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I have a u_char array, which changes each time I enter the loop, and a structure with a member as a u_char array. I am trying to create a vector of structures, containing all iterations of the u_char array. But I get either a segfault (when I use memcpy) or the same value duplicated (when I don't). Help? Here's the code

u_char * data; //contains the data I want to copy from each iteration
unsigned char mem1[bytes];
for(int i=0; i<bytes; i++) {
    mem1[i] = data[i];
}
struct load l1; //contains one member (pload of type u_char*)
l1.pload = mem1; //this gives me the same value in all elements of the vector
//I also tried using memcpy, like this:
//memcpy(&l1.pload, mem1, bytes); //this gave me a segfault 
recd.push_back(l1) //recd is a std::vector<load>

I print it out like so:

for(std::vector<load>::iterator it1 = recd.begin(); it1 != recd.end(); ++it1) {
     cout<<"\nhere\n"<<endl;
     cout<<it1->pload<<endl;
}
ROARK
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  • What is the definition of `struct load`? – Thomas Matthews Mar 18 '13 at 14:30
  • If `l1.pload` is a `u_char*` as documented and thus no backed memory, then `memcpy (&l1.pload, ...)` will segfault as `bytes` exceeds `sizeof (u_char*)` which is 4-8 bytes, and `memcpy (l1.pload, ..)` which is what you really meant will segfault as it doesn't point anywhere. – Steve-o Mar 18 '13 at 14:33
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    Before using `memcpy()` you need to ensure there is enough memory allocated for `l1.pload`. Note you could use a `std::vector`, as you are already use `std::vector` elsewhere, and avoid any explicit dynamic memory management. – hmjd Mar 18 '13 at 14:33

1 Answers1

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You get a segmentation fault on memcpy for what Steve-o and hmjd said in the comments: li.pload is probably just a pointer, and it's not pointing to any space of its own, so memcpy fails. Depending on your compiler, it's uninitialized, or points to 0x0, and in either case, copying in to that should properly cause that error.

It's ok for li.pload to just be a pointer. The trick is, mem1 should point to new, valid data each time:

`u_char * mem1 = new u_char[bytes];`

Then your for loop (which could be replaced by memcpy there) will work fine, and you can keep your l1.pload = mem1, since you'll be storing a pointer to different memory areas each time. As it is right now, each li.pload is pointing to the same memory area, which is why you're seeing the same value in each record.

Of course, since you're allocating memory, when you're done with your vector, you'll have to remember to delete pload for each load in your vector.

Scott Mermelstein
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