A format string in a print function call allows the following format specifiers preceded by a %: d, i, o, u, x, X, f, F, e, E, g, G, a, A, c, s, p, n, C (XCI), S (XSI), and %. The SV.FMT_STR.UNKNOWN_FORMAT checker flags the code if there is an invalid format specifier in the format string.

Vulnerability and risk

When the syntax of the conversion specification is incorrect, the resulting behavior is undefined, and it may cause unexpected output.

Vulnerable code example

1  void foo(FILE* f, char* pc, int i, char c) {
2      fprintf(f, "%c %s %k", c, pc);
3      fprintf(f, "%o", i);
4  }

Klocwork reports an error for line 2, since %k is an invalid format specifier.

Fixed code example

1  void foo(FILE* f, char* pc, int i, char c) {
2      fprintf(f, "%c %s", c, pc);
3      fprintf(f, "%o", i);
4  }

In the fixed code, the invalid format specifier has been removed.

Related checkers

External guidance

CWE-686: Function Call with Incorrect Argument Type