Linked List : Adding and removing issues - c

Question

First of all, sorry for my english.
For a project, I have to make a garage management program. There's a linked list of mechanics. I made tho function at the moment : one for adding mechanics (addMec) in this list and one for removing mechanics (remMec).
Here is the addMec function :
void addMec(mechanics* first)
{
mechanics *new = malloc(sizeof(mechanic));
int speciality();
if(first==NULL)
{
return(0);
} else
{
printf("Add mechanic : \n");
// Ask for needed informations.
printf("Employee-Number : ");
scanf("%5d",&new->number);
printf("\nName : ");
scanf("%s",&new->name);
printf("\nFirst Name : ");
scanf("%s",&new->first_name);
new->speciality = speciality();
// Add to end -> new->next = NULL
new->next = NULL;
printf("%d %s %s %d\n",new->number,
new->name, new->first_name, new->speciality);
mechanics *current = first;
while(current->next!=NULL)
{
current = current->next;
}
current->next = new;
}
}
This function doesn't add anything in the list. If I try to debug the program, I have a SIGSEGV message.
function remMec
int remMec(mechanics* first)
{
int numrem, ret = 0;
mechanics *current, *intercale;
printf("Supprimer l'employe numero : ");
scanf("%5d",&numrem);
// On parcoure la liste à la recherche du numero d'employe.
current = first;
while(current->next != NULL)
{
// Nécéssaire si l'élément à supprimer est le premier de la liste.
if(current->number == first->number)
{
if(current->number == numrem && current != NULL)
{
intercale = first;
first = first->next;
free(intercale);
ret = 1;
}
}
if(current->number == numrem)
{
intercale = current->next;
current->next = intercale->next;
free(intercale);
ret = 1;
}
current = current->next;
}
if(ret)
{
return 1;
} else
{
return 0;
}
}
The issue with this function is that it removes the next element of the searched element.
example : in the list :
15264 John Chris 1
12569 Chris John 2
If I try to remove John Chris, it'll remove Chris John
Here's the struct :
typedef struct{
char name[31],first_name[31];
int speciality, number;
struct mechanic *next;
}mechanic;
EDIT : It seems that the SIGSEGV happen when i try to display the list. Here is that function :
void displayList(mechanic *first, int n)
{
int i;
mechanic *courant = malloc(sizeof(mechanic));
current = first;
printf("Mechanics :\n");
for(i = 1; i<n; i++)
{
printf("%5d %s %s",current->number,current->name, current->first_name);
switch(current->speciality)
{
case 1: printf(" Wheels\n");break;
case 2: printf(" Motor\n");break;
case 3: printf(" Carrosserie\n");break;
case 4: printf(" Chassis\n");break;
default: printf(" Maintenance\n");break;
}
current = current->next;
}
}
Main :
int main()
{
int nbMec = 1,i,choise;
int menu();
int remMec(mechanics*);
void afficherListe(mechanics*, int);
void addMec(mechanics*);
mechanics *first, *current, *nextM;
first = malloc(sizeof(mechanics));
current = first;
FILE *fdat, *fdat2;
fdat = fopen("clients.dat","r");
fdat2 = fopen("employe.dat","r");
// Lecture du fichier contenant les mécaniciens
fscanf(fdat2,"%5d",&current->number);
while(!feof(fdat2))
{
fscanf(fdat2,"%s%s%1d",&current->name, &current->first_name, &current->speciality);
nextM = malloc(sizeof(mechanics));
current->next = nextM;
nbMec++;
current = nextM;
fscanf(fdat2,"%5d",&currentM->number);
}
//Finir la liste avec NULL
current = first;
for(i = 1; i<nbMec; i++){
current = current->next;
}
current->next = NULL;
free(nextM);
DisplayList(first,nbMec);
// Redirection en fonction du choix dans le menu
choise = menu();
if(choise == 1)
{
// Ajout d'un mécano
addMec(first);
nbMec++;
displayList(first,nbMec);
} else if (choise == 2)
{
if(remMec(first))
{
printf("Mecanicien efface de la liste.\n");
nbMec--;
} else
{
printf("Impossible.\n");
}
afficherListe(first,nbMec);
} else
{
//Ajout d'un rendez-vous
}
return 0;
}
Here is the employe.dat file :
12345 Ric Hochet 1
13456 Jean Valjean 2
14567 Rick Grimes 3
15007 James Bond 4
16789 Rogge Jacquie 5
example of record :
XXXXX CCCCC CCCCC X
number name first_name speciality
Can you help me ?
Thank you !

