I'm trying to make a dynamic char array, but I'm running into issues. I get segmentation fault when I try to add anything to the array.
static int arrays()
{
int INITIAL = 1;
char **all_words;
all_words = (char**)malloc(INITIAL*sizeof(*all_words));
int currentSize = INITIAL;
for(int i = 0; i < counter; i++){
all_words = realloc(all_words,currentSize*sizeof(*all_words));
strcpy(all_words[i], "hello");
currentSize++;
}
for(int i = 0; i < counter; i++){
printf("%s ", all_words[i]);
}
}
I pretty much copied this from a guide online, so I'm not sure why it wouldn't work.
You've correctly allocated an array of char *, however those pointers remain uninitiaized. So when you then do this:
strcpy(all_words[i], "hello");
You're dereferencing an invalid pointer.
Each element of all_words needs to point to allocated space. This simplest way to do this is to use strdup if your system supports it instead of strcpy:
all_words[i] = strdup("hello");
Otherwise you would use malloc to allocate the space, then use strcpy
all_words[i] = malloc(sizeof("hello"));
strcpy(all_words[i], "hello");
I have created double pointer char to be used as a 2d array to store strings. The append function is meant to add the string provided to the end of the array, the num_strings pointer is provided to keep track of the elements in the array (since I can't use sizeof). It seems that at some point, the function isn't allocating enough memory but I can't seem to figure out where and can't find any other issues.
I have already tried giving both the outer array and the inner array large amounts of memory, much more than they need. The issue persists. I have also tried copying the string to the array after the function had run.
int main(int argc, char *argv[]) {
char **strings = NULL;
int num_strings = 0;
append(&strings, &num_strings, "Alex");
append(&strings, &num_strings, "Edward");
// Do things with array
for (int i = 0; i < num_strings; i++) {;
printf("%s\n", strings[i]);
}
// Free memory after use
for (int i = 0; i < num_strings; i++) {
free(strings[i]);
}
free(strings);
strings = NULL;
return 0;
}
void append(char ***array, int * num_strings, char *string) {
if (*array == NULL) {
*array = malloc(sizeof(*array)); // start with enough room for 1 item (pointer)
} else {
// reallocate memory for new item
*array = realloc(*array, (((*num_strings) + 1) * sizeof(*array)));
}
printf("Char Size: %lu\n", sizeof(char));
printf("Given Size: %lu\n", sizeof(***(array)));
*(array[*num_strings]) = malloc((strlen(string) + 1) * sizeof(***(array + 0)));
strcpy(*(array[*num_strings]), string);
(*num_strings)++; // increment the number of strings
}
The output of the program should be the two strings, at the moment it only prints the first and then crashs due to the segmentation fault.
The problem is there are a couple instances of *(array[*num_strings]) that should be (*array)[*num_strings].
The difference is that the first form tries to index through the pointer passed to the function, as if the passed strings were an array, corrupting the caller's stack. The corrected version first derefernces the pointer, then indexed through the target as desired.
There are also a few places where sizeof(*array) is used where it should be sizeof(**array). x = malloc(sizeof(x)) is never correct. But this isn't causing a visible problem.
In the main method , i am creating an array of pointers to string
in the add method i am reallocating the array size and adding x elements which i dont know
while coming back to main how can i know the new size of the array , i mean the number of elements int the array ?
Here is my code .. (it has some bugs)
#include <stdio.h>
void add(char ***x);
int main()
{
char **s;
s = (char **) malloc(sizeof(char *));
int i;
add(&s);
for( i=1;i<=?????(**find the new size of the array** );i++)
puts(*s[i]);
return 0;
}
void add(char ***x)
{
- ** // alter arry add x random datas to the array of string pointer**
/*
s[1]="Hello";
s[2]="Need";
s[3]="a help";
s[4]="and help";
s[5]="for the help";
*/
char **data;
int i = 0;
for (i = 1; i <= 5; i++)
{
data = (char **) realloc(*x, 1 * sizeof(char *));
data[i] = (char *) malloc(i * sizeof(char *));
strcpy(data[i], "first");
}
}
can some one please point and fix the bug in the code..
(Sidenote:
can some one please point and fix the bug in the code..
hey, isn't that what debuggers are for?)
Long story short, keep track of it manually:
char **func_that_reallocs(char **ptr, size_t *sz)
{
char **tmp = realloc(ptr, new_size * sizeof(*ptr));
*sz = new_size;
return tmp;
}
And please do not cast the return value of malloc()!
Always add one entry more to the array as needed and set this additional last entry to NULL.
Then write a function which scans the array until it find this NULL-pointer and return the number of entries counted up until then and you are done.
