Reusing structs in loop (in C)

Question

I'm trying to insert data into a Data struct first, then parse it to an insertFirst function that can put it into my linked list. This is all done in a while loop.

while(fgets(line, 8, file) != NULL)
{
    x= (Data*)malloc(sizeof(Data)); 
    sscanf(line, "%s %s", line, val);

    x->c = line; 
    x->v =val; 

    insertFirst(list, x);
}

However I'm trying to reuse the Data struct. Mallocing it every time. The problem I have is that despite the lines being read in correctly. e.g. LOREM 1 and IPSUM 3, the linked list will always contain IPSUM 3. How do i use a struct like this repeatedly in the loop?

you're not reusing the struct. You're just assigning pointers pointing to the *same* character array all over. Consider having `command` and `val` as *array members* in the `Data` and `sscanf`ing directly to these instead. — Antti Haapala -- Слава Україні, Oct 05 '18 at 06:43
Also [don't cast malloc](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — mschuurmans, Oct 05 '18 at 06:44
Data is a struct that holds the two strings called `command` and `value`. — Kevin, Oct 05 '18 at 06:52

score 4 · Accepted Answer · answered Oct 05 '18 at 06:44

4

The problem here is you are assigning cmd and val as pointers to x->command and x->value, hence x->command and x->value will always points to updated value in cmd and val.

So you change the code as below.

while(fgets(line, 15, inputFile) != NULL)
{
    x= malloc(sizeof(Data)); 
    sscanf(line, "%s %s", cmd, val);

    x->command = strdup(cmd); 
    x->value = strdup(val); 

    insertFirst(list, x);
}

Where strdup calculates the space needed and allocates the memory dynamically and copies the input string and returns the newly allocated pointer.

answered Oct 05 '18 at 06:44

kiran Biradar

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1

Thank you! Saw what I was doing wrong, needed to allocate memory for command and value. And your use of strdup was more better than my extra mallocs. – Kevin Oct 05 '18 at 06:56
@KevinLe `strdup` does internally the same thing. – kiran Biradar Oct 05 '18 at 06:57
Was just neater than what i had. – Kevin Oct 05 '18 at 06:59
@KevinLe Yes for sure. – kiran Biradar Oct 05 '18 at 06:59
@Kevin Please note though that strdup is not standard C, so it isn't portable. – Lundin Oct 05 '18 at 08:37

score 0 · Answer 2 · answered Oct 05 '18 at 07:06

Another possibility is this:

typedef struct Data
{
    char command[100]; 
    char value[100]; 
} Data; 

while(fgets(line, 15, inputFile) != NULL)
{
    x= (Data*)malloc(sizeof(Data)); 
    sscanf(line, "%s %s", x->command, x->value);  

    insertFirst(list, x);
}

Disclaimer: no erro checking or bounds checking for the strings is done here for brevity.

Reusing structs in loop (in C)

2 Answers2