I'm having a problem in line: novo->conta.nome_cliente = valor2;. In this dynamic stack structure. Empilhar is a function to push a new element to the stack. Why i cant assign a new value to the structure inside the stack?
struct conta_de_luz
{
int cod_cliente;
char nome_cliente[40];
};
typedef struct Item {
struct conta_de_luz conta;
struct Item *anterior;
} Elemento;
void Inicializar(Elemento **topo) {
*topo = NULL;
}
int EstaVazia(Elemento **topo) {
if(*topo == NULL)
return VERD;
else
return FALSO;
}
void Empilhar(Elemento **topo, int valor, char valor2) {
Elemento *novo;
novo = (Elemento *) malloc(sizeof(Elemento));
novo->conta.cod_cliente = valor;
novo->conta.nome_cliente = valor2;
novo->anterior = *topo;
*topo = novo;
}
The problem is that line novo->conta.nome_cliente = valor2; assigns a char to something that is a string defined as char nome_cliente[40];.
So your function probably should have been void Empilhar(Elemento **topo, int valor, char* valor2). But that would still assume the value of valor2 remains in memory for the lifetime of topo.
Better would be void Empilhar(Elemento **topo, int valor, const char* valor2) and then use strcpy(novo->conta.nome_cliente, valor2);.
But this assumes valor2 including its NULL-terminator always fits within 40 characters. If this assumption is not guaranteed it's better to duplicate the string using strdup(). You will then also need to free() this seperately this in your cleanup function.
Related
Situation: I'm currently working with chained lists and I am adding new elements through a function called inputRegistering(). I am positive that at the end of this function, an element have been added to the correct place.
Issue1: The added element modifies all the other to take its value.
Issue2: When exiting the function, and calling the inputReadingAll() function (which displays the list in its entirety), the elements are now all "empty", or replaced with gibberish (as can do the pointers).
Test1: I have tested to do a manual adding in the main() function and it seems everything worked fine. I can't understand what is the fundamental difference with my code though...
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXLEN 256
typedef struct
{
struct elt *first;
struct elt *last;
} Tfile;
typedef struct elt
{
char *val;
struct elt *next;
} Telt;
int fileAdd(Tfile *, char *);
void fileInit(Tfile *);
void inputReadingAll(Tfile *);
void inputRegistering(Tfile *);
int main(int argc, char **argv){
Tfile file;
fileInit(&file);
inputRegistering(&file);
inputRegistering(&file);
inputReadingAll(&file);
}
int fileAdd(Tfile *F, char *newVal){
Telt *newElt;
newElt = (Telt*)malloc(sizeof(Telt));
if(newElt == NULL){
printf("Error\n");
return 0;
}
newElt->val = newVal;
newElt->next = NULL;
if(fileTaille(F) == 0)
F->first = newElt;
else
F->last->next= newElt;
F->last = newElt;
return 1;
}
void fileInit(Tfile *F){
F->first = NULL;
F->last = NULL;
}
void inputReadingAll(Tfile *file){
printf("> Reading all function\nBEGIN\n");
Telt *currElt = file->first;
while(currElt != NULL){
printf("%p, %s\n", currElt, currElt->val);
currElt = currElt->next;
}
printf("END\n");
}
void inputRegistering(Tfile *file){
printf("> Registering function\n> What to register : \n> ");
char temp[MAXLEN];
fgets(temp, MAXLEN, stdin);
temp[strcspn(temp, "\n")] = 0;
printf("Registering %s\n", temp);
fileAdd(file, temp);
}
The (pointer to) local array temp of the function inputRegistering() is passed to the function fileAdd() and the pointer is directly stored to the structure.
This is bad because the array is local and it is invalidated on returning from the function inputRegistering().
Instead of this, the function fileAdd() should copy the passed string and store the pointer to the copy in the structure.
This can be done like this:
newElt->val = malloc(strlen(newVal) + 1); /* +1 for terminating null-character */
if (newElt->val == NULL) {
printf("Error\n");
free(newElt);
return 0;
}
strcpy(newElt->val, newVal);
instead of this:
newElt->val = newVal;
Well I am wanting to change the way my structures are written, currently I use array and I need to limit its use, but I wanted a way to create a dynamic array that is the size of the reading done, without always having to edit the array value.
Current Code:
struct sr_flag {
int value_flag;
};
struct er_time {
int value_time;
};
struct se_option {
struct sr_flag flag[50];
struct er_time time[50];
};
struct read_funcs
struct se_option *option;
void (*option_func) (void);
...
