posix.cfg: Improved configuration for ffs().

cfg/posix.cfg

@@ -4966,7 +4966,8 @@ The function 'mktemp' is considered to be dangerous due to race conditions and s
   <!-- http://man7.org/linux/man-pages/man3/ffs.3.html -->
   <!-- int ffs(int i); -->
   <function name="ffs">
-    <returnValue type="int"/>
+    <!-- It returns the position of the first bit set, or 0 if no bits are set in i. -->
+    <returnValue type="int">arg1==0 &amp;0</returnValue>
     <use-retval/>
     <noreturn>false</noreturn>
     <leak-ignore/>

test/cfg/posix.c

@@ -32,6 +32,16 @@
 #include <wchar.h>
 #include <string.h>
 
+#include <strings.h>
+void knownConditionTrueFalse_ffs(int i)
+{
+    // ffs() returns the position of the first bit set, or 0 if no bits are set in i.
+    const int x = ffs(0);
+    // cppcheck-suppress knownConditionTrueFalse
+    if (x == 0) {}
+    if (ffs(i) == 0) {}
+}
+
 ssize_t nullPointer_readlink(const char *path, char *buf, size_t bufsiz)
 {
     // cppcheck-suppress nullPointer