linked list not working properly, head updated instead - c++

Question

I have a problem with the following linked list code, but i'm not sure what it is. Could someone point me in the right direction? I'm using this code in some larger code where i update records, but it never reaches the "creating new record" section. It is as if the main code is updating the head pointer instead thus always resulting in a favourable comparison.
Thanks in advance. I've been racking my brains out trying to figure out what the problem is.
struct l_list *find_name(const char *name)
{
struct l_list *tmp=0;
if(records==0) { // First record
head=new l_list;
head->name=name;
head->next=0;
tail=head;
records++;
return head;
}
else {
tmp=head;
while(tmp!=0)
{
if(!std::strcmp(tmp->name,name))
{
cout << "Returning existing record with value: " << tmp->number << " name:" << tmp->name << endl;
return tmp;
}
tmp=tmp->next;
}
// No first and no existing records
cout << "Creating new record" << endl;
tail->next=new l_list;
tail=tail->next;
tail->name=name;
tail->next=0;
records++;
return tail;
}
I'm calling this from main with:
struct records *tmp=find_name("Max");
then :
tmp=find_name("Eva");
Once i get the struct i update it like so:
tmp->number=1;
Or even updating the name:
tmp->name="Peter";
So by passing a string to the function it will either create a new record and return it or give an existing record and return that. Problems might not be apparent in the output, but when you put it in a for(;;) loop in main it will
mess up.
The struct is as follows:
struct records {
const char *name;
struct records *next;
}
The relevant program code is:
struct record {
const char *name;
struct record *next;
};
struct record *head;
struct record *tail;
struct record *find_name(const char *name)
{
struct record *tmp=0;
if(record_count==0) { // First record
cout << "Creating first record" << endl;
head=new record;
head->name=name;
head->next=0;
tail=head;
record_count++;
return head;
} else {
tmp=head;
while(tmp!=0) {
if(!std::strcmp(tmp->name,name)) {
cout << "Returning existing record with value: " << "name: " << name << "tmp->name: " << tmp->name << endl;
return tmp;}
tmp=tmp->next;
}
// No first and no existing records
cout << "Creating new record" << endl;
tail->next=new record;
tail=tail->next;
tail->name=name;
tail->next=0;
record_count++;
return tail;
}
}
int main(int argc, const char *argv[])
{
struct record *tmp=0;
if(something is true) {
//Return or create a new user
tmp=find_name("Peter");
} else {
tmp=find_name("Unknown"); // Hold 1 unknown person in database
}
}
I know it's not compilable as-is but i had to extract it from a larger part.

Answer 1

Since you have not told us what the records structure is, there is no way for anyone to give a correct answer. You have made it even more impossible by not giving an example of code that will cause your function to behave incorrectly.
If the name element is a char * pointer then you could easily get this behaviour. For example:
The calling code puts a name into a buffer, and calls find_name.
find_name stores the address of the buffer into the name element of a records object. name therefore points to whatever the buffer happens to contain, now and in the future.
The calling code puts a new name into the same buffer. This automatically means that the name element now points to that new name, since it is pointing to the buffer.
The calling code calls find_name again.
find_name compares the contents of the buffer to the string pointed to by the name element of the first records object. Since the name element contains the address of the buffer passed by the caller (from step 2), this means that it is comparing the buffer to itself. So the result is always "equal".
But it may be that name is not a pointer at all, in which case this entire answer is irrelevant.

Answer 2

First of all do not use the following code formatting
if(record_count==0) { // First record
cout << "Creating first record" << endl;
//...
} else {
tmp=head;
//...
It is difficult to read such a code. It is just a bad style of programming.
The function itself can look the following way
struct l_list * find_name( const char *name )
{
struct l_list *tmp = head;
wjile ( tmp != nullptr && std::strcmp( tmp->name, name ) != 0 ) tmp = tmp->next;
if ( tmp == nullptr )
{
cout << "Creating new record" << endl;
tmp = new l_list;
tmp->name = name;
tmp->next = nullptr;
if ( tail == nullptr )
{
head = tail = tmp;
}
else
{
tail = tail->next = tmp;
}
records++;
}
return tmp;
}
Take into account that the nodes can contain pointers to strings either with the static storage duration as for example string literals or allocated in the heap.