Sscanf 1 byte data without overflow
I need to scan a 6 byte hexadecimal data representation and store it in a 6 byte array.
Please note, I am on an embedded system, so each byte counts.
Using sscanf like this:
uint8_t buf[6];
char hex[13] = "123456789ABC";
sscanf(hex, "%02X%02X%02X%02X%02X%02X", &buf[0], &buf[1], &buf[2], &buf[3], &buf[4], &buf[5]);
overflows because each% 02X specifier is loaded into uint32_t.
- % 02lX will be loaded into uint32_t
- % 02hX will be loaded into uint16_t
Is there a specifier that gets loaded into uint8_t? (I don't find anything)
Is there any other way to do this?
I tried to do:
sscanf(hex, "%08lX%04hX", &buf[0], &buf[4]);
it does not overflow, but since I am in a small structure the order is mixed ...
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Please note, I am on an embedded system, so each byte counts.
If so, then sscanf is probably not recommended; its stack usage and code space will overshadow any savings you might perceive when using the smallest data type possible. Consider:
uint8_t hexdigit( char hex )
{
return (hex <= '9') ? hex - '0' :
toupper(hex) - 'A' + 10 ;
}
uint8_t hexbyte( const char* hex )
{
return (hexdigit(*hex) << 4) | hexdigit(*(hex+1)) ;
}
Then your code will look like this:
char hex[13] = "123456789ABC";
for( int b = 0; b < 6; b++ )
{
buf[b] = hexbyte( &hex[b * 1] ) ;
}
If you must use sscanf()
, but your library does not support a format specifier hh
(as many built-in or older C libraries cannot), you can use an intermediate integer:
char hex[13] = "123456789ABC";
for( int b = 0; b < 6; b++ )
{
unsigned byte ;
buf[b] = hexbyte( &hex[b * 1] ) ;
sscanf( &hex[b * 2], "%02X", byte ) ;
buf[b] = (unit8_t)byte ;
}
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