I've got a project that involves creating a text game. I'm creating a struct for each player and putting them in an array. I'm then trying to pass in data and then pass by pointer the array to other functions, however I keep on getting segmentation faults (Although on the odd occasion working fine). I've summarised below.
#include <stdio.h>
#include <stdlib.h>
typedef struct
{
char name[9];
int cardsHeld;
int hand[8];
} Player;
void printNames(Player** playerArray)
{
for (int i = 0; i < 3; i++)
{
fprintf(stdout, "%s\n", playerArray[i]->name);
}
}
void gamesetup()
{
int count;
fprintf(stdout, "How many players will be partaking in 'The Game'? ( 1 - 5)\n");
fscanf(stdin, "%d", &count);
Player** playerArray = (Player**)malloc(sizeof(Player*) * count);
for(int i = 0; i < count; i++)
{
playerArray[i] = (Player*) malloc(sizeof(Player));
fprintf(stdout, "Please enter the name for player %d.\n\n", i + 1);
fscanf(stdin, "%s", playerArray[i]->name);
}
printNames(playerArray);
}
int main(int argc, char** argv)
{
gamesetup();
return 0;
}
My questions are;
Is the fscanf getting the address of the Player.name member? I'm getting confused whether the -> operator should deference the value of the struct member or since its in an array the address?
I'm not sure why it works sometimes but not others. If it works sometimes fundamentally it should be ok. Is the malloc function allocating memory it should not or is the fscanf putting data in the wrong place.
Thank you.
-EDIT-
Changed the code so it is in a complete program that appears to work without seg faults. I think that my issues arise from not freeing the memory before termination is messing it up next time I run it without compiling first. I'm still not sure why fscanf works as in my mind the argument playerArray[i]->name is returning the value, not the address.
I've worked it out where I was confused. Thank you for all your help in the comments.
The member I am accessing in my array is a string of chars so the first member is a pointer. By using fscanf(stdin, "%s",playerArray[i]->name); This deferenced the pointer (an address) so it works. I was getting in a muddle as it was an member of an array of structs. The segfaults were caused by me messing with the code to try and fix what already worked.
Related
I'm new to the C programming language and beginning to mess around with pointers and allocating memory to objects. I wanted to make a simple program that would read in (x) elements from the user and simply print them out using functions.
This was my inital code at the beginning. The code was reading in the user input properly but it gave the wrong output and crashed when displaying elements within the main function.
int main() {
int *myArr;
int myArrSize;
getInputArr(&myArrSize, &myArr);
for (int i = 0; i < myArrSize; i++) {
printf("Element No.%i: %i\n", i, myArr[i]);
}
free(myArr);
return 0;
}
void getInputArr(int *arrSize, int *arr) {
printf("Please Enter Length of Array: \n");
scanf("%i", arrSize);
arr = (int *) malloc(*arrSize * sizeof(int));
printf("Enter %i Numbers!\n", *arrSize);
for (int i = 0; i < *arrSize; i++) {
scanf("%i", &arr[i]);
}
}
After messing around, I finally got it to work using double pointers but I am unsure how it completely works, could someone explain why the code below behaves as expected?
int main() {
int *myArr;
int myArrSize;
getInputArr(&myArrSize, &myArr);
for (int i = 0; i < myArrSize; i++) {
printf("Element No.%i: %i\n", i, myArr[i]);
}
free(myArr);
return 0;
}
void getInputArr(int *arrSize, int **myArr) {
printf("Please Enter Length of Array: \n");
scanf("%i", arrSize);
*myArr = (int *) malloc(*arrSize * sizeof(int));
printf("Enter %i Numbers!\n", *arrSize);
for (int i = 0; i < *arrSize; i++) {
scanf("%i", &((*myArr)[i]));
}
}
There are several surprising things in your implementation, but in the end they all make sense, and indeed they must be present in order for this implementation to work.
You ultimately want int *myArr in main to simulate an array of int, but in getInputArr you refer to it using a "double pointer" int **myArr. But this makes sense, because any time you want to return something from a function "by reference" like this, you need to use a pointer -- an extra pointer. To return an int by reference, you'd use an int *. But to return an int * by reference, you need an int **. The fact that you (correctly) call getInputArr(&myArrSize, &myArr) in main shows that getInputArr's second argument is going to be an int **.
In getInputArr, when you call scanf, you do not have an & next to the argument you pass to scanf for %d to read into. This is highly unusual, but in this case it's absolutely correct, because arrSize is already a pointer.
