Initialize the length of a buffer array with a variable - c++

Question

I am reading buffer bytes from a socket but I don't know how to initialize the buffer array with the length info.
uint32_t len;
int lengthbytes = 0;
int databytes = 0;
// receive the length info of an image data
lengthbytes = recv(clientSocket, (char *)&len, sizeof(len), 0);
// convert hexadecimal data to length in bytes
len = ntohl(len);
// ????? how to initialize the buffer array with the length info ????
char buf[len]; -----> this is illegal in C
// read the image data
databytes = recv(clientSocket, buf, sizeof(buf), 0);

Answer 1

len = ntohl(len);
char buf[len]; //----> this is illegal in C
This is valid in C99 and it is called a variable length array. If you are not using C99 use malloc to allocate the array (and declare buf as a char *).

Answer 2

When you declare buf you declare a variable length array. This is legal in C (from the C99 standard) but illegal in C++. In C++ you can instead use std::vector:
std::vector<char> buf(len);
You can use this vector in the call to recv as well:
databytes = recv(clientSocket, &buf[0], buf.size(), 0);
To use the vector inside a loop, you have two choices:
Declare the variable outside the loop, and use clear and resize when needed:
std::vector<char> buf;
// ...
for (int i = 0; i < number_of_images; i++)
{
std::cout << "Fetching image #" << (i + 1) << '\n';
// Get the image length
size_t length = get_image_length();
buf.clear(); // Clear the buffer
buf.resize(length); // Set the size to the image length
// Receive the image
databytes = recv(clientSocket, &buf[0], buf.size(), 0);
}
Declare the vector to be local inside the loop:
for (int i = 0; i < number_of_images; i++)
{
std::cout << "Fetching image #" << (i + 1) << '\n';
// Get the image length
size_t length = get_image_length();
std::vector<char> buf(length);
// Receive the image
databytes = recv(clientSocket, &buf[0], buf.size(), 0);
}

Answer 3

You have to use dynamic memory allocation;
char* buf = new char[len];
If you're done using buf, don't forget to call delete to free the memory.
delete[] buf;

Answer 4

Please allocate the buffer through malloc i.e. buf = malloc(sizeof(char) * len);

Answer 5

You can do it with new or malloc.
Don't forget to delete the buffer when done!

Answer 6

You can use an std::vector<char>, and then use it's data() as an array buffer:
#include <vector>
std::vector<char> buf(len);
databytes = recv(clientSocket, buf.data(), buf.size(), 0); // access underlying char array
databytes = recv(clientSocket, &buf[0], buf.size(), 0); // as above, C++03 version

Answer 7

I wrote a class called tempbuf precisely for this purpose in C++.
You can find it here:
small_lib.cpp
small_lib.h
These two files are MIT-licensed, so you can use it anyway you like.
How to use this class?
tempbuf buf(len);
databytes = recv(clientSocket, buf.get(), buf.size(), 0); // if you want char* returned
databytes = recv(clientSocket, buf.constchar(), buf.size(), 0); // if you want constchar* returned
And guess why I wrote this class? You don't need to delete or deallocate the dynamically allocated memory because it is done in the destructor of the class.
Why didn't I use std::auto_ptr? Because from my understanding, that's only for non-arrays as in it supports new X but not new X[10].