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csegmentation-faultscanfstrcmp

Broken C code for reading database

I have a snippet of code, in C for reading entries from a passwd-style database:

do {
    if (fscanf(db, "%s:%s:%d", uname, passwd, &gid) == EOF) {
        return NULL;
    }
} while (strcmp(uname, username));

However, when running it, it gives a segmentation fault. After running it in the debugger, I found that the strcmp part is the part raising the segfault. Data types:

  • db is FILE *, opened already.
  • uname and passwd are char *, both initialised to NULL
  • username is char [64]
  • gid is int

How can I fix this problem, and alternatively how can I parse the database without needing to use this ugly hack?

Solution

  • If both the string variables uname and passwd are just pointers, that doesn't point anywhere, what do you think happens when fscanf tries to use those pointers and write the string into memory?

    That will lead to undefined behavior, as will the dereferencing done in the strcmp call.

    You should use arrays for the uname and passwd variables as well.