You then have *myArr = (int *) malloc(*arrSize * sizeof(int)). This was the first thing I spotted in your initial (nonworking) implemenation that was quite wrong. In getInputArr, myArr is a pointer to the pointer that you want to set. So *myArr is the pointer that you want to set.
Finally, you have the jawbreaker call scanf("%i", &((*myArr)[i]));. This looks pretty ugly, and there are probably simpler ways to write it, but it's correct. Let's break it down. Again, myArr is a pointer to the pointer you want to work with. So *myArr is the pointer you want to work with. So (*myArr)[i] is one element of the simulated array (pointed to by the pointer) that you want to work with. You need explicit parentheses, because if you wrote *myArr[i] this would mean, "take the i'th element pointed to by myArr, interpret it as a pointer, and take the contents." But what you want (and, with the parentheses, you have) is "take myArr, interpret it as a pointer, take the thing that it points to, which is *myArr, and interpret that as a pointer, and finally take the i'th element that it (the second pointer) points to."
You've got multiple levels of pointers confusing you. But what if the dynamically allocated array you're dealing with lives inside a struct? Then we only have to deal with the pointer passing ("reference") semantics on that struct.
Consider the below. See the comments for explanation.
/* Basic stuff. */
#include <stdio.h>
#include <stdlib.h>
/* Here's our struct.
* It contains the size of the array,
* and the pointer to the memory allocated for the array.
*/
typedef struct dyn_int_array {
size_t size;
int * array;
} dyn_int_array_t;
/* Forward declarations for a function which creates and
* returns our dynamic_int_array struct.
*/
dyn_int_array_t * create_dyn_int_array();
/* ... and here's where you see that we don't want to
* pass the struct by value, but rather effectively by
* reference by passing a pointer to it.
*/
void scan_into_dyn_int_array(dyn_int_array_t * da);
int main(void) {
dyn_int_array_t * da = create_dyn_int_array();
/* I never bothered to free up the allocated memory,
* because it's not really critical for demonstration here.
*/
}
The implementations of those functions are below, but aren't really critical to this demonstration, as you've hopefully see the pass by reference use of pointers, without having to directly worry about or get confused by two levels of pointer indirection.
dyn_int_array_t * create_dyn_int_array() {
dyn_int_array_t * result = malloc(sizeof(dyn_int_array_t));
fprintf(stdout, "Input an array size: ");
fscanf(stdin, "%zd", &(result->size));
result->array = malloc(sizeof(int) * result->size);
/* Because "da" is already a pointer to dye_int_array_t
* there's no need to get its address.
*/
scan_into_dyn_int_array(result);
return result;
}
void scan_into_dyn_int_array(dyn_int_array_t * da) {
for (int i = 0; i < da->size; i++) {
/* We do have to pass the address of the current
* element of the array to fscanf.
*/
fscanf(stdin, "%d", &(da->array[i]));
}
}
I want to order an array of structs by the first letter of a student´s name. The code that I made so far is the following:
#include <stdio.h>
#include <stdlib.h>
typedef struct{
int cod;
char* name;
int mark;
}student;
void print(student* class){
int i;
for (i=0;i<4;i++){
printf("%d\n",class[i].cod);
printf("%s\n",class[i].name);
printf("%d\n",class[i].mark);
printf("\n");
}
}
int main(int argc, char *argv[])
{
int ind,i;
int cod=1000;
student class[4];
student temp;
int lengthData=10;
for (i=0;i<4;i++)
{
class[i].name=malloc(sizeof(char)*lengthData);
}
class[0].cod=cod;
class[0].name="Joseph";
class[0].mark=15;
cod++;
class[1].cod=cod;
class[1].name="Jonathan";
class[1].mark=16;
cod++;
class[2].cod=cod;
class[2].name="Karen";
class[2].mark=17;
cod++;
class[3].cod=cod;
class[3].name="Anna";
class[3].mark=20;
print(class);
for (ind=1;ind<4;ind++){
temp=class[ind];
i=ind-1;
while (i>=0){
if (temp.name[0]<class[i].name[0]){
class[i+1]=class[i];
class[i]=temp;
i--;
}
else break;
}
}
printf("ordered data\n");
print(class);
system("PAUSE");
return 0;
}
I am using DevC++ and when I run it the program hangs, but when I add the following lines before the loop for the bubble sort (only for testing):
class[3]=class[2];
printf("%s\n",class[3].name);
for (ind=1;ind<4;ind++){
...
