Why do I get a segmentation fault accessing a struct? - c

Question

Edit: added the rest of my code so it's easier to see
I'm receiving a segmentation fault when trying to access certain values of a struct in a header file.
Here's courses.c:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include "structs.h"
void createStudents () {
int random, i;
for (i = 0; i < 12; i++) {
students[i].firstName = firstName[i];
students[i].lastName = lastName[i];
random = 10000 + rand() % 89999;
students[i].num.studentNum = random;
printf("%d - %s, %s \n", students[i].num.studentNum, students[i].lastName, students[i].firstName);
}
}
void createCourses () {
int numbers[999];
int numbersLeft = 999;
char courseCode[512];
char courseCode1[512];
char courseCode2[512];
int numCourses = 3;
int random, i, j;
for (i = 0; i < 999; i++) {
numbers[i] = i;
}
for (j = 0; j < 1; j++) {
random = rand() % numbersLeft;
if (random < 10) {
snprintf(courseCode, sizeof courseCode, "CS00%d", random);
courses[0].cCode = courseCode;
}
else if (random < 100 && random > 9) {
snprintf(courseCode, sizeof courseCode, "CS0%d", random);
courses[0].cCode = courseCode;
}
else if (random > 99){
snprintf(courseCode, sizeof courseCode, "CS%d", random);
courses[0].cCode = courseCode;
}
courses[0].cName = courseName[0];
courses[0].cDescription = courseDescription[0];
numbers[random] = numbers[numbersLeft-1];
numbersLeft--;
random = 4 + rand() % 4;
courses[0].maxRegister = random;
}
for (j = 0; j < 1; j++) {
random = rand() % numbersLeft;
if (random < 10) {
snprintf(courseCode1, sizeof courseCode1, "CS00%d", random);
courses[1].cCode = courseCode1;
}
else if (random < 100 && random > 9) {
snprintf(courseCode1, sizeof courseCode1, "CS0%d", random);
courses[1].cCode = courseCode1;
}
else if (random > 99){
snprintf(courseCode1, sizeof courseCode1, "CS%d", random);
courses[1].cCode = courseCode1;
}
courses[1].cName = courseName[1];
courses[1].cDescription = courseDescription[1];
numbers[random] = numbers[numbersLeft-1];
numbersLeft--;
random = 4 + rand() % 4;
courses[1].maxRegister = random;
}
for (j = 0; j < 1; j++) {
random = rand() % numbersLeft;
if (random < 10) {
snprintf(courseCode2, sizeof courseCode2, "CS00%d", random);
courses[2].cCode = courseCode2;
}
else if (random < 100 && random > 9) {
snprintf(courseCode2, sizeof courseCode2, "CS0%d", random);
courses[2].cCode = courseCode2;
}
else if (random > 99){
snprintf(courseCode2, sizeof courseCode2, "CS%d", random);
courses[2].cCode = courseCode2;
}
courses[2].cName = courseName[2];
courses[2].cDescription = courseDescription[2];
numbers[random] = numbers[numbersLeft-1];
numbersLeft--;
random = 4 + rand() % 4;
courses[2].maxRegister = random;
}
}
void regiserStudents () {
int checkSum = 0, checkSum1 = 0, checkTemp = 0, count0 = 0, count1 = 0, count2 = 0;
int wCount0 = 0, wCount1 = 0, wCount2 = 0;
int v, i, j, random, max0, max1, max2;
max0 = courses[0].maxRegister;
max1 = courses[1].maxRegister;
max2 = courses[2].maxRegister;
for (i = 0; i < 2; i++) {
checkTemp = count0;
for (j = 0; j < 12; j++) {
random = rand() % 3;
if (random == 0) {
if (count0 == 0) {
courses[random].registered[count0] = &students[j];
count0++;
}
else {
checkSum1 = students[j].num.studentNum;
for (v = 0; v < checkTemp; v++) {
checkSum = courses[0].registered[v]->num.studentNum;
if (checkSum == checkSum1) {
/*Do Nothing*/
}
else if (count0 == max0) {
courses[random].waitlisted[count0] = &students[j];
wCount0++;
}
else {
courses[random].registered[count0] = &students[j];
count0++;
}
/*}*/
}
}
}
if (random == 1) {
if (count1 == 0) {
courses[random].registered[count1] = &students[j];
count1++;
}
else {
checkSum1 = students[j].num.studentNum;
for (v = 0; v < checkTemp; v++) {
checkSum = courses[1].registered[v]->num.studentNum;
if (checkSum == checkSum1) {
/*Do Nothing*/
}
else if (count1 == max1) {
courses[random].waitlisted[count1] = &students[j];
wCount1++;
}
else {
courses[random].registered[count1] = &students[j];
count1++;
}
}
}
}
if (random == 2) {
if (count2 == 0) {
courses[random].registered[count2] = &students[j];
count2++;
}
else {
checkSum1 = students[j].num.studentNum;
for (v = 0; v < checkTemp; v++) {
checkSum = courses[2].registered[v]->num.studentNum;
if (checkSum == checkSum1) {
/*Do Nothing*/
}
else if (count2 == max2) {
courses[random].waitlisted[count2] = &students[j];
wCount2++;
}
else {
courses[random].registered[count2] = &students[j];
