Question

I want to use this program to write a hash table to a file
but i get

cannot convert parameter 1 from ‘fpinfo’ to ‘const void *’

error at compile time

if i change struct fpinfo e; with struct fpinfo *e i get the run time error:

The variable ‘e’ is being used without being initialized.

I tried to initialize e by declaring it as struct fpinfo *e=NULL;. This either doesn’t work.

Please give me your help as usual.

WriteHTtoFile(struct fpinfo t[][B_ENTRIES],int this_tanker,tanker_record tr[][BUCKETS])
{
    //struct fpinfo *e;
    struct fpinfo e;
    int i=0, mask;
    char filename[sizeof ("file0000.txt")];
    sprintf(filename, "filee%d.txt", this_tanker);
    curr_tanker++;
    //fp = fopen(filename,"w");
    FILE *htfile=fopen(filename,"w+b");
     system("cls");
if (htfile != NULL)  
        { 
         for (int j = 0; j < BUCKETS; ++j) 
        {
            for (int k = 0; k < tr[this_tanker][j].bkt.num_of_entries; ++k)
            {
            printf("%d\t%d\t%s\n",t[j][k].chunk_offset,t[j][k].chunk_length,t[j][k].fing_print);
            (e).chunk_offset=t[j][k].chunk_offset;
            (e).chunk_length=t[j][k].chunk_length;
            strcpy((char*)((e).fing_print),(char*)t[j][k].fing_print);
            fwrite(e,sizeof(fpinfo),1,htfile);
            }
        } 
            fclose(htfile); 
         } 
        else 
        { 
            std::cout<<"File could not be opend for writing";
            printf("Error %d\t\n%s", errno,strerror(errno));
        }

    fclose(htfile);
    return 1;
}

Answer 1

The first argument to fwrite() is a const void*. This passes a struct fpinfo:

fwrite(e, sizeof(fpinfo), 1, htfile);

Change to:

fwrite(&e, sizeof(fpinfo), 1, htfile);

I am unsure what the members of struct fpinfo are so this may be unsafe (if, for example, it contains any pointer members). Any future reordering of the members in struct fpinfo or a change that results in a size increase of struct fpinfo (like new members being added) means any attempt to read previously written struct fpinfo data will be incorrect.

When the declaration of e was changed to a struct fpinfo* e; the unitialised error was due to the pointer not being NULL or being assigned to a dynamically allocated struct fpinfo.

When changed to struct fpinfo *e = NULL; this would cause a segmentation fault when an attempt to access any member of e occurred as it is not pointing to a struct fpinfo.