The program works even though one record (3) has been replaced by the data of record (2).
Any help?
You have quite a few problems:
First of all you leak memory, since you allocate memory and make name point to that, then you make name point somewhere else. You need to copyinto the memory you allocate.
Secondly, also with the name member, once you copy into the memory, you will go out of bounds since you only allocate five bytes for each string, but you have strings of up to at least nine characters (ten with the terminator).
Thirdly, and more about going out of bounds, your class array only have three elements, yet you access four elements of the array.
The reassignment of the name pointer won't cause more problems than a temporary memory leak, since you don't attempt to pass the pointer to free. The second problem isn't really an issue because you don't copy the strings yet. The third problem on the other hand, that will lead to undefined behavior as soon as you execute that code.
I have a function that looks through a number of matches from an array and find all teams in matches, that meet some conditions. When found they need to be assigned to a new array. The new array should be used as an output parameter.
I get segmentation fault: 11 when I call it. I have tried to debug but cannot seem to get why. Following is declared in main:
TEAM team_least_viewers;
double spectators = 99999;
solve_task_four(round, team, &team_least_viewers, &spectators);
And the function itself:
void solve_task_four(ROUND *round, TEAM *team, TEAM *team_least_viewers, double *spectators) {
int i, j, k = 0;
for(i=0; i<ROUNDS_PR_SEASON; i++) {
for(j=0; j<MATCH_PR_ROUND; j++) {
if(round[i].match[j].year == 2015) {
/* Searching for team name in team[]*/
for(k=0; k<NUMBER_OF_TEAMS; k++) {
/* If it matches */
if (round[i].match[j].home_team == team[k].name) {
team[k].spectators_home_last_year += round[i].match[j].spectators;
}
}
}
}
for(k=0; k<NUMBER_OF_TEAMS; k++) {
if(team[k].spectators_home_last_year < *spectators) {
*spectators = team[k].spectators_home_last_year;
}
}
}
}
The structs as requested:
typedef struct {
char weekday[WEEKDAY_SIZE], start_time[START_TIME_SIZE],
home_team[TEAM_SIZE], away_team[TEAM_SIZE];
double spectators;
int day, month, year, round, home_team_score, away_team_score;
} MATCH;
typedef struct {
MATCH match[MATCH_PR_ROUND];
} ROUND;
typedef struct {
char *name;
int points, matches_played,
matches_won, matches_draw, matches_lost,
matches_won_home, matches_won_away,
goals_for, goals_against, goal_difference;
double spectators_home_last_year;
} TEAM;
Any help is much appreciated.
I infer your questions is: How do I figure out what is causing the segmentation fault? If that's right, then one answer is to use a debugger. Another answer would be to add print statements throughout the code. The segfault is almost certainly one of the array indexings, like round[i] or round[i].match[j], so be sure to print the i and j values. You may be indexing past the end of an array or dereferencing a null pointer or an uninitialized pointer, so print the pointer values, like printf("round[%d] at %p\n", i, &round[i]).
SIGSEGV on several operating systems is signal 11, and is delivered to the process on a segmentation fault.
Segmentation faults occur when your program accesses memory in a way which isn't allowed, usually by attempting to dereference a null pointer or running off the end of an array.
In your program, the most likely culprits are are array indexes, round[i].match[j] and team[k]. (Another possibility would be if the spectator argument passed were not a valid location for writing, but this is unlikely in this particular case.) You may wish to insert code/run in a debugger to check whether each access is correct.
In particular, assuming that your ROUNDS_PR_SEASON &c. values are correct, it seems most likely that some round[i].match contains a null, if your round array was not fully initialized.
I've been struggling with this one for a few hours now and I'm at a loss as to what's happening. This is the code for program.c:
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#define SPACE 32
#define INITIAL 4
typedef struct {
char *town;
char *country;
} town_t;
typedef struct {
int num_towns, current_size;
town_t **towns_list;
} index_t;
int main(int argc, char *argv[]) {
index_t town_index;
town_index.current_size = INITIAL;
town_index.towns_list = malloc(town_index.current_size * sizeof(*(town_index.towns_list)));
assert(town_index.towns_list != NULL);
printf("Step: %d\n", 1);
town_index.towns_list[0]->town = malloc(4 * sizeof(*(town_index.towns_list[0]->town)));
printf("Step: %d\n", 2);
assert(town_index.towns_list[0]->town != NULL);
return 0;
}
On Linux this is how it runs:
./program
Step: 1
Segmentation fault
but on Windows it prints out
program.exe
Step: 1
Step: 2
as I'd expect, which really isn't helping. For the Linux output, however, clearly the first print statement is being executed but not the second, which would lead me to think that the line between is that one at fault. Particularly, I think doing town_index.towns_list[0] is causing me issues, but I cannot say why.