Answer 1

I found the solution :
Concerning addMec, thanks to CiaPan.
After these issues, I choose to add a new mechanic at the beginning of the list.
I've also created a list struct, containing the first element.
Here is the correct addMec (Please note that for the project, i've added a rep (reparation) array).:
void addMec(list *list, int number, char name[], char first_name[], int speciality)
{
int i;
mechanics *new = malloc(sizeof(mechanics));
if(list == NULL || new == NULL)
{
exit(EXIT_FAILURE);
}
new->number = number;
strcpy(new->name,name);
strcpy(new->first_name,first_name);
new->speciality = speciality;
for(i = 1; i<=50; i++)
{
new->rep->year=0;
new->rep->day = 0;
new->rep->month = 0;
new->rep->price = 0;
new->rep->emergency = 0;
strcpy(new->rep->brkdwn,"");
}
new->next = list->first;
list->first = new;
list->nbElmt++;
}
Concerning remMec, i searched on a other site, and an user, called pagrette explained me.
With these lines,
intercale = current->next;
current->next = intercale->next;
free(intercale);
I just free the next element
Thank you for helping !

Answer 2

Your first function begins like this:
void addMec(mechanics* first)
{
mechanics *new = malloc(sizeof(mechanic));
int speciality();
if(first==NULL)
{
return(0);
} else
{
......
......
THIS WILL NOT COMPILE! First because of return with a value from a void function, second for an undefined type name.
And even if it did, it would not make any sense. First you allocate a block of memory and store the pointer in the new variable. Then you test if first is NULL and you return from the function if so.
Look carefully: you return from the function. So you do not input any data for the new variable and do not append it to the list. That means your program can not build any list from the user's input if it is not filled from the file before.
Additionally, when you return from the function, all the local, automatic variables are lost. With the new among them. That means you no longer have access to the just allocated block—it is lost. You allocated some memory but can not utilize it in any way. That's what we call a memory leak.
To make the function more readable (and easier to further development) I'd split it into two parts: one to create a new object and put it into the list and a subroutine to fill it with the user input:
void inputMec(mechanic *mec)
{
int speciality();
printf("Add mechanic : \n");
// Ask for needed informations.
printf("Employee-Number : ");
scanf("%5d", & mec->number);
printf("\nName : ");
scanf("%s", & mec->name);
printf("\nFirst Name : ");
scanf("%s", & mec->first_name);
mec->speciality = speciality();
}
void addMec(mechanic* first)
{
if(first != NULL) // only if the list's head exists
{
mechanic *new = malloc(sizeof(mechanic));
if(new != NULL) // a new object created sucessfully
{
inputMec(new);
printf("%d %s %s %d\n",new->number,
new->name, new->first_name, new->speciality);
// find the last item in the list
mechanic *last = first;
while(last->next != NULL) // still not the last one?
{
last = last->next; // step forward
}
last->next = new; // append after the last
new->next = NULL; // terminate the list
}
}
}
If the ordering in the list is not important (as it seems from your code), then you can easier add the new item at the beginning of the list:
// insert the item just after the list's head
new->next = first->next; // append the list after the new item
first->next = new; // link the new item to the head
In a real programming task you would have to consider also restricting and validating input, both for internal limitations (are strings non-empty? aren't they too long? is the speciality value in the dictionary range?) and for external consistency (are employees' identifiers distinct? are their name+first_name pairs distinct?).
In case of internal inconsistency you might ask for correct data, and loop reading input until it is correct, or abandon input operation; in the latter case the inputMec routine should return some status. Then the caller routine addMec should test the status to decide whether insert a new item into a list if it is correct, or free() it in case of input failure.
In case of external conflicts you might print appropriate message, retain temporarily the new item and iterate reading input to obtain non-conflicting data or you may abandon the operation, free() the object and return to the main menu, where the user can enter adding mechanics again.