Friday, 21 May 2021

Adjacent C string concatenation gotcha

C has the useful feature of adjacent allowing literal strings to be automatically concatenated. This is described in K&R "The C programming language" 2nd edition, page 194, section A2.6 "String Literals":

"Adjacent string literals are concatenated into a single string."

Unfortunately over the years I've seen several occasions where this useful feature has led to bugs not being detected, for example, the following code:


A simple typo in the "Does Not Compute" error message ends up with the last two literal strings being "silently" concatenated, causing an array out of bounds read when error_msgs[7] is accessed.

This can also bite when a new literal string is added to the end of the array.  The previous string requires a comma to be added when a new string is added to the array, and sometimes this is overlooked.  An example of this is in ACPICA commit 81eb9c383e6dee0f1b6620e91e5c3dbb48234831  - fortunately static analysis detected this and it has been fixed with commit 3f8c79fc22fd64b739f51268654a6783a874520e

The concerning issue is that this useful string concatenation feature can produce hazardous outcomes with coding mistakes that are simple to make but hard to notice.