This is a relatively complex data structure, so maybe I'm getting lost at some point. Basically town_index is meant to be a index struct that contains the current number of towns in towns_list and current_size which reflects the space currently available to save towns. It also contains an array of pointers to town_ts which contain the name and country as strings.
I've tried to use Valgrind, but it's really not helping out much. Here's a Pastebin for those who want to see.
This is a simplified scenario of what I was experiencing in another program, so don't any mind magic numbers and whatnot.
This is on VirtualBox Linux Mint 64-bit.
Unrelated question, if anyone can: How do I get Valgrind to display the precise lines? I see that everywhere else online, but my output just tells me the folder in which the program and function is, which isn't much help.
You initialized town_index.towns_list, but not town_index.towns_list[0], so town_index.towns_list[0]->town is undefined behaviour.
You missed something like
for (int i = 0; i < town_index.current_size; ++i)
town_index.towns_list[i] = malloc(sizeof **town_index.towns_list);
for the second dimension.
town_index.towns_list and town_index.towns_list[0] are not the same. You initialize town_index.towns_list but town_index.towns_list[0] is equal to 0. The crash caused by dereferencing town_index.towns_list[0]
I'm just starting on the road the learning C, and ran into some difficulty:
The code listed below is giving me the following error:
Attaching to program: `/workfolder/cocoa/c_stuff/bookshelf/build/Debug/bookshelf', process 1674.
Cannot access memory at address 0xa0df194
Cannot access memory at address 0xa0df194
// code start
#define MAX_NAME_LENGTH 200
#define MAX_AUTHOR_LENGTH 200
#define MAX_DESCRIPTION_LENGTH 1000
#define MAX_PUBLISHER 200
#define MAX_ISBN 50
//structures<
typedef struct {
char title[MAX_NAME_LENGTH];
char author[MAX_AUTHOR_LENGTH];
char ISBN[MAX_ISBN];
char description[MAX_DESCRIPTION_LENGTH];
char publisher[MAX_PUBLISHER];
} Book;
void getUserInput(Book *s[])
{
printf("what is the book's title ?\n");
fgets(s[book_count]->title, MAX_NAME_LENGTH, stdin);
printf("what is the author's name?\n");
fgets(s[book_count]->author, MAX_AUTHOR_LENGTH, stdin);
printf("what is the ISBN?\n");
fgets(s[book_count]->ISBN, MAX_ISBN, stdin);
printf("write a short description\n");
fgets(s[book_count]->description, MAX_DESCRIPTION_LENGTH, stdin);
printf("what is the book's publisher\n");
fgets(s[book_count]->publisher, MAX_PUBLISHER, stdin);
printf("want to add another book ? Y\\N\n");
book_count++;
if(tolower(fgetc(stdin)) == 'y')
{
return getUserInput(s);
}
else
{
return;
}
}
int main (int argc, const char * argv[]) {
// insert code here...
Book *book_shelf[100];
if((book_shelf[0] = (Book *)malloc(sizeof(Book))) == NULL)
{
exit(1);
}
getUserInput(book_shelf);
return 0;
}
The code compiles properly, and the function runs fine the first time (all the questions get asked and the struct receives the data); but when the user types 'y' to add another book, the mem error occurs.
Any ideas where the error is happening?
Thanks in advance!
You've only ever allocated memory for the first book in main - after that it tries to write to the next slot in the array, which doesn't point to an allocated block of memory, giving you a seg-fault. You're going to have to allocate memory for each book you want to read in.
In addition, since C doesn't know how long an array is, you have to pass that information along into function calls. (And I don't see where you're defining book_count.)
You might try something along these lines:
void getUserInput(Book *s[], int *book_count, int max_book_count)
{
if (book_count == max_book_count) return; // If we've filled all the slots, we can't add anymore without causing trouble.
s[book_count] = malloc(sizeof(Book));
..
if(tolower(fgetc(stdin)) == 'y')
{
(*book_count)++;
getUserInput(s, book_count, max_book_count);
}
return;
}
int main (int argc, const char * argv[]) {
// insert code here...
