How to avoid sprintf when joining variables - c

Question

I am working on code to get USB device details into single String, and have following code,
struct usb_bus *bus;
struct usb_device *dev;
usb_init();
usb_find_busses();
usb_find_devices();
for (bus = usb_busses; bus; bus = bus->next)
for (dev = bus->devices; dev; dev = dev->next)
{
// working outputs
printf("Trying device %s/%s\n", bus->dirname, dev->filename);
printf("Trying device2 %0x\n", dev->descriptor.idVendor);
printf("Trying device3 %0x\n", dev->descriptor.idProduct);
char deviceDetailsStr[150];
sprintf(deviceDetailsStr, "%s_%s_%0x_%0x", bus->dirname,
dev->filename,dev->descriptor.idVendor,dev->descriptor.idProduct);
... have other code here that works on "deviceDetailsStr"
}
Been reading thatt "sprintf" has performance issues, since it supports lots of transforms.
Can you please suggest what is better alternative to using "sprintf", so that all 4 variables data gets read into variable "deviceDetailsStr"
End goal is "deviceDetailsStr" char array needs to have all 4 entires as single string.
Thanks

Answer 1

If you want the best performance, I would say you need to write something custom. Here's an example for your specific requirements.
uint32_t printHex( char * buffer, uint32_t value ) {
uint32_t sz = value <= 0xF ? 1 :
value <= 0xFF ? 2 :
value <= 0xFFF ? 3 :
value <= 0xFFFF ? 4 :
value <= 0xFFFFF ? 5 :
value <= 0xFFFFFF ? 6 :
value <= 0xFFFFFFF ? 7 : 8;
for( uint32_t i=sz-1; i; i-- ) {
buffer[ i ] = ((value & 0xF) <= 9 ? '0' : 'a'-10 ) + (value & 0xF);
value=value>>4;
}
return sz;
}
char buffer[150];
unsigned bi=0;
for( char * ptr = bus->dirname ; *ptr; ptr++ ) buffer[bi++] = *ptr;
buffer[bi++] = '_';
for( char * ptr = bus->filename; *ptr; ptr++ ) buffer[bi++] = *ptr;
buffer[bi++] = '_';
bi += printHex( buffer + bi, dev->descriptor.idVendor );
buffer[bi++] = '_';
bi += printHex( buffer + bi, dev->descriptor.idProduct );
buffer[bi] = '\0';
Note: There are no size checks (just like sprintf). Again if you want best performance, this is a trade off.

Answer 2

Step 1: Determine the maximum buffer size needs.
Assume bus->dirname, dev->filename are arrays.
#define Mark_SZ ((sizeof bus->dirname - 1) + 1 + \
(sizeof dev->filename - 1) + 1 + \
((sizeof dev->descriptor.idVendor * CHAR_BIT + 3) /4) + 1 + \
((sizeof dev->descriptor.idProduct * CHAR_BIT + 3) /4) + 1)
#defined Extra (depends on: "other code here that works on "deviceDetailsStr"")
char deviceDetailsStr[Mark_SZ + Extra];
Step 2: Copy in each part
// Some untested code to give you an idea.
char *p = deviceDetailsStr;
size_t n = strlen(bus->dirname);
memcpy(p, bus->dirname, n);
p += n;
*p++ = '_';
n = strlen(dev->filename);
memcpy(p, dev->filename, n);
p += n;
*p++ = '_';
p += sprintf(p, "%x", dev->descriptor.idVendor);
*p++ = '_';
sprintf(p, "%x", dev->descriptor.idProduct);
I coded sprintf(p, "%x", dev->descriptor.idVendor) and sprintf(p, "%x", dev->descriptor.idProduct) by themselves as hoping even a modest compiler will recognize this and replace with the equivalent itoa() like function calls. Otherwise, simply code up a replacement unsigned to string.
I see no value with "0" in "%0x"