count2++;
}
}
}
}
}
}
courses[0].studentRegistered = count0;
courses[1].studentRegistered = count1;
courses[2].studentRegistered = count2;
courses[0].studentWaitlisted = wCount0;
courses[1].studentWaitlisted = wCount1;
courses[2].studentWaitlisted = wCount2;
}
void printCourses () {
int i;
printf("\n%s - %s: %s\nRegistered Students (%d/%d):\n", courses[0].cCode, courses[0].cName, courses[0].cDescription, courses[0].studentRegistered, courses[0].maxRegister);
for (i = 0; i < courses[0].studentRegistered; i++) {
printf("* %d - %s, %s \n", courses[0].registered[i]->num.studentNum, courses[0].registered[i]->lastName, courses[0].registered[i]->firstName);
}
printf("Waitlisted Students (%d)", courses[0].studentWaitlisted);
if (courses[0].studentWaitlisted == 0) {
printf("\n");
}
else {
for (i = 0; i < courses[0].studentWaitlisted; i++) {
printf("* %d - %s, %s \n", courses[0].waitlisted[i]->num.studentNum, courses[0].waitlisted[i]->lastName, courses[0].waitlisted[i]->firstName);
}
}
printf("\n%s - %s: %s\nRegistered Students (%d/%d):\n", courses[1].cCode, courses[1].cName, courses[1].cDescription, courses[1].studentRegistered, courses[1].maxRegister);
for (i = 0; i < courses[1].studentRegistered; i++) {
printf("* %d - %s, %s \n", courses[1].registered[i]->num.studentNum, courses[1].registered[i]->lastName, courses[1].registered[i]->firstName);
}
printf("\n%s - %s: %s\nRegistered Students (%d/%d):\n", courses[2].cCode, courses[2].cName, courses[2].cDescription, courses[2].studentRegistered, courses[2].maxRegister);
for (i = 0; i < courses[2].studentRegistered; i++) {
printf("* %d - %s, %s \n", courses[2].registered[i]->num.studentNum, courses[2].registered[i]->lastName, courses[2].registered[i]->firstName);
}
}
And here's my header file
structs.h
#ifndef STRUCTS_H_
#define STRUCTS_H_
char *firstName[] = {
"Emma", "Liam", "Olivia",
"Noah", "Ava", "Logan",
"Sophia", "Lucas", "Isabella",
"Mason", "Shaylyn", "Jack"
};
char *lastName[] = {
"Smith", "Johnson", "Williams",
"Brown", "Jones", "Miller",
"Davis", "Garcia", "Rodriguez",
"Wilson", "Seguin", "Loveday"
};
typedef struct{
int studentNum;
}studentNumber;
typedef struct{
char *firstName;
char *lastName;
studentNumber num;
}studentID;
studentID students[12];
char *courseName[] = {"Web Programming", "Technical Communication", "Processor Architecture"};
char *courseDescription[] = {"Learn the language of HTML, and how to create websites.", "Learn the essentials of communication skills, and how to apply them on the job.", "Learn the basics of circuits and Machine Language coding."};
typedef struct {
int maxRegister;
char *cCode;
char *cName;
char *cDescription;
studentID *registered[8];
studentID *waitlisted[12];
int studentRegistered;
int studentWaitlisted;
}course;
course courses[3];
#endif
Printing the values of the registered students works fine, but when I print the waitlisted students I get a segmentation error. I used gdb and found it was on this line but couldn't figure out why:
printf("%d - %s, %s \n", courses[0].waitlisted[i]->num.studentNum, courses[0].waitlisted[i]->lastName, courses[0].waitlisted[i]->firstName);

Answer 1

studentID *waitlisted[12]; That means they will hold the address of variable of type structure studentID. You simply didn't do this but start to access those pointer variables. You have passed them to scanf which invokes undefined behavior due to trying to read an unintialized pointer variable.
So what you need to do here -
courses[i].waitlisted = malloc(sizeof *courses[i].waitlisted * courses[i].studentWaitlisted);
Same goes for the studentID also. Here fistName is a char* you need to allocate some meory to access it otherwise it's UB.
Or
typedef struct{
char firstName[100]; //you are considering that name length would be 100
//at max.
char lastName[100];
studentNumber num;
}studentID;
After OP edited question:
The seg fault you get is because of accessing a local variable after it's lifetime ended.
courses[0].cCode = courseCode;
What you can do is
courses[0].cCode = strdup(courseCode);
Initialize array int numbers[999]={0}; otherwise you might access garbage value as the indexing is result of random number geneartion.
if( random < 999){
numbers[random] = numbers[numbersLeft-1];
numbersLeft--;
}
else{
fprintf(stderr, "%s\n","Error" );
}
Lots of repeated code. You should remove repeated code. And also there is no need for single-iteration-for-loop.