Malloc Array of structs, data in fields not saving properly

Question

I have the following struct:

struct message {
    int id;
    int ack;
    int data_len;
    char *data;
    time_t timer;
} *messages, *temp;

I am allocating a chunk of memory to hold upto numMessages amount of these structs:

messages = malloc(sizeof(messages)*numMessages);

And then n messages can be added by calling the function ReadFromFile(int), with count initialized to 0

void readFromFile(int n) {

    char input_buff[4096];
    int size = sizeof(struct message);
    for (int i = 0; i < n; i++) {
        bzero(input_buff, sizeof(input_buff));
        int nread = fread(input_buff, 1 , msgSize, fp);

        if (nread > 0) { 
           printf("adding message: %d\n", count);

           temp = (struct message *) malloc (sizeof (struct message)); 
           temp->data_len = nread;
           temp->id = count; // set integer id
           temp->data = malloc( sizeof(char) * ( nread ));
           temp->ack = 0;

           memcpy(temp->data, input_buff, nread); 
           memcpy(&messages[count],temp,sizeof(temp));

           count++;
           free(temp);

        } 

        if (nread < sizeof(input_buff)) {
            if (feof(fp))
            printf("End of file\n");
            free(filename);
            close(fp);
            break;
        }

        if (ferror(fp)) {
            printf("Error reading\n");
            break;
        }
    }
}

HOWEVER, messages[count].data isn't being stored. However, if I swap the lines:

temp->data_len = nread;

and

temp->data = malloc( sizeof(char) * ( nread ));

The data is properly stored, but now data_len isn't stored? What am I doing wrong? Besides the fact I'm sure that having a pointer temp and than coping the memory is redundant...

Thank you!

[don't cast the result of malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, May 06 '16 at 01:37
`memcpy(&messages[count],temp,sizeof(temp));` should also be `sizeof(*temp)` — user253751, May 06 '16 at 02:33
"I have the following struct:" - No! You have pointers to that `struct` — too honest for this site, May 06 '16 at 03:23

score 2 · Answer 1 · answered May 06 '16 at 01:36

2

messages is a pointer to struct message, its size is not the same as the struct, change to this:

messages = malloc(sizeof(struct message) * numMessages);

answered May 06 '16 at 01:36

fluter

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Thank you! However, it's still not properly saving. when I add some print statements I get: adding message: 0, data_len: 0, nread 4096, Saved data: (null), Saved data_len: 0, adding message: 1, data_len: 0, nread 4096, Saved data: (null), Saved data_len: 0, adding message: 2, data_len: 53, nread 2048, Saved data: (null), Saved data_len: 53, End of file – mdibound May 06 '16 at 01:55
@mdibound what's the expected behavior? – fluter May 06 '16 at 02:26
each struct should store the data that is read from disk to input_buffer, as well as store the value nread as data_len so that messages[0].data contains the first chunk read from file, and messages[0].data_len = the number of bytes read... however neither of these are being saved. – mdibound May 06 '16 at 02:34
@mdibound as @immibis pointed, please change to `memcpy(&messages[count],temp,sizeof(temp));` and try again to see if it solved the problem. – fluter May 06 '16 at 02:39
strangely, after memcpy(temp->data, input_buff, nread); messages[count].data is still NULL in all cases :/ – mdibound May 06 '16 at 04:24
How is 'count' defined? Also check it after the loop? – fluter May 06 '16 at 04:26

Malloc Array of structs, data in fields not saving properly

1 Answers1