Book *book_shelf[100];
int book_count = 0;
getUserInput(book_shelf, &book_count, 100);
// Make sure to free all the malloc'd data
}
Even better in this situation, would just be using a loop and skipping the whole recursion step.
int main (int argc, const char * argv[]) {
// insert code here...
Book *book_shelf[100];
char response = 'y';
int book_count = 0;
while (book_count < 100 && response == 'y')
{
book_shelf = malloc(sizeof(Book));
response = getUserInput(book_shelf[book_count++]);
}
// make sure to free all the allocated data!
}
char getUserInput(Book *book)
{
// write input straight to book
printf("what is the book's title ?\n");
fgets(book->title, MAX_NAME_LENGTH, stdin);
...
return tolower(fgetc(stdin));
}
Unless I'm reading something wrong, you haven't defined book_count before using it as an array subscript.
Within main, you allocated on the stack an array of 100 pointers to the Book Structure. I believe it was your intent to allocate 100 structures and then pass the address to that block of structures to getUserInput
Change main to:
Book book_shelf[100];
...
getUserInput(book_shelf);
...
EDIT: OOPS Missed the single Book malloc mentioned in the earlier post. That ones Correct for the first book. If you edit as above and eliminate the
if (book_shelf[0]...) check, you'll accomplish your intended results
You allocate just space for the firstbook, not for the others (malloc in main)
I guess there is some code missing, no declaration and initialization of book_count
You should use loops instead of recursion
Use not recursion but loops for this kind of repetition
Recursion is probably overkill for this problem where a simple do { ... } while(user keeps answering yes) would do. However the problem you having is in main with your Book *book_shelf[100]. There are several ways you could solve this problem.
First change it to an array of Book's like samills suggests:
Book book_shelf[100];
and then change your getUserInput to something like this:
getUserInput(Book *book_shelf, int offset, int length) {
if(offset < 0 || offset >= length) {
return;
}
//...
return getUserInput(book_shelf, offset + 1, length)
}
Or you could use your existing code and change you getUserInput function to look something like this and remove the malloc from main:
getUserInput(Book *book_shelf) {
book_shelf[book_count] = (Book*)malloc(sizeof(Book));
// ...
}
props for correct use of the sizeof operator (I see that thing misused so often it makes my eyes bleed).
As in Josh's answer, by adding the following lines to your code should make it work:
book_count++;
if(tolower(fgetc(stdin)) == 'y')
{
if((book_shelf[book_count] = (Book *)malloc(sizeof(Book))) == NULL)
{
printf("Cannot allocate memory for Book");
exit(1);
}
return getUserInput(s);
}
else
{
return;
}
However, I encourage you not to use the recursive function for getting input. Recursive can lead to difficulties in debugging. You may consider using normal loop instead.
Note: I'm assuming the book_count is global variable which has been initialized to 0
thanks a lot for the replies!
I realized that I hadn't malloc-ed enough memory to handle more then one element of the struct array (Exactly what Josh is saying). So essentially:
Book *book_shelf;
if(book_shelf = (Book*)malloc(sizeof(Book)) == NULL)//exit code
so the second time around I would hit a memory issue.
thanks again!
Looks like your still doing it wrong:
Book *book_shelf;
if(book_shelf = (Book*)malloc(sizeof(Book)) == NULL)//exit code
book_shelf is only the size of a pointer. When you do the malloc you only allocate one Book at a time. This is wrong. You need to allocate contiguous memory for an array of Book objects all in one instanciation of an array.
Like
Book book_shelf[100];
not
Book *book_shelf[100];
or using malloc, use your pointer to point to an array instanciated using
100*malloc(sizeof(Book)).
You may get lucky that no other heap memory is allocated in between your malloc(sizeof(Book)) calls and that the memory management system is alocating contiguous memory by default. Also, book_shelf will only point to the last malloced Book structure, not the first one as you indicated you want in your original question.
Josh is also not allocating enough memory at one time. Use a linked list if you want to keep extending elements to the end of your book_shelf one-by-one.
factorial with pointer and recursion
#include<iostream.h>
#include<conio.h>
int show(int *p)
{
int f;
int x=*p;
if(*p==1) //boundry checking for recursion
return 1;
else
f=x*show(&(--*p)); //this code is similar to f=x*show(n-1); with non-pointers
return f;
}
void main()
{
int a=6;
int b=show(&a);
cout<<b;
getch();
}