}
struct read_funcs func_;
struct read_funcs *func;
int sr_flags(int i, int fg, int val) {
if(i < 0)
return 0;
return func->option[i].flag[fg].value_flag = val;
}
void option_func(void) {
struct se_option fnc;
fnc.option = malloc(500 * sizeof(*(fnc.option)));
}
void read_fnc() {
func = &func_;
func->option = NULL;
func->option_func = option_func;
}
I look for a way to remove the array amount [50] instead each time the sr_flags function is executed the limit is raised
Example: sr_flags function executed 1x array would be [1] if executed 2x would be [2]
I also think about doing the same with the option_func function
I tried using the following more unsuccessfully
struct se_option {
struct sr_flag *flag;
struct er_time time[50];
};
int sr_flags(int i, int fg, int val) {
if(i < 0)
return 0;
func->option[i].flag = malloc(1 * sizeof(*(func->option[i].flag)));
return func->option[i].flag[fg].value_flag = val;
}
int main () {
for(int i < 0; i < 10; i++)
sr_flags(i, 1, 30);
return 0;
}
I'm not 100% certain on what it is you want but I think you just want to call realloc and increase the size by the amount you provide. And that's very easy to do, as for the values you want with the arrays I'm not sure so I just used a placeholder value.
#include <stdio.h>
#include <stdlib.h>
struct sr_flag {
int value_flag;
};
struct er_time {
int value_time;
};
struct se_option {
struct sr_flag* flag;
struct er_time* time;
};
void allocateflags(struct se_option* options, int size, int val){
options->flag = realloc(options->flag, size*sizeof(struct sr_flag));
struct sr_flag* flag = options->flag+size-1;
flag->value_flag = val;
}
void allocatetime(struct se_option* options,int size, int val){
options->time = realloc(options->time, size*sizeof(struct er_time));
struct er_time* time = options->time+size-1;
time->value_time = val;
}
void displayflagvalues(struct se_option* options,int size){
for(int index = 0; index < size ; ++index){
printf("flag: %i\n",options->flag[index].value_flag);
}
}
void displaytimevalues(struct se_option* options, int size){
for(int index = 0; index < size ; ++index){
printf("time: %i\n",options->time[index].value_time);
}
}
int main(){
struct se_option options = {0};
for(int index = 0; index < 10; ++index){
allocateflags(&options, index,index);
allocatetime(&options, index,index);
}
displayflagvalues(&options, 10);
displaytimevalues(&options,10);
return 0;
}
The code creates an se_option structure wheren sr_flag and er_time pointers are null. Then there's two functions one allocateflags and the other allocatetime, both of which call realloc with the size you provide. When you call realloc, all previous memory is copied over to the new array. Also free is called automatically by realloc.
This step
struct sr_flag* flag = options->flag+size-1;
flag->value_flag = val;
struct er_time* time = options->time+size-1;
time->value_time = val;
Is slightly redundant but it was just to show the newest array can hold the value. If you understand pointer arithmetic, all its doing is incrementing the pointer to the last position then subtracting 1 struct size and setting that value. Basically setting the value of the final array in the pointer.
I apologise if this seems simple, I'm still learning and I'm new to C.
I have this as my struct:
struct Game{
char id;
char name[50];
char genre[20];
char platform[15];
char company[30];
float price;
int quantity = 10;
};
And this declared as a struct array:
struct Game gList[30];
I have a function where I'm passing all of 'gList' to search through values in the gList[i].name variables.
So my question is, is it possible to send only the gList[i].name part of the struct to the function as a parameter?(ie All the 30 name values only).
No.
But you could make an array of pointers that point to the name field and pass it to the function:
char* ptr[30];
for(int i = 0; i < 30; i++)
ptr[i] = gList[i].name;
func(ptr);
No you can't. However, you can pass iterators to functions just fine. Typical pattern:
struct context { struct Game *gList; int nList; int i; }
char *iter_names(void *baton)
{
struct context *ptr = baton;
if (ptr->i == ptr->nList) return NULL;
return ptr->gList[ptr->i++].name;
}
void wants_name_array(char (*nextname)(void *), void *baton)
{
while (char *name = nextname(baton))
{
printf("%s\n", name);
/* and whatever else you are doing */
}
}
/* ... */
struct context baton = { gList, 30, 0 };
wants_name_array(iter_names, baton);
Yeah it looks kinda bad. Thankfully, gcc has an extension that makes this much better.
void wants_name_array(char (*nextname)())
{
while (char *name = nextname())
{
printf("%s\n", name);
/* and whatever else you are doing */
}
}
/* ... */
{
int i = 0;
char *nextname()
{
if (i == 30) return NULL;
return gList[i++].name;
}
wants_name_array(nextname);
}
When using this particular gcc extension, never ever return nested functions. Undefined behavior.
