c - Valgrind Warning: Should I Take It Seriously

Question

Background: I have a small routine that mimics fgets(character, 2, fp) except it takes a character from a string instead of a stream. newBuff is dynamically allocated string passed as a parameter and character is declared as char character[2].

Routine:

character[0] = newBuff[0];

character[1] = '';

strcpy(newBuff, newBuff+1);

The strcpy replicates the loss of information as each character is read from it.

Problem: Valgrind does warns me about this activity, "Source and destination overlap in strcpy(0x419b818, 0x419b819)".

Should I worry about this warning?

Answer 1

Probably the standard does not specify what happens when these buffers overlap. So yes, valgrind is right to complain about this.

In practical terms you will most likely find that your strcpy copies in order from left-to-right (eg. while (*dst++ = *src++);) and that it's not an issue. But it it still incorrect and may have issues when running with other C libraries.

One standards-correct way to write this would be:

memmove(newBuff, newBuff+1, strlen(newBuff));

Because memmove is defined to handle overlap. (Although here you would end up traversing the string twice, once to check the length and once to copy. I also took a shortcut, since strlen(newBuff) should equal strlen(newBuff+1)+1, which is what I originally wrote.)