick to fixed-sized char arrays for the moment and spend some time reading through a tutorial like Ted Jensen's "Tutorial on Pointers and Arrays".

Bye, Andreas

This is not how you allocate each student's name:

Code:

---

    student->first_name = malloc(sizeof((char) * MAX_CHARS_IN_NAME)*num_students);
    student->last_name = malloc(sizeof((char) * MAX_CHARS_IN_NAME)*num_students);

---

Each student[j] has its own name pointer. Like Tclausx says, diagram your data.

To understand your compiler error, parse malloc's argument:
sizeof(
(char) - a cast to char type, supposed to be one of the multiplication operator's argument.
* - supposed to be the multiplication operator
MAX_CHARS_IN_NAME) - supposed to be one of the multiplication operator's argument.

The compiler errors out because you cannot multiply something like this. A cast is an operation that also needs an operand, so (char)operand * MAX_CHARS_IN_NAME makes sense but what you wrote does not.

sizeof(char) is always 1 because of the C standard's requirements. Thus, multiplying when you are allocating many characters is unnecessary.

Quote:

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sizeof(char) is always 1 because of the C standard's requirements

---

I'm on my iPad and can't "control f" to search the standards draft - What section is that in?

In C99 5.2.4.2.1 Sizes of integer types <limits.h>

Quote:

---

Originally Posted by Click_here

I'm on my iPad and can't "control f" to search the standards draft - What section is that in?

---

6.5.3.4 The sizeof operator in PDF n1362.pdf (ISO/IEC 9899:201x)
in numbered sub-section/paragraph 3. In PDF n1570.pdf its is in numbered sub-section/paragraph 2.

Quote:

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3 When applied to an operand that has type char, unsigned char,or signed char,
(or a qualified version thereof) the result is 1.

---

Tim S.

I've always thought that the size of char in bytes is not guaranteed to be 1 byte.

I learn something new every day :)

Quote:

---

Originally Posted by Click_here

I've always thought that the size of char in bytes is not guaranteed to be 1 byte.

I learn something new every day :)

---

A year ago, I thought that but this sites mentions the topic about once every two months.

NOTE: I still believe it can be something other that 1 on NON C standard compilers.
Just to be a C standard compiler is must be 1.

Can anyone confirm this idea?

Tim S.

On a non-standard compiler, a char can be an old wooden ship.

If you start getting old wooden ships instead of one byte, change your compiler to a standard one.

Quote:

---

A year ago, I thought that but this sites mentions the topic about once every two months.

NOTE: I still believe it can be something other that 1 on NON C standard compilers.
Just to be a C standard compiler is must be 1.

Can anyone confirm this idea?

---

Well, on such a system, char would not be one byte in size. The unit sizeof uses is bytes. That doesn't really mean anything by itself.

In some ways if you actually have something other than 8-bit bytes, though, you're warring with historical precedent.

No matter what I try I keep getting a segmentation fault 11 error. This is very frustrating as I cannot continue with writing this program :( I have tried multiplying by num_students, putting the mallocs in a loop so that it allocates space everytime, etc. I cannot continue I am very frustrated with programming right now and my TA is inaccessible as it is spring break.

Quote:

---

Originally Posted by C0__0D

No matter what I try I keep getting a segmentation fault 11 error. This is very frustrating as I cannot continue with writing this program :( I have tried multiplying by num_students, putting the mallocs in a loop so that it allocates space everytime, etc. I cannot continue I am very frustrated with programming right now and my TA is inaccessible as it is spring break.

---

If you have given up; no reason to post.
If not, post your current code using either fixed size name C-String (char array) or using a for loop with dynamic memory for name C-string.

Tim S.

