CMake: Extend TIMESTAMP sub-commands with new unix time format specifier

The new `%s` format specifier is substituted by file()/string()
`TIMESTAMP` sub-commands with the number of seconds since unix-epoch
(1970-01-01 00:00:00 UTC).

Co-Author: Nils Gladitz <nilsgladitz@gmail.com>

2 files changed, 66 insertions, 9 deletions

diff --git a/Source/cmTimestamp.cxx b/Source/cmTimestamp.cxx
index 6fd6ab7..1c795c4 100644
--- a/Source/cmTimestamp.cxx
+++ b/Source/cmTimestamp.cxx
@@ -12,9 +12,11 @@
 #include "cmTimestamp.h"
 
 #include <cstring>
+#include <cstdlib>
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <sstream>
 
 //----------------------------------------------------------------------------
 std::string cmTimestamp::CurrentTime(
@@ -44,7 +46,7 @@ std::string cmTimestamp::FileModificationTime(const char* path,
 
 //----------------------------------------------------------------------------
 std::string cmTimestamp::CreateTimestampFromTimeT(time_t timeT,
-    std::string formatString, bool utcFlag)
+  std::string formatString, bool utcFlag) const
 {
   if(formatString.empty())
     {
@@ -79,12 +81,12 @@ std::string cmTimestamp::CreateTimestampFromTimeT(time_t timeT,
   for(std::string::size_type i = 0; i < formatString.size(); ++i)
     {
     char c1 = formatString[i];
-    char c2 = (i+1 < formatString.size()) ?
-      formatString[i+1] : static_cast<char>(0);
+    char c2 = (i + 1 < formatString.size()) ?
+      formatString[i + 1] : static_cast<char>(0);
 
     if(c1 == '%' && c2 != 0)
       {
-      result += AddTimestampComponent(c2, timeStruct);
+      result += AddTimestampComponent(c2, timeStruct, timeT);
       ++i;
       }
     else
@@ -97,8 +99,40 @@ std::string cmTimestamp::CreateTimestampFromTimeT(time_t timeT,
 }
 
 //----------------------------------------------------------------------------
+time_t cmTimestamp::CreateUtcTimeTFromTm(struct tm &tm) const
+{
+#if defined(_MSC_VER) && _MSC_VER >= 1400
+  return _mkgmtime(&tm);
+#else
+  // From Linux timegm() manpage.
+
+  std::string tz_old = "TZ=";
+  if (const char* tz = cmSystemTools::GetEnv("TZ"))
+    {
+    tz_old += tz;
+    }
+
+  // The standard says that "TZ=" or "TZ=[UNRECOGNIZED_TZ]" means UTC.
+  // It seems that "TZ=" does NOT work, at least under Windows
+  // with neither MSVC nor MinGW, so let's use explicit "TZ=UTC"
+
+  cmSystemTools::PutEnv("TZ=UTC");
+
+  tzset();
+
+  time_t result = mktime(&tm);
+
+  cmSystemTools::PutEnv(tz_old);
+
+  tzset();
+
+  return result;
+#endif
+}
+
+//----------------------------------------------------------------------------
 std::string cmTimestamp::AddTimestampComponent(
-  char flag, struct tm& timeStruct)
+  char flag, struct tm& timeStruct, const time_t timeT) const
 {
   std::string formatString = "%";
   formatString += flag;
@@ -117,6 +151,26 @@ std::string cmTimestamp::AddTimestampComponent(
     case 'y':
     case 'Y':
       break;
+    case 's': // Seconds since UNIX epoch (midnight 1-jan-1970)
+      {
+      // Build a time_t for UNIX epoch and substract from the input "timeT":
+      struct tm tmUnixEpoch;
+      memset(&tmUnixEpoch, 0, sizeof(tmUnixEpoch));
+      tmUnixEpoch.tm_mday = 1;
+      tmUnixEpoch.tm_year = 1970-1900;
+
+      const time_t unixEpoch = this->CreateUtcTimeTFromTm(tmUnixEpoch);
+      if (unixEpoch == -1)
+        {
+        cmSystemTools::Error("Error generating UNIX epoch in "
+          "STRING(TIMESTAMP ...). Please, file a bug report aginst CMake");
+        return std::string();
+        }
+
+      std::stringstream ss;
+      ss << static_cast<long int>(difftime(timeT, unixEpoch));
+      return ss.str();
+      }
     default:
       {
       return formatString;
diff --git a/Source/cmTimestamp.h b/Source/cmTimestamp.h
index 24c1869..7c4b216 100644
--- a/Source/cmTimestamp.h
+++ b/Source/cmTimestamp.h
@@ -16,7 +16,7 @@
 #include <time.h>
 
 /** \class cmTimestamp
- * \brief Utility class to generate sting representation of a timestamp
+ * \brief Utility class to generate string representation of a timestamp
  *
  */
 class cmTimestamp
@@ -30,10 +30,13 @@ public:
     const std::string& formatString, bool utcFlag);
 
 private:
-  std::string CreateTimestampFromTimeT(time_t timeT,
-      std::string formatString, bool utcFlag);
+  time_t CreateUtcTimeTFromTm(struct tm& timeStruct) const;
 
-  std::string AddTimestampComponent(char flag, struct tm& timeStruct);
+  std::string CreateTimestampFromTimeT(
+    time_t timeT, std::string formatString, bool utcFlag) const;
+
+  std::string AddTimestampComponent(
+    char flag, struct tm& timeStruct, time_t timeT) const;
 };