Set increment data to Avro array type using C language

Question

I am programming in Objective-C. I use Apache Avro to serialize data.

My avro scheme is this:

{ "name": "School", "type":"record", "fields":[ { "name":"Employees", "type":["null", {"type": "array", "items":{ "name":"Teacher", "type":"record", "fields":[ {"name":"name", "type":"string"} {"name":"age", "type":"int"} ] } } ], "default":null } ] } 

In my Objective-C code, I have an Array object from Teacher , each teacher object contains a value of name and age .

I want to write teacher array data to a file using Avro with the above diagram. I'm mostly worried about how to write data to the Employees array defined in the above diagram.

Here is my code (for this I have to use C style code, I follow the Avro C documentation):

 // I don't show this function, it constructs the a `avro_value_t` based on the schema. No problem here. avro_value_t school = [self constructSchoolValueForSchema]; // get "Employees" field avro_value_t employees; avro_value_get_by_name(school, "employees", &employees, 0); int idx = 0; for (Teacher *teacher in teacherArray) { // get name and age NSString *name = teacher.name; int age = teacher.age; // set value to avro data type. // here 'unionField' is the field of 'Employees', it is a Avro union type which is either null or an array as defined in schema above avro_value_t field, unionField; avro_value_set_branch(&employees, 1, &unionField); // based on documentation, I should use 'avro_value_append' avro_value_append(&employees, name, idx); // I get confused here!!!! // in above line of code, I append 'name' to 'employees', //which looks not correct, // because the 'Employees' array is an array of 'Teacher', not arrary of 'name' // What is the correct way to add teacher to 'employees' ? idx ++; } 

The question I want to ask is actually in the code comment above.

I follow the Avro C documentation, but am lost, how can I add each Teacher to Employees ? In my code above, I added only each name teacher to the Employees array.

Answer 1

I think there are two things with your code, but I am not familiar with Avro, so I cannot guarantee one of them. I just glanced quickly at the documentation you linked, and here is how I understood avro_value_append :

Creates a new element, that is, Teacher, and returns this via the link in the second parameter (so that it returns by reference). I assume that you need to use other avro... methods avro... to populate this element (i.e. set the name of the teacher, etc.). In the end, do the following:

 avro_value_t teacher; size_t idx; avro_value_append(&employees, &teacher, &idx); // note that idx is also returned by reference and now contains the new elements index 

I'm not sure if you set up the employees correctly, by the way, I did not have time for this.

A second problem will arise with your use of name at some point. I assume Avro is expecting a C string, but you are using NSString here. You will need to use the getCString:maxLength:encoding: method to populate the prepared buffer to create a C string that you can pass to Avro. You may also be able to use UTF8String , but read your documentation: you will most likely have to copy the memory ( memcpy shenanigans), otherwise your Avro container will extract its data from underneath.