It's the same concept as for a C-"string", with the only difference of using a NULL instead of '\0' as (array-)terminator.
Some people call this last element also the "stopper"-element.
The positive thing about this approach is, one does not have to keep the array's size in a different variable, which might get out of sync with the real size of the array: The size is implicitly given by the array itself.
I'm relatively new to C in general and I'm having a problem with some code. It's pretty simple code: The objective of the code is to copy a given array of char pointers, or char **source, in other words, to a given char **destination.
The issue I'm having is that sometimes (usually when I have more than 2 strings in source) the first element gets completely corrupted and when I end up printing out destination, it will print out something like ";#?" for the first element, with the other elements printing fine.
The code that performs the copying is:
void CopyArrayOfStrings(char **source, int numStrings)
{
char **destination = malloc(numStrings);
for (int i = 0; i < numStrings; i++)
{
destination[i] = malloc(strlen(source[i] + 1);
strcpy(destination[i], source[i]);
}
}
Note that I left out the code that checks if the result of malloc is NULL.
You need to change your allocation of destination as :
char **destination = malloc(numStrings*(sizeof(char*)));
to allocate number of char * pointers to hold strings.
Also verify you are appropriately passing char ** as source array of strings.
You're doing it wrong.
void CopyArrayOfStrings(char **source, int numStrings)
{
char **destination = malloc(numStrings * sizeof(char *));
for (int i = 0; i < numStrings; i++)
{
destination[i] = malloc(strlen(source[i]) + 1);
strcpy(destination[i], source[i]);
//alternatively you can use strdup() as suggested by #Christoffer
}
}
This will give you storage space for numStrings arrays. Each element of which, will point to a null-terminated string.
I know how to create an array of structs but with a predefined size. However is there a way to create a dynamic array of structs such that the array could get bigger?
For example:
typedef struct
{
char *str;
} words;
main()
{
words x[100]; // I do not want to use this, I want to dynamic increase the size of the array as data comes in.
}
Is this possible?
I've researched this: words* array = (words*)malloc(sizeof(words) * 100);
I want to get rid of the 100 and store the data as it comes in. Thus if 76 fields of data comes in, I want to store 76 and not 100. I'm assuming that I don't know how much data is coming into my program. In the struct I defined above I could create the first "index" as:
words* array = (words*)malloc(sizeof(words));
However I want to dynamically add elements to the array after. I hope I described the problem area clearly enough. The major challenge is to dynamically add a second field, at least that is the challenge for the moment.
I've made a little progress however:
typedef struct {
char *str;
} words;
// Allocate first string.
words x = (words) malloc(sizeof(words));
x[0].str = "john";
// Allocate second string.
x=(words*) realloc(x, sizeof(words));
x[1].FirstName = "bob";
// printf second string.
printf("%s", x[1].str); --> This is working, it's printing out bob.
free(x); // Free up memory.
printf("%s", x[1].str); --> Not working since its still printing out BOB even though I freed up memory. What is wrong?
I did some error checking and this is what I found. If after I free up memory for x I add the following:
x=NULL;
then if I try to print x I get an error which is what I want. So is it that the free function is not working, at least on my compiler? I'm using DevC??
Thanks, I understand now due to:
FirstName is a pointer to an array of char which is not being allocated by the malloc, only the pointer is being allocated and after you call free, it doesn't erase the memory, it just marks it as available on the heap to be over written later. – MattSmith
Update
I'm trying to modularize and put the creation of my array of structs in a function but nothing seems to work. I'm trying something very simple and I don't know what else to do. It's along the same lines as before, just another function, loaddata that is loading the data and outside the method I need to do some printing. How can I make it work? My code is as follows:
# include <stdio.h>
# include <stdlib.h>
# include <string.h>
# include <ctype.h>
typedef struct
{
char *str1;
char *str2;
} words;
void LoadData(words *, int *);
main()
{
words *x;
int num;
LoadData(&x, &num);
printf("%s %s", x[0].str1, x[0].str2);
printf("%s %s", x[1].str1, x[1].str2);
getch();
}//
void LoadData(words *x, int * num)
{
x = (words*) malloc(sizeof(words));
x[0].str1 = "johnnie\0";
x[0].str2 = "krapson\0";
x = (words*) realloc(x, sizeof(words)*2);
x[1].str1 = "bob\0";
x[1].str2 = "marley\0";
*num=*num+1;
}//
This simple test code is crashing and I have no idea why. Where is the bug?
You've tagged this as C++ as well as C.
If you're using C++ things are a lot easier. The standard template library has a template called vector which allows you to dynamically build up a list of objects.
