etl/docs/getting-started/interfacing-with-c.md

title	weight
Interfacing with C	3

Interfacing ETL containers with contiguous storage to a C API is fairly easy to do.

This is a C function that copies count characters , starting from a, to a buffer pointed to by start.

size_t CFunctionCopyToBuffer(char* start, size_t count)
{
  size_t i = 0;
  while (i < count)
  {
    *p++ = 'a' + i;
    ++i;
  }
  return i;
}

---

Array
The ETL array is simply a C array wrapped in a C++ interface.

size_t length;

etl::array<char, 26> buffer;

length = CFunctionCopyToBuffer(buffer.data(), buffer.max_size() / 2);

Array contains [ a, b, c, d, e, f, g, h, i, j, k, l, m ]

---

Vector
To use a vector with C, you must make room for the expected new data, and then adjust to the new size after copying.

etl::vector<char, 26> buffer;

// Make the vector as big as it can be.
buffer.uninitialized_resize(buffer.max_size());

size_t newLength = CFunctionCopyToBuffer(buffer.data(), buffer.max_size() / 2);

 // Now adjust it back to the actual new size.
buffer.resize(newLength);

Vector contains [ a, b, c, d, e, f, g, h, i, j, k, l, m ]

---

String
Strings are terminated with null characters. We can use this to easily adjust the length after copying.

etl::string<26> buffer;

// Ensure that the string's free space are terminator characters.
buffer.initialize_free_space();

CFunctionCopyToBuffer(buffer.data(), buffer.max_size() / 2);

// Trim the string length at the first null terminator.
buffer.trim_to_terminator();

String contains "abcdefghijklm"

// Lets add some more characters.
// Start at data_end() so we append to what's already there.
// We'll use the alternative method to adjust the string length.
// We could still use trim_to_terminator() if we wanted to.

// Remember the current size.
size_t oldSize = buffer.size();

// Make the string as big as it can be.
buffer.uninitialized_resize(buffer.max_size()); 

// Append the characters.
size_t appendedSize = CFunctionCopyToBuffer(buffer.data_end(), buffer.max_size() / 2);

// Now adjust it back to the actual new size.
buffer.uninitialized_resize(oldSize + appendedSize); 

String contains "abcdefghijklmabcdefghijklm"