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Here's another example of this fine feature:

  #include <stdio.h>
  #include <string.h>
  #include <stdlib.h>
  #define LENGTH 128

  int main(int argc, char **argv) {
      char *string = NULL;
      int length = 0;
      if (argc > 1) {
          string = argv[1];
          length = strlen(string);
          if (length >= LENGTH) exit(1);
      }

      char buffer[LENGTH];
      memcpy(buffer, string, length);
      buffer[length] = 0;

      if (string == NULL) {
          printf("String is null, so cancel the launch.\n");
      } else {
          printf("String is not null, so launch the missiles!\n");
      }

      printf("string: %s\n", string);  // undefined for null but works in practice

      #if SEGFAULT_ON_NULL
      printf("%s\n", string);          // segfaults on null when bare "%s\n"
      #endif

      return 0;
  }

  nate@skylake:~/src$ clang-3.8 -Wall -O3 null_check.c -o null_check
  nate@skylake:~/src$ null_check
  String is null, so cancel the launch.
  string: (null)

  nate@skylake:~/src$ icc-17 -Wall -O3 null_check.c -o null_check
  nate@skylake:~/src$ null_check
  String is null, so cancel the launch.
  string: (null)

  nate@skylake:~/src$ gcc-5 -Wall -O3 null_check.c -o null_check
  nate@skylake:~/src$ null_check
  String is not null, so launch the missiles!
  string: (null)
It appear that Intel's ICC and Clang still haven't caught up with GCC's optimizations. Ouch if you were depending on that optimization to get the performance you need!

But before picking on GCC too much, consider that all three of those compilers segfault on printf("string: "); printf("%s\n", string) when string is NULL, despite having no problem with printf("string: %s\n", string) as a single statement. Can you see why using two separate statements would cause a segfault? If not, see here for a hint: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25609




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