Reusing structs in loop (in C) [duplicate]

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 sscanfing directly to these instead. — Antti Haapala
– Antti Haapala, Commented Oct 5, 2018 at 6:43

kiran Biradar · Accepted Answer · 2018-10-05 06:44:24Z

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 5, 2018 at 6:44

kiran Biradar

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5 Comments

Kevin Over a year ago

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.

kiran Biradar Over a year ago

@KevinLe strdup does internally the same thing.

Kevin Over a year ago

Was just neater than what i had.

kiran Biradar Over a year ago

@KevinLe Yes for sure.

Lundin Over a year ago

@Kevin Please note though that strdup is not standard C, so it isn't portable.

Jabberwocky · Accepted Answer · 2018-10-05 07:06:46Z

0

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.

answered Oct 5, 2018 at 7:06

Jabberwocky

51.3k18 gold badges71 silver badges127 bronze badges

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