Question

I am new to C and reviewing some source code. But I am not sure what is happening with this code snippet.

I really don’t think it is doing anything, as in the debugging the output seems to be the same for tempstr.

This is what I think, correct if I am wrong.
*(tempstr + strlen(line)) is adding the length of line to tempstr and dereferencing and assigning the 0x0 converted to a char?

char line[128], tempstr[128]

strcpy(line, "AUTO_ANSWER_CALL = 1");
strcpy(tempstr,line);
*(tempstr + strlen(line)) = (char) 0x0; // Confusing part

Answer 1

This is a pointer value:

tempstr

This is another pointer value (which points to 5 elements beyond the tempstr pointer value):

tempstr + 5

This is an integer:

strlen(line)

Therefore, this is a pointer value (which points to strlen(line) elements beyond the tempstr pointer value):

tempstr + strlen(line)

And this is dereferencing that pointer:

*(tempstr + strlen(line))

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This is what I think, correct if I am wrong. *(tempstr + strlen(line)) is adding the length of line to tempstr and dereferencing and assigning the 0x0 converted to a char?

It’s ensuring that the character at index 20 of tempstr, immediately beyond the “AUTO_ANSWER_CALL = 1” characters, is null: i.e. it’s ensuring that the string is null-terminated.

That string already is null-terminated, by the way (so that last statement is redundent): because strcpy copies the string including the implicit null-termination character.

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Isn’t easier just to do the following tempstr[sizeof(tempstr)-1] = ‘\0’; this is much easier to understand.

These aren’t the same thing: strlen(line) equals 20, but sizeof(tempstr) equals 128.

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would this work: tempstr[strlen(tempstr)] = ‘\0’

That’s exactly the same things as:

*(tempstr + strlen(tempstr)) = '\0'

Just a different way of writing it.

However, if tempstr is NOT a null terminated string, the length would be 128 as well.

If tempstr is NOT a null terminated string then strlen(tempstr) is undefined (‘undefined’ means that it’s meaningless and dangerous, a bug, and shouldn’t be used): the strlen function isn’t valid except when it’s used on a string that’s already null-terminated.