Question

Possible Duplicate:
Getting segmentation fault SIGSEGV in memcpy after mmap

I’m using mmap() in my cpp code to map a large size area (100,000,000 bytes ~ 100MB).

From man mmap I understand that I can only know whether it succeeded or not, I cannot know how much size it succeeded to map.

In my case, I could read that area iteratively with a buffer of 8192 bytes, but after I read ~24MB I get SIGSEGV – means that mmap didn’t map the entire area successfully?

I’m reading by a memcpy function to copy from the mapped area to my buffer on the heap. (I see the same behavior also when the buffer is on the stack).

How can I know if it mapped the entire area or not? And if it mapped the entire area then why do I get the SIGSEGV after around 24MB of bytes read?

Thanks!

Answer 1

int *  addr = reinterpret_cast<int *>(mmap(NULL, length , PROT_READ, flags , fd, 0));
// ...    
int * initaddr = addr;

char buffer[jbuffer_size];

void *ret_val = buffer;
int read_length = length;

while(ret_val == buffer || read_length<jbuffer_size) {
    ret_val = memcpy(buffer, addr,jbuffer_size);
    addr+=jbuffer_size;
    read_length -= jbuffer_size;
}

So, your termination condition looks wrong: you decrement read_length and loop until it is less than buffer_size.

Also, you’re incrementing addr by buffer_size integers (it is an int*), not bytes. So, you’re advancing addr 4 times too fast.

BTW, in other code: lseek takes and returns an off_t, not a size_t.

---

Edit: most of these bugs are pointed out in the other question already, so I’m not sure this one adds anything.