Namaste! I want to initialize my struct array position 0 myList.items[0] with a pointer to my item struct, but it prints out jibberish on the relevant positions when I print it out from my print function. It changes what was previously initialized (for test) so I know it works partially, but what's causing the bad output and what should be changed?
Before:
----My Shopping list---------
1 - Chocolate 40 100g
2 - Fishsauce 9 l
After:
----My Shopping list---------
1 - c┴®¶²` 128565603 ■ lüIv
2 - Fishsauce 9 l
typedef struct{
char name[20];
int amount;
char amountType[10];
}item;
typedef struct{
item *items[5];
int length;
}list;
int addItemToList(list *myList);
main(void)
{
list myList;
myList.length = 0;
for(int i; i<5;i++)
{
myList.items[i] = NULL;
}
addItemToList(&myList);
return 0;
}
int addItemToList(list *myList)
{
item newItem = {"Potatoes",2, "kg"};
myList->items[myList->length]=&newItem; //Something wrong here?
myList->length++;
printf ("Added [%s %i %s] as #%i.", newItem.name, newItem.amount,newItem.amountType, myList->length);
return 0;
}
This:
int addItemToList(list *myList)
{
item newItem = {"Potatoes",2, "kg"};
That allocates newItem on the stack. That means the memory for it will go away when addItemToList is completed, so &newItem will be pointing to gibberish later. It will be fine while still running code in addItemToList, but then after that the memory contents will be replaced in any further functions that are called.
You can either use malloc to allocate some memory for newItem, or you can allocate newItem on the stack in your main function and pass the pointer to newItem to any other called functions.
You cannot initialize a struct on the stack and then pass the pointer to that struct to a different struct that lives longer than that stack frame is open. Once the function returns, the stack frame where your item was allocated will close and the assigned values to the fields will be gone.
You need an initializer function with a heap allocated struct like this:
item* item_new(const char* name, int amount, const char* amountType);
Then your initialization function should:
call Malloc for the size of struct,
Copy the name and amountType strings to the struct.
copy the amount to the struct.
Then you can do this: myList->items[myList->length]=newItem;, where newItem is created by your init function.
You are mixing stack and heap memory, I rewriten it for you, it should give you some sense what went wrong. Also always free allocated memory its no java:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct{
char name[20];
int amount;
char amountType[10];
}item;
typedef struct{
item *items[5];
int length;
}list;
int addItemToList(list *myList);
int main(void)
{
list myList;
myList.length = 0;
for(int i; i<5;i++)
{
myList.items[i] = 0;
}
addItemToList(&myList);
for(int i; i<myList.length; i++)
{
free(myList.items[i]);
}
return 0;
}
int addItemToList(list *myList)
{
item* newItem = malloc(sizeof(item));
strcpy(newItem->name, "Potatoes");
newItem->amount = 2;
strcpy(newItem->amountType, "kg");
myList->items[myList->length++] = newItem; //Something wrong here?
printf ("Added [%s %i %s] as #%i.\n", newItem->name, newItem->amount, newItem->amountType, myList->length);
return 0;
}
Why am I getting a segmentationfault when it tries to print the second member in the list?
After printing the first element of the list, the debugger opens the stdio.h and says:
At C:\TDM-GCC-32\include\stdio.h:255
At C:\TDM-GCC-32\include\stdio.h:256
At C:\TDM-GCC-32\include\stdio.h:258
At C:\TDM-GCC-32\include\stdio.h:259
Here is the code.
