Introduce pam_str_skip_prefix_len and pam_str_skip_prefix

Every time I see a code like
  if (!strncmp(*argv,"user_readenv=",13))
    *user_readenv = atoi(13+*argv);
my eyes are bleeding.

Introduce a new helper inline function pam_str_skip_prefix_len() and
a new macro pam_str_skip_prefix() on top of it, to be used in subsequent
commits to cleanup the ugliness.

* libpam/include/pam_inline.h: Include <string.h>.
(pam_str_skip_prefix_len): New function.
(pam_str_skip_prefix): New macro.

1 files changed, 15 insertions, 0 deletions

diff --git a/libpam/include/pam_inline.h b/libpam/include/pam_inline.h
index d93f8474..2e3a8d6a 100644
--- a/libpam/include/pam_inline.h
+++ b/libpam/include/pam_inline.h
@@ -9,6 +9,7 @@
 #define PAM_INLINE_H
 
 #include "pam_cc_compat.h"
+#include <string.h>
 
 /*
  * Evaluates to
@@ -34,4 +35,18 @@
 /* Evaluates to the number of elements in the specified array.  */
 #define PAM_ARRAY_SIZE(a_)		(sizeof(a_) / sizeof((a_)[0]) + PAM_MUST_BE_ARRAY(a_))
 
+/*
+ * Returns NULL if STR does not start with PREFIX,
+ * or a pointer to the first char in STR after PREFIX.
+ * The length of PREFIX is specified by PREFIX_LEN.
+ */
+static inline const char *
+pam_str_skip_prefix_len(const char *str, const char *prefix, size_t prefix_len)
+{
+	return strncmp(str, prefix, prefix_len) ? NULL : str + prefix_len;
+}
+
+#define pam_str_skip_prefix(str_, prefix_)	\
+	pam_str_skip_prefix_len((str_), (prefix_), sizeof(prefix_) - 1 + PAM_MUST_BE_ARRAY(prefix_))
+
 #endif /* PAM_INLINE_H */