Function sscanf must be assigned to variable otherwise strange behavior - c

Question

Consider this code:
#define TRANSLATOR_requestElectricityMeterWrite() do{addr = word_getAddress(); value = word_getValue(); }while(0)
uint16_t value;
uint8_t addr;
bool dispatcher(void)
{
TRANSLATOR_requestElectricityMeterWrite();
return true;
} // AFTER this point (during debug) program goes to default handler
int main(void)
{
if(dispatcher())
continue;
. . . .
. . . .
}
uint16_t word_getValue(void)
{
uint16_t value;
sscanf("ABCD", "%4x", (unsigned int *)&value);
return value;
}
uint8_t word_getAddress(void)
{
uint8_t address;
sscanf("00", "%2x", (unsigned int *)&address);
;
return address;
}
When the code above is run, the statement inside if causes program to crash(goes to some default handler).
But when I change the two(word_getValue and word_getAddres) functions to this:
uint16_t word_getValue(void)
{
uint16_t value;
int i = 0;i++;
i = sscanf(WORD_getValueString(), "%4x", (unsigned int *)(&value));
return value;
}
uint8_t word_getAddress(void)
{
uint8_t address;
int i = 0;i++;
i = sscanf(WORD_getNameString(), "%2x", (unsigned int *)(&address));
return address;
}
It works. The addition if the dummy i seems to solve that problem. But why doesn't it work the other way?
GNU ARM v4.8.3 toolchain

Answer 1

Both functions invoke undefined behavior, hence anything can happen. Adding an extra local variable changes the location of the destination variable, hiding the effect of its incorrect size.
sscanf("ABCD", "%4x", (unsigned int *)&value);
sscanf will store sizeof(unsigned int) bytes (probably 4) into variable value, which has only 2 bytes.
sscanf(WORD_getNameString(), "%2x", (unsigned int *)(&address));
Will store sizeof(unsigned int) bytes into variable address, which has only 1 byte.
The easiest way to fix this problem is to parse into an unsigned int and store the parsed value to the destination separately, or simply return the value:
uint16_t word_getValue(void) {
unsigned int value;
if (sscanf(WORD_getValueString(), "%4x", &value) == 1)
return value;
// could not parse a value, return some default value or error code
return 0;
}
uint8_t word_getAddress(void) {
unsigned int address;
if (sscanf(WORD_getNameString(), "%2x", &address) == 1)
return address;
// could not parse a value, return some default value or error code
return 0;
}
You might also want to verify if the parsed value is within range for the destination type, but since you limit the parse to respectively 4 and 2 hex digits, overflow cannot happen.

Answer 2

%x format requires unsigned argument (suppose it's uint32_t on your platform). If you pass uint16_t or uint8_t it can corrupt memory. In your case it corrupt stack and overwrites return address. Try use %4hx for uint16_t and %2hhx for uint8_t.