#include <stdio.h>
#include <stdlib.h>
struct Student {
char *Name;
char *Adresse;
unsigned long Mtnr;
short Kurse;
struct Student *next;
struct Student *previous;
};
typedef struct Student Student;
Student *liste = NULL, *ende = NULL;
void add(char Name, char Adresse, unsigned long Mtnr, short Kurse) {
Student *add;
ende->next = malloc(sizeof(Student));
add = ende->next;
add->Name = Name;
add->Adresse = Adresse;
add->Mtnr = Mtnr;
add->Kurse = Kurse;
add->previous = ende;
add->next = NULL;
ende = ende->next;
}
void Ausgabe(Student *Anfang) {
while (Anfang != NULL) {
printf("%s %s %d %d \n", Anfang->Name, Anfang->Adresse, Anfang->Mtnr, Anfang->Kurse);
Anfang = Anfang->next;
}
}
int main() {
liste = malloc(sizeof(Student));
ende = liste;
liste->Name = "Anna Musterfrau";
liste->Adresse = "Am Schwarzberg-Campus 3";
liste->Mtnr = 22222;
liste->Kurse = 2;
liste->next = NULL;
liste->previous = NULL;
add("Hans Peter", "Kasernenstrasse 4", 4444, 4);
Ausgabe(liste);
return 0;
}
The error is in the declaration of the add() function. The strings should be char pointers, not chars.
void add(char *Name, char *Adresse, unsigned long Mtnr, short Kurse){
The signature of the function add is inconsistent to the declaration and usage to the members of Student. Change the signature as follows.
void add(char* Name, char* Adresse, unsigned long Mtnr, short Kurse)
On the long run, it might also be necessary to create copies of Name and Adresse in add, as the caller of add might deallocate them, perhaps causing undesired behaviour.
While Marc Is correct with his observation, there is one more thing you may want to fix here.
When you add a record, you allocate it, but you do not allocate the content of it's pointers (Specifically = for the name and address pointers). The add function just point them to the input's address. this is a problem because the data in the address supplied to the add function is likely to change if, for example, it's a user input, or some other external buffer.
in the code snipped below I fixed the 'name', but left the address as is. please run it and see what happens. (assaf's record displays david's address)
hope this helps
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Student {
char *Name;
char *Adresse;
unsigned long Mtnr;
short Kurse;
struct Student *next;
struct Student *previous;
};
typedef struct Student Student;
Student *liste = NULL, *ende = NULL;
void add(char *Name, char *Adresse, unsigned long Mtnr, short Kurse) {
Student *add;
ende->next = malloc(sizeof(Student));
add = ende->next;
add->Name = malloc(strlen(Name)+1);
strcpy(add->Name, Name);
add->Adresse = Adresse;
add->Mtnr = Mtnr;
add->Kurse = Kurse;
add->previous = ende;
add->next = NULL;
ende = ende->next;
}
void Ausgabe(Student *Anfang) {
while (Anfang != NULL) {
printf("%s %s %d %d \n", Anfang->Name, Anfang->Adresse, Anfang->Mtnr, Anfang->Kurse);
Anfang = Anfang->next;
}
}
int main() {
char name_buf[100];
char address_buf[100];
liste = malloc(sizeof(Student));
ende = liste;
liste->Name = "Anna Musterfrau";
liste->Adresse = "Am Schwarzberg-Campus 3";
liste->Mtnr = 22222;
liste->Kurse = 2;
liste->next = NULL;
liste->previous = NULL;
add("Hans Peter", "Kasernenstrasse 4", 4444, 4);
sprintf(name_buf,"assaf stoler");
sprintf(address_buf,"maria 8");
add(name_buf, address_buf, 8888, 8);
sprintf(name_buf,"david david");
sprintf(address_buf,"some street 9");
add(name_buf, address_buf, 9999, 9);
Ausgabe(liste);
return 0;
}
EDIT: Op asked some questions, and the comment space is limited, so I'll add below:
A pointer is just an object pointing somewhere in memory. it's size is fixed. the content it's pointing to will vary.
When you include a pointer to a string in a structure, the space where the string is kept need to be allocated / accounted for. it is not part of the sizeof(struct).
In your original example the pointers were pointing to constant strings (which reside in the static code, usually the data section, allocated by the compiler), which is why your original code was able to access the strings.
In a more realistic case, the input data is not part of the program data, but is received by some input method (which my *buf was to emulate). as such, pointing your name and address at it would break the program, as the pointer you point to may have it's content changed. therefor copying of the data (string / array) is needed, and since we copy the data, we need to allocate space for it, and point our (name/address) pointer at it.
Alternate option is to use non-pointer array for name and address as in:
struct Student {
char Name[20];
char Adresse[60];
unsigned long Mtnr;
...
}
In this scenario, sizeof (struct Student) would actually include all the space for those fields. you still need to use strcpy or memcpy, as well as check for and handle strings that are too long to fit in your pre-defined length.
Hope this helps