Hey I have a snippet of assembly code that does not make sense to me
.text:004016C5 prompt_uid: ; "[ User %d ]\t"
.text:004016C5 mov [esp+0Ch+var_C], offset aUserD
.text:004016CC mov ecx, [ebp+arg_0]
.text:004016CF mov [esp+0Ch+var_8], ecx
.text:004016D3 call printf
.text:004016D8 mov [esp+0Ch+var_C], esi
.text:004016DB call gets
.text:004016E0 test eax, eax ; test eax=0
.text:004016E2 jz short prompt_uid
At first I was confused by the test eax,eax and jz following a gets.
"Why would any input be 0?". Then I thought "Ok. strings are terminated by a nullbyte so an empty string should result in the test setting ZF"
But when I tested it in practice the jz was never taken. Even when I just hit enter at the prompt.
Where is my mistake?
The gets() reference gives the answer to when it will return NULL;
RETURN VALUE
Upon successful completion, gets() shall return s. If the stream is at end-of-file, the end-of-file indicator for the stream shall be set and gets() shall return a null pointer. If a read error occurs, the error indicator for the stream shall be set, gets() shall return a null pointer, [CX] and set errno to indicate the error.
In other words, gets() may return NULL on end of file or a read error, not in normal "press return" operation.