I have not given up. I don't give up THAT easily :p but am frustrated beyond imagination. Okay, I put the name arrays in the loop, here's where I am at:

Code:

---

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


# define MAX_CHARS_IN_NAME 30


typedef struct
{
    char *first_name;
    char *last_name;
    char month[10];
    int day;
    int year;
} person;


int main()
{
    // Create a file pointer and open the file
    FILE *ifp = fopen("birthday.txt", "r");


    // Test to see if the file opens
    if (!ifp)
    {
        printf("The file is unable to open!\n");
        return -1;
    }


    // Create variables for the number of classes, number of students in the class, birthday month/day/year, number of queried students, queried students
    int i, num_classes, num_students, num_of_queried;
    char queried_student;


    // Read in the number of classes. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_classes);


    // Read in the number of students in the class. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_students);


    // Allocating space for the students in the classroom
    person *student = malloc(sizeof(person)*num_students);
   
    /*
    // Allocating space for the queried student
    person *queried_student = malloc(sizeof(person));
    queried_student->first_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    queried_student->last_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    */
   
    // Loop through the text file
    for (i=0; i< num_classes; i++)
    {
        int j;


        for (j=0; j< num_students; j++)
        {


            student->first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);
            student->last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);


            printf("j = %d\n", j);


            // Scan in the student's first name
            fscanf(ifp, "%s", student[j].first_name);
            printf("student[%d]->first_name = %s\n", j, student->first_name);
           
            // Scan in the student's last name
            fscanf(ifp, "%s", student[j].last_name);
            printf("student[%d]->last_name =  %s\n", j, student->last_name);


            // Scan in the student's birth month
            fscanf(ifp, "%s", student[j].month);
            printf("student[%d]->month = %s\n", j, student->month);


            // Scan in the student's birth day
            fscanf(ifp, "%d", &student[j].day);
            printf("student[%d]->day = %d\n", j, student->day);
           
            // Scan in the student's birth year
            fscanf(ifp, "%d", &student[j].year);
            printf("student[%d]->year = %d\n", j, student->year);


            // Scan in the number of queried students
            //fscanf(ifp, "%d", &num_of_queried);
       
            /* int k;


            for (k=0; k< num_of_queried; k++)
            {


            } */


        }
       
       
    }




    // Sort the list in order of birthday, IGNORING THE YEAR!!!


    // print output




    fclose(ifp);
   
    return 0;
}

---

Also tried this:

Code:

---

for (i=0; i< num_classes; i++)
        {
                int j;


                for (j=0; j< num_students; j++)
                {


                        // Allocating space for the students in the classroom
                        person *student = malloc(sizeof(person)*num_students);
                        student->first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);
                        student->last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);


                        printf("j = %d\n", j);


                        // Scan in the student's first name
                        fscanf(ifp, "%s", student[j].first_name);
                        printf("student[%d]->first_name = %s\n", j, student->first_name);
                       
                        // Scan in the student's last name
                        fscanf(ifp, "%s", student[j].last_name);
                        printf("student[%d]->last_name =  %s\n", j, student->last_name);


                        // Scan in the student's birth month
                        fscanf(ifp, "%s", student[j].month);
                        printf("student[%d]->month = %s\n", j, student->month);


                        // Scan in the student's birth day
                        fscanf(ifp, "%d", &student[j].day);
                        printf("student[%d]->day = %d\n", j, student->day);
                       
                        // Scan in the student's birth year
                        fscanf(ifp, "%d", &student[j].year);
                        printf("student[%d]->year = %d\n", j, student->year);


                        // Scan in the number of queried students
                        //fscanf(ifp, "%d", &num_of_queried);
               
                        /* int k;


                        for (k=0; k< num_of_queried; k++)
                        {


                        } */


                }
               
               
        }

---

If it matters, my text file is:

Code:

---

2
3
JOHN SMITH MARCH 7 1968
PETER THOMPSON MAY 12 1977
STEVEN WILSON FEBRUARY 18 1959
2
JOHN SMITH
STEVEN WILSON
4
DANIEL MARNER MAY 12 1968
JOHNSON ELLINGTON JUNE 28 1960
BILLY CLINT AUGUST 10 1950
PETER FOUTS MARCH 10 1951
1
BILLY CLINT

---

Fixed so it does not seg fault for me; still has logic errors.

The two lines that were obvious wrong to me.

