Thread: Problem with allocating memory for a char pointer

Hi, this program I wrote is about creating a notebook using linked list as a final requirement. A notebook with a page number, line number and also a text on each line number.

Here is my insert note function where I will enter the Page#, Line#, and the Text and the inputs will just be passed to another functions where the nodes will be inserted.

When calling this function for the first time, there is no problem, the node will be inserted. but on the second call of this function, the program will stop on the statement which allocates memory for variable txt which is a char pointer.

I can't figure out what is the problem with the code. I tried inserting the code "free(txt);" at the bottom of the function before the "return;" statement but it will cause an error even on the first call of this function.

Can anyone help me? Thanks in advance.
Here is my code below:

Code:

void InsertNote(){
int pn,ln,i;
char *txt,c;
struct notebook *temp;
temp=note;

txt = (char *) malloc(sizeof(char));
printf("\n\nINSERT NOTE");
printf("\nWhat Page Number? ");
scanf("%d",&pn);
printf("What Line Number? ");
scanf("%d",&ln);

fflush(stdin);
printf("Enter Your Note: ");
gets(txt);

if(temp==NULL){
AddFirst(txt,pn,ln);
}
else{
while(temp!=NULL){
if(temp->pagenum <= pn){
if(temp->linenum < ln){
c++;
}
}
temp=temp->next;
}
if(c==0){
AddFirst(txt,pn,ln);
}
else if(c<count()){
AddAfter(txt,pn,ln,++c);
}
else{
Append(txt,pn,ln);
}
}
//free(txt);
return;
}

Thanks guys.. the above problem is solved. and i found again another problem. i have two function the AddFirst() and Append() both will insert a node. there is no problem with the AddFirst function but if i will add another node which will call the Append() function the error will occur on reading the statement "strcpy(temp->line,str);". and again i can't figure out what is the cause of the error.

Code:

void Append(char *str, int p, int l){
    struct notebook *temp,*right;
    temp=(struct notebook *)malloc(sizeof(struct notebook));
    //temp=malloc(sizeof(struct notebook));
    strcpy(temp->line,str);
    temp->pagenum=p;
    temp->linenum=l;
    
    right=(struct notebook *)note;
    while(right->next != NULL){
       right=right->next;
    }
    
    right->next = temp;
    right=temp;
    right->next=NULL;
    
    //return;
}

void AddFirst(char *str, int p, int l){
    struct notebook *temp;
    temp=(struct notebook *)malloc(sizeof(struct notebook));
    
    strcpy(temp->line,str);
    temp->pagenum=p;
    temp->linenum=l;
    
    if(note == NULL){
       note=temp;
       note->next=0;
    }
    else{
       temp->next=note;
       note=temp;
    }
    //return;
}

Code:

void Append(char *str, int p, int l){
    struct notebook *temp,*right;
    temp=(struct notebook *)malloc(sizeof(struct notebook));
    //temp=malloc(sizeof(struct notebook));
    strcpy(temp->line,str);

How is your "struct notebook" defined?

Don't cast the return value of malloc!

Bye, Andreas

Here is my struct:

Code:

struct notebook{
    char *line;
    int pagenum, linenum;
    struct notebook *next; 
}*note;

oh i'm using wxDev-C++ compiler version number build 7.4.2.569
also using .c extension

Originally Posted by jmyrrah

there is no problem with the AddFirst function

Wrong, you have the same problem as in Append():

Code:

void AddFirst(char *str, int p, int l){
    struct notebook *temp;
    temp=(struct notebook *)malloc(sizeof(struct notebook));
    
    strcpy(temp->line,str);    // temp->line doesn't point to valid memory

Bye, Andreas

but why is it that when i call the function display, it will display page number, line number, and the text.

sorry i'm was wrong.. because it already works when i put the temp->line = malloc(strlen(str)+1); before strcpy(temp->line,str); on the AddFirst() function

thanks you so much guys..

i'll post again if there's another problem i will encounter..