Store more data in iov in C++ - c++

Question

I am writing a C++ program (see below). My goal is to store data in iov struct. I have allocated buffer of fixed length in constructor. Every time that buffer gets filled, I want to transfer data in iov and allocated new buffer of fixed length. Finally when done with data processing, I want to return iov struct. My intension here is to store all these data into iov so that if it's required in future, I can send data easily. I have written sample code. But it seems it's not working. I got an "Bus error: 10". Can someone help me?
Sample code:
#include <iostream>
#include <string>
#include <sys/uio.h>
#include <cstdlib>
using namespace std;
#define MAX_LEN 1000
#define MIN_LEN 20
class MyClass
{
public:
MyClass();
~MyClass();
void fillData(std::string &data);
private:
struct iovec *iov;
unsigned int count;
unsigned int len;
char *buf;
unsigned int total_len;
unsigned int tmp_len;
};
MyClass::MyClass()
{
cout << "Inside constructor" << endl;
total_len = MIN_LEN;
buf = (char *)malloc(MAX_LEN);
if (buf == NULL) {
cout << "Error: can’t allocate buf" << endl;
exit(EXIT_FAILURE);
}
}
MyClass::~MyClass()
{
free(buf);
}
void MyClass::fillData(std::string &data)
{
unsigned int d_len, tmp_len, offset;
d_len = data.size();
const char* t = data.c_str();
total_len += d_len;
tmp_len += d_len;
if (total_len > MAX_LEN) {
/* Allocate memory and assign to iov */
tmp_len = d_len;
}
memcpy(buf + offset, t, d_len);
/* Adjust offset */
}
int main()
{
MyClass my_obj;
int i;
std::string str = "Hey, welcome to my first class!";
for (i = 0; i < 10; i++) {
my_obj.fillData(str);
}
return 0;
}

Answer 1

Without understanding the intent of your program in detail, it is very clear that you forgot to reserve memory for the iov-objects themselfes.
For example, in your constructor you write iov[0].iov_base = buf, yet iov has not been allocated before.
To overcome this, somewhere in your code, before the first access to iov, you should write something like iov = calloc(100,sizeof(struct iovev)) or a c++ equivalent using new[].
Consider the following program:
struct myStruct {
char *buf;
int len;
};
int main() {
struct myStruct *myStructPtr;
myStructPtr->buf = "Herbert"; // Illegal, since myStructPtr is not initialized; So even if "Herbert" is valid, there is no place to store the pointer to literal "Herbert".
myStructPtr[0].buf = "Herbert"; // Illegal, since myStructPtr is not initialized
// but:
struct myStruct *myStructObj = new (struct myStruct);
myStructObj->buf = "Herbert"; // OK, because myStructObj can store the pointer to literal "Herbert"
myStructObj->buf = "Something else"; // OK; myStructObj can hold a pointer, so just let it point to a different portion of memory. No need for an extra "new (struct myStruct)" here
}

Answer 2

I took your code, which didn't exactly use anything with the iovec, and I modified it a little.
I am not sure why developers prefer buffers of char* instead of std::string
or why use a pointer which should be allocated and then deleted instead of using a std::vector
I also added a function which uses the iovec. It is called void MyClass::print_data(). It prints all the data in the vector iovecs
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include <vector>
#include <unistd.h>
#include <sys/uio.h>
using namespace std;
class MyClass
{
vector<struct iovec> iovs;
vector<string> bufs;
public:
MyClass();
~MyClass();
void fill_data(const string &data);
void print_data();
};
MyClass::MyClass()
{
cout << "Inside constructor" << endl;
}
MyClass::~MyClass()
{
}
void MyClass::fill_data(const string &data)
{
stringstream stream;
stream << setw(2) << setfill(' ') << (this->bufs.size() + 1) << ". "
<< data << endl;
this->bufs.push_back(stream.str());
iovec iov = {&(bufs.back()[0]), bufs.back().size()};
this->iovs.push_back(iov);
}
void MyClass::print_data() {
writev(STDOUT_FILENO, iovs.data(), iovs.size());
}
int main() {
MyClass my_obj;
string str = "Hey, welcome to my first class!";
for (int i = 0; i < 10; ++i) {
my_obj.fill_data(str);
}
my_obj.print_data();
return 0;
}
compile it like so: g++ test.cpp