Question

I want to code vsnprintf function, using NSString.

This is my implementation of vsnprintf :

int my_vsnprintf(char *buffer, size_t count, const char *format, ...)
{
    int iRet;
    va_list ap;
    NSString *pnssBuffer=NULL;
    NSString *pnssFormat;
    const char *pcszBuffer;
    /* Format with NSString */
    pnssFormat=[NSString stringWithCString:format 
    encoding:NSUTF8StringEncoding];
    va_start(ap, format);
    pnssBuffer=[[NSString alloc] initWithFormat:pnssFormat arguments:ap];
    /* Copy to char * */
    pcszBuffer=[pnssBuffer cStringUsingEncoding:NSUTF8StringEncoding];
    memcpy(buffer, pcszBuffer, count);
    iRet=strlen(buffer);
    /* Free */
    [pnssBuffer release];
    va_end(ap);
    return iRet;
}

When I call it with char * param, it works :

    my_vsnprintf(buffer, 256, "hello %s", "world");
NSLog(@"buffer = %s", buffer);

print correctly this : buffer = hello world

But when I call it with wchar_t * param, I only get fist character of world

    my_vsnprintf(buffer, 256, "hello %ls", L"world");
NSLog(@"buffer = %s", buffer);

print this : buffer = hello w

I don’t understand why, any help for my function ?

Answer 1

According to String Format Specifiers in the “String Programming Guide”, initWithFormat:arguments: and friends do not support the “%ls” format. In fact, the “l” modifier is ignored here.

There is a “%S” format, but that is for a sequence of 16-bit Unicode characters, so you cannot use it for wchar_t either.

ADDED

wchar_t is 4-byte Unicode (little endian) on the iPhone and the Simulator, therefore you can convert it to NSString as follows:

wchar_t *w = L"Hello world";
NSString *str = [[NSString alloc] initWithBytes:w length:4*wcslen(w)
    encoding:NSUTF32LittleEndianStringEncoding];