Question

For some reasons I evaluate the option to exchange data between my iOS-App and my C++-Lib (used by the iOS-App) by file.

C++ (works fine on a local machine):

    void LibFacadeTest::testFileAccess()
    {
      this->log->info("start testFileAccess");

        char filename[] = "./test.txt";
        std::string result;

        LibFacade *e = new LibFacade();

        try
        {
            result = e->testFileAccess(filename);
        }
        catch(...)
        {
            this->log->error("error");
        }
        this->log->info("Result:" + result);

        delete(e);
  }

Recompiled for ARM and run on device I can see that the filename parameter is passed correctly (via log), but nothing is returned.

Here is the Objective-C code:

LibFacade *tlb = new LibFacade();
char *testName = "blub.txt";

std::string str = tlb->testFileAccess(testName);

// Check, if file exists
NSFileManager *filemgr;

filemgr = [NSFileManager defaultManager];

if ([filemgr fileExistsAtPath: @"blub.txt" ] == YES)
    NSLog (@"File exists");
else
    NSLog (@"File not found");

NSError *error = nil;
NSString *myString = [NSString stringWithContentsOfFile:@"blub.txt" encoding:NSUTF8StringEncoding error:&error];

UIActionSheet *actionSheet = [[UIActionSheet alloc] initWithTitle:myString                                               
delegate:nil                                                      cancelButtonTitle:nil                                                destructiveButtonTitle:myString                                                      otherButtonTitles:nil];
    [actionSheet showInView:self.view];

A bit quick and dirty, but what I can see is that there no file has been created on device although the lib could be used.

My question:
Are there any restrictions to access a device file system from an included C++ lib? Do I have to use any special directories for file exchange?

Answer 1

Your application is “sandboxed”, meaning that it cannot access files outside of its temporary and document file space. "./test.txt" is outside the “sandbox area”, so iOS correctly does not let you access that file. In a nutshell, if you would like to transfer data using temporary files, you’d need something like this for a file name:

NSString *tempDirectoryName =
    [NSTemporaryDirectory() stringByAppendingPathComponent:@"test.txt"];

This document provides further reading on the subject.