#include <stdio.h>
#include <vector>
typedef std::vector<char*> words;
int main(int argc, char** argv) {
words myWords;
myWords.push_back("Hello");
myWords.push_back("World");
words::iterator iter;
for (iter = myWords.begin(); iter != myWords.end(); ++iter) {
printf("%s ", *iter);
}
return 0;
}
If you're using C things are a lot harder, yes malloc, realloc and free are the tools to help you. You might want to consider using a linked list data structure instead. These are generally easier to grow but don't facilitate random access as easily.
#include <stdio.h>
#include <stdlib.h>
typedef struct s_words {
char* str;
struct s_words* next;
} words;
words* create_words(char* word) {
words* newWords = malloc(sizeof(words));
if (NULL != newWords){
newWords->str = word;
newWords->next = NULL;
}
return newWords;
}
void delete_words(words* oldWords) {
if (NULL != oldWords->next) {
delete_words(oldWords->next);
}
free(oldWords);
}
words* add_word(words* wordList, char* word) {
words* newWords = create_words(word);
if (NULL != newWords) {
newWords->next = wordList;
}
return newWords;
}
int main(int argc, char** argv) {
words* myWords = create_words("Hello");
myWords = add_word(myWords, "World");
words* iter;
for (iter = myWords; NULL != iter; iter = iter->next) {
printf("%s ", iter->str);
}
delete_words(myWords);
return 0;
}
Yikes, sorry for the worlds longest answer. So WRT to the "don't want to use a linked list comment":
#include <stdio.h>
#include <stdlib.h>
typedef struct {
char** words;
size_t nWords;
size_t size;
size_t block_size;
} word_list;
word_list* create_word_list(size_t block_size) {
word_list* pWordList = malloc(sizeof(word_list));
if (NULL != pWordList) {
pWordList->nWords = 0;
pWordList->size = block_size;
pWordList->block_size = block_size;
pWordList->words = malloc(sizeof(char*)*block_size);
if (NULL == pWordList->words) {
free(pWordList);
return NULL;
}
}
return pWordList;
}
void delete_word_list(word_list* pWordList) {
free(pWordList->words);
free(pWordList);
}
int add_word_to_word_list(word_list* pWordList, char* word) {
size_t nWords = pWordList->nWords;
if (nWords >= pWordList->size) {
size_t newSize = pWordList->size + pWordList->block_size;
void* newWords = realloc(pWordList->words, sizeof(char*)*newSize);
if (NULL == newWords) {
return 0;
} else {
pWordList->size = newSize;
pWordList->words = (char**)newWords;
}
}
pWordList->words[nWords] = word;
++pWordList->nWords;
return 1;
}
char** word_list_start(word_list* pWordList) {
return pWordList->words;
}
char** word_list_end(word_list* pWordList) {
return &pWordList->words[pWordList->nWords];
}
int main(int argc, char** argv) {
word_list* myWords = create_word_list(2);
add_word_to_word_list(myWords, "Hello");
add_word_to_word_list(myWords, "World");
add_word_to_word_list(myWords, "Goodbye");
char** iter;
for (iter = word_list_start(myWords); iter != word_list_end(myWords); ++iter) {
printf("%s ", *iter);
}
delete_word_list(myWords);
return 0;
}
If you want to dynamically allocate arrays, you can use malloc from stdlib.h.
If you want to allocate an array of 100 elements using your words struct, try the following:
words* array = (words*)malloc(sizeof(words) * 100);
The size of the memory that you want to allocate is passed into malloc and then it will return a pointer of type void (void*). In most cases you'll probably want to cast it to the pointer type you desire, which in this case is words*.
The sizeof keyword is used here to find out the size of the words struct, then that size is multiplied by the number of elements you want to allocate.
Once you are done, be sure to use free() to free up the heap memory you used in order to prevent memory leaks:
free(array);
If you want to change the size of the allocated array, you can try to use realloc as others have mentioned, but keep in mind that if you do many reallocs you may end up fragmenting the memory. If you want to dynamically resize the array in order to keep a low memory footprint for your program, it may be better to not do too many reallocs.
This looks like an academic exercise which unfortunately makes it harder since you can't use C++. Basically you have to manage some of the overhead for the allocation and keep track how much memory has been allocated if you need to resize it later. This is where the C++ standard library shines.
For your example, the following code allocates the memory and later resizes it:
// initial size
int count = 100;
words *testWords = (words*) malloc(count * sizeof(words));
// resize the array
count = 76;
testWords = (words*) realloc(testWords, count* sizeof(words));
Keep in mind, in your example you are just allocating a pointer to a char and you still need to allocate the string itself and more importantly to free it at the end. So this code allocates 100 pointers to char and then resizes it to 76, but does not allocate the strings themselves.