Code:

---

            student->first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);
            student->last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);

---

Code:

---

            student[j].first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME); /* Tim S Fixed */
            student[j].last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME); /* Tim S Fixed */

---

Tim S.

Code:

---

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


# define MAX_CHARS_IN_NAME 30


typedef struct
{
    char *first_name;
    char *last_name;
    char month[10];
    int day;
    int year;
} person;


int main()
{
    // Create a file pointer and open the file
    FILE *ifp = fopen("birthday.txt", "r");


    // Test to see if the file opens
    if (!ifp)
    {
        printf("The file is unable to open!\n");
        return -1;
    }


    // Create variables for the number of classes, number of students in the class, birthday month/day/year, number of queried students, queried students
    int i, num_classes, num_students, num_of_queried;
    char queried_student;


    // Read in the number of classes. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_classes);


    // Read in the number of students in the class. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_students);


    // Allocating space for the students in the classroom
    person *student = malloc(sizeof(person)*num_students);

    /*
    // Allocating space for the queried student
    person *queried_student = malloc(sizeof(person));
    queried_student->first_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    queried_student->last_name = malloc(sizeof(char) *MAX_CHARS_IN_NAME);
    */

    // Loop through the text file
    for (i=0; i< num_classes; i++)
    {
        int j;


        for (j=0; j< num_students; j++)
        {


            student[j].first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME); /* Tim S Fixed */
            student[j].last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME); /* Tim S Fixed */


            printf("j = %d\n", j);


            // Scan in the student's first name
            fscanf(ifp, "%s", student[j].first_name);
            printf("student[%d]->first_name = %s\n", j, student->first_name);

            // Scan in the student's last name
            fscanf(ifp, "%s", student[j].last_name);
            printf("student[%d]->last_name =  %s\n", j, student->last_name);


            // Scan in the student's birth month
            fscanf(ifp, "%s", student[j].month);
            printf("student[%d]->month = %s\n", j, student->month);


            // Scan in the student's birth day
            fscanf(ifp, "%d", &student[j].day);
            printf("student[%d]->day = %d\n", j, student->day);

            // Scan in the student's birth year
            fscanf(ifp, "%d", &student[j].year);
            printf("student[%d]->year = %d\n", j, student->year);


            // Scan in the number of queried students
            //fscanf(ifp, "%d", &num_of_queried);

            /* int k;


            for (k=0; k< num_of_queried; k++)
            {


            } */


        }


    }




    // Sort the list in order of birthday, IGNORING THE YEAR!!!


    // print output




    fclose(ifp);

    return 0;
}

---

I would use my normal fgets and sscanf instead of fscanf as you are doing to get input from file.

The only example I found on this site of it in the FAQs FAQ > Validate user input - Cprogramming.com

Note: The project directions might rule this method out.

Tim S.

This

Code:

---

student[j].first_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);
student[j].last_name = malloc(sizeof(char) * MAX_CHARS_IN_NAME);

---

is worse than this

Code:

---

typedef struct
{
    char first_name[MAX_CHARS_IN_NAME];
    char last_name[MAX_CHARS_IN_NAME];
    char month[10];
    int day;
    int year;
} person;

---

You are gaining nothing by allocating a maximum space, and therefore wasting efficiency by the extra calls to malloc. I recommend you abandon the allocation for first_name and last_name, stop worrying about efficiency, and just define the struct with fixed length arrays.

Next is this

Code:

---

// Read in the number of classes. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_classes);


    // Read in the number of students in the class. PROPERLY READ IN (TESTED)
    fscanf(ifp, "%d", &num_students);


    // Allocating space for the students in the classroom
    person *student = malloc(sizeof(person)*num_students);


---

My guess is that your file contains X classes each with Y students? If so, then your allocation is only for Y students (num_students).

So when you do this

Code:

---

for (i=0; i< num_classes; i++)
    {
        int j;


        for (j=0; j< num_students; j++)
        {
...

---

On each iteration of the outer loop, you simply overwrite the previous population of num_students structs. So you end up with just the last num_students students. Which, by the way, makes your first_name/last_name allocation all the worse, as you just abandon that allocated memory.