Question

I want to get the first 8 bytes or so of a file without reading the whole file. I’m using NSData to operate on the data and such, but I don’t want to slow down my application with excessive file reads and writes because in some cases I’m having to read a 200 kilobyte file just to extract the first 2 bytes of data from the file. Is there any way to only read or write a part of the file without reading or overwriting the whole thing in Xcode with the iOS SDK?

The file system that I’m using is just the default one that’s accessible through the NSFileManager class (I don’t know of any other iOS file system).

Answer 1

Use the standard C file API (either FILE* or int file descriptors). The caveat is that you have to properly convert the string path to a correct char* file path. Also, don’t forget to close the file when done. Consider a category on NSData, something kinda like this…

+ (id)dataWithContentsOfFile:(NSString *)filePath numBytes:(NSUInteger)numBytes
{
    void *bytes = malloc(numBytes);
    NSData *result = [NSData dataWithBytesNoCopy:bytes length:numBytes];
    char const *path = [[NSFileManager defaultManager] fileSystemRepresentationWithPath:filePath];
    int fd;
    if ((fd = open(path, O_RDONLY)) < 0 || read(fd, bytes, numBytes) != numBytes) {
        result = nil;
    }
    close(fd);
    return result;
}