fd-util: introduce parse_fd()

It's a simple wrapper for safe_atoi() that returns error if the parsed
fd is < 0 .

src/basic/parse-util.c

@@ -333,6 +333,21 @@ int parse_errno(const char *t) {
         return e;
 }
 
+int parse_fd(const char *t) {
+        int r, fd;
+
+        assert(t);
+
+        r = safe_atoi(t, &fd);
+        if (r < 0)
+                return r;
+
+        if (fd < 0)
+                return -ERANGE;
+
+        return fd;
+}
+
 static const char *mangle_base(const char *s, unsigned *base) {
         const char *k;
 

src/basic/parse-util.h

@@ -21,6 +21,7 @@ int parse_size(const char *t, uint64_t base, uint64_t *size);
 int parse_sector_size(const char *t, uint64_t *ret);
 int parse_range(const char *t, unsigned *lower, unsigned *upper);
 int parse_errno(const char *t);
+int parse_fd(const char *t);
 
 #define SAFE_ATO_REFUSE_PLUS_MINUS (1U << 30)
 #define SAFE_ATO_REFUSE_LEADING_ZERO (1U << 29)