I have a suspicion that you actually want to allocate the number of characters in a string which is very similar to the above, but change word to char.
EDIT: Also keep in mind it makes a lot of sense to create functions to perform common tasks and enforce consistency so you don't copy code everywhere. For example, you might have a) allocate the struct, b) assign values to the struct, and c) free the struct. So you might have:
// Allocate a words struct
words* CreateWords(int size);
// Assign a value
void AssignWord(word* dest, char* str);
// Clear a words structs (and possibly internal storage)
void FreeWords(words* w);
EDIT: As far as resizing the structs, it is identical to resizing the char array. However the difference is if you make the struct array bigger, you should probably initialize the new array items to NULL. Likewise, if you make the struct array smaller, you need to cleanup before removing the items -- that is free items that have been allocated (and only the allocated items) before you resize the struct array. This is the primary reason I suggested creating helper functions to help manage this.
// Resize words (must know original and new size if shrinking
// if you need to free internal storage first)
void ResizeWords(words* w, size_t oldsize, size_t newsize);
In C++, use a vector. It's like an array but you can easily add and remove elements and it will take care of allocating and deallocating memory for you.
I know the title of the question says C, but you tagged your question with C and C++...
Another option for you is a linked list. You'll need to analyze how your program will use the data structure, if you don't need random access it could be faster than reallocating.
Your code in the last update should not compile, much less run. You're passing &x to LoadData. &x has the type of **words, but LoadData expects words* . Of course it crashes when you call realloc on a pointer that's pointing into stack.
The way to fix it is to change LoadData to accept words** . Thi sway, you can actually modify the pointer in main(). For example, realloc call would look like
*x = (words*) realloc(*x, sizeof(words)*2);
It's the same principlae as in "num" being int* rather than int.
Besides this, you need to really figure out how the strings in words ere stored. Assigning a const string to char * (as in str2 = "marley\0") is permitted, but it's rarely the right solution, even in C.
Another point: non need to have "marley\0" unless you really need two 0s at the end of string. Compiler adds 0 tho the end of every string literal.
For the test code: if you want to modify a pointer in a function, you should pass a "pointer to pointer" to the function. Corrected code is as follows:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
typedef struct
{
char *str1;
char *str2;
} words;
void LoadData(words**, int*);
main()
{
words **x;
int num;
LoadData(x, &num);
printf("%s %s\n", (*x[0]).str1, (*x[0]).str2);
printf("%s %s\n", (*x[1]).str1, (*x[1]).str2);
}
void LoadData(words **x, int *num)
{
*x = (words*) malloc(sizeof(words));
(*x[0]).str1 = "johnnie\0";
(*x[0]).str2 = "krapson\0";
*x = (words*) realloc(*x, sizeof(words) * 2);
(*x[1]).str1 = "bob\0";
(*x[1]).str2 = "marley\0";
*num = *num + 1;
}
Every coder need to simplify their code to make it easily understood....even for beginners.
So array of structures using dynamically is easy, if you understand the concepts.
// Dynamically sized array of structures
#include <stdio.h>
#include <stdlib.h>
struct book
{
char name[20];
int p;
}; //Declaring book structure
int main ()
{
int n, i;
struct book *b; // Initializing pointer to a structure
scanf ("%d\n", &n);
b = (struct book *) calloc (n, sizeof (struct book)); //Creating memory for array of structures dynamically
for (i = 0; i < n; i++)
{
scanf ("%s %d\n", (b + i)->name, &(b + i)->p); //Getting values for array of structures (no error check)
}
for (i = 0; i < n; i++)
{
printf ("%s %d\t", (b + i)->name, (b + i)->p); //Printing values in array of structures
}
scanf ("%d\n", &n); //Get array size to re-allocate
b = (struct book *) realloc (b, n * sizeof (struct book)); //change the size of an array using realloc function
printf ("\n");
for (i = 0; i < n; i++)
{
printf ("%s %d\t", (b + i)->name, (b + i)->p); //Printing values in array of structures
}
return 0;
}
If you want to grow the array dynamically, you should use malloc() to dynamically allocate some fixed amount of memory, and then use realloc() whenever you run out. A common technique is to use an exponential growth function such that you allocate some small fixed amount and then make the array grow by duplicating the allocated amount.
Some example code would be:
size = 64; i = 0;
x = malloc(sizeof(words)*size); /* enough space for 64 words */
while (read_words()) {
if (++i > size) {
size *= 2;
x = realloc(sizeof(words) * size);
}
}
/* done with x */
free(x);
Here is how I would do it in C++
size_t size = 500;
char* dynamicAllocatedString = new char[ size ];
Use same principal for any struct or c++ class.