[SOLVED] GCC: Specified bound depends on the length of the source argument

Issue

The following code:

while (node)
{
    if (node->previous== NULL) break;
    struct Node* prevNode = node->previous;
    len = strlen(prevNode->entity);
    //pp is a char* fyi
    pp-=len;
    strncpy(pp, prevNode->entity, len+1);
    *(--pp) = '/';
    node = prevNode;
}

Generates the following warning/error in GCC (I treat all warnings as errors):

../someFile.C:1116:24: error: 'char* strncpy(char*, const char*, size_t)' specified bound depends on the length of the source argument [-Werror=stringop-overflow=]
 1116 |                 strncpy(pp, prevNode->entity, len+1);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../someFile.C:1114:29: note: length computed here
 1114 |                 len = strlen(prevNode->entity);
      |                       ~~~~~~^~~~~~~~~~~~~~~~~~~~

Why is GCC giving me a warning? What is wrong with relieing on the size of a source argument for the buffer size? Can someone give an example of what issues this may cause? Code does what it should I'm just curious why I'm getting a warning.

Solution

The point is that the length bound passed to strncpy should depend on the size of the destination argument, not the source argument. Otherwise, what is it even for? The compiler correctly recognises that there is no point to using strncpy here, and gives you an informative error message to that effect.

Answered By - TonyK

Answer Checked By - Terry (WPSolving Volunteer)