Why doesn't the compiler report a missing semicolon?
C is a free-form language. That means you could format it in many ways and it will still be a legal program.
For example a statement like
a = b * c;
could be written like
a=b*c;
or like
a
=
b
*
c
;
So when the compiler see the lines
temp = *a
*a = *b;
it thinks it means
temp = *a * a = *b;
That is of course not a valid expression and the compiler will complain about that instead of the missing semicolon. The reason it's not valid is because a is a pointer to a structure, so *a * a is trying to multiply a structure instance (*a) with a pointer to a structure (a).
While the compiler can't detect the missing semicolon, it also reports the totally unrelated error on the wrong line. This is important to notice because no matter how much you look at the line where the error is reported, there is no error there. Sometimes problems like this will need you to look at previous lines to see if they are okay and without errors.
Sometimes you even have to look in another file to find the error. For example if a header file is defining a structure the last it does in the header file, and the semicolon terminating the structure is missing, then the error will not be in the header file but in the file that includes the header file.
And sometimes it gets even worse: if you include two (or more) header files, and the first one contains an incomplete declaration, most probably the syntax error will be indicated in the second header file.
Related to this is the concept of follow-up errors. Some errors, typically due to missing semicolons actually, are reported as multiple errors. This is why it's important to start from the top when fixing errors, as fixing the first error might make multiple errors disappear.
This of course can lead to fixing one error at a time and frequent recompiles which can be cumbersome with large projects. Recognizing such follow-up errors is something that comes with experience though, and after seeing them a few times it's easier to dig out the real errors and fix more than one error per recompile.
Why doesn't the compiler detect the missing semicolon?
There are three things to remember.
- Line endings in C are just ordinary whitespace.
*in C can be both a unary and a binary operator. As a unary operator it means "dereference", as a binary operator it means "multiply".- The difference between unary and binary operators is determined from the context in which they are seen.
The result of these two facts is when we parse.
temp = *a /* Oops, missing a semicolon here... */
*a = *b;
The first and last * are interpreted as unary but the second * is interpreted as binary. From a syntax perspective, this looks OK.
It is only after parsing when the compiler tries to interpret the operators in the context of their operand types that an error is seen.
Some good answers above, but I will elaborate.
temp = *a *a = *b;
This is actually a case of x = y = z; where both x and y are assigned the value of z.
What you are saying is the contents of address (a times a) become equal to the contents of b, as does temp.
In short, *a *a = <any integer value> is a valid statement. As previously pointed out, the first * dereferences a pointer, while the second multiplies two values.