Passing a preallocated char* buffer to a c function

Question

I have a function in c that receives a preallocated string and fills it with data - I'd like to call this function from swift. I've found many examples on how to pass strings from swift to c, but I found none showing c functions that write the data on a preallocated string.

The c function:

int GetData(char *dataJson, int maxSize);

On the swift side, this code compiles, but I can't find how to preallocate dataBuffer

let dataBuffer = UnsafePointer<UInt8>
GetData(dataBuffer, 2048)

Is there a way to do so in Swift?

Martin R · Accepted Answer · 2017-09-19 12:10:10Z

7

An easy way is to pass an array as inout expression with &, this calls the function with a pointer to the contiguous element storage:

var dataBuffer = Array<Int8>(repeating: 0, count: 2048)
let result = GetData(&dataBuffer, numericCast(dataBuffer.count))

The advantage is that you don't have to manage the memory manually, it is released automatically when dataBuffer goes out of scope.

edited Sep 19, 2017 at 12:10

answered Sep 19, 2017 at 11:59

Martin R

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Comments

Nitay · Accepted Answer · 2017-09-19 11:56:00Z

0

I've figured it out, here's the code for future reference:

let dataBuffer = UnsafeMutablePointer<Int8>.allocate(capacity: 2048)
GetData(dataBuffer, 2048)
let jsonData = String(cString: dataBuffer)

answered Sep 19, 2017 at 11:56

Nitay

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1 Comment

Martin R Over a year ago

That's correct, but you should deallocate the memory eventually, otherwise you have a memory leak.

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