char arrays in PROGMEM

The AVR PROGMEM fix can be hidden behind a macro like:

#ifdef __AVR__
  #define MEM PROGMEM
#else
  #define MEM /* dummy macro */
#endif

Regarding performance and allocation:

The advantage of having a pointer-based look-up table as in the first example is execution speed - you'll be able to grab each string quickly with no run-time overhead.

The down-side is that you have to allocate the pointers themselves too, so it wastes extra flash or in worst case RAM+flash in case the pointer table is copied down from ROM to RAM during start-up.

This answer is only applicable in case you really need to save flash over everything else. But also if you want to centralize code maintenance to a single list. You can then cook up something evil-looking using "X macros", optionally with a name for each string:

#define STR_LIST(X)     \
  X(d1, "first")        \
  X(d2, "second")       \
  X(d3, "third")        \

You can then allocate this stuff adjacently in the same big array, like this:

static const char STRINGS[] MEM =
{
  #define STR_ALLOC(name, str) str "\0"
  STR_LIST(STR_ALLOC)
};

This uses string concatenation for convenience, but with manually added null terminators since string concatenation would otherwise remove them. It expands to:

"first" "\0" "second" "\0" "third" "\0"

And gets concatenated to:

 "first\0second\0third\0"

(We'll get an extra null terminator at the end, but that might be neat for "sentinel value" purposes.)

Various dirty hacks for "named" access in run-time (flash size over speed optimization) - full code example:

#define STR_LIST(X)     \
  X(d1, "first")        \
  X(d2, "second")       \
  X(d3, "third")        \

#ifdef __AVR__
  #define MEM PROGMEM
#else
  #define MEM /* dummy macro */
#endif

static const char STRINGS[] MEM =
{
  #define STR_ALLOC(name, str) str "\0"
  STR_LIST(STR_ALLOC)
};

typedef enum
{
  #define STR_ENUM(name, str) STR_ENUM_##name,
  STR_LIST(STR_ENUM)
  
  STR_ENUM_N
} str_enum_t;

static str_enum_t key;
#define STR_COUNT(name, str) +(STR_ENUM_##name<key ? sizeof(str) : 0)

#define STR_GET_POS(name) (key=STR_ENUM_##name, STR_LIST(STR_COUNT))

#include <stdio.h>
int main (void)
{
  puts("Memory dump, | marks null terminators:");
  for(size_t i=0; i<sizeof(STRINGS); i++)
  {
    printf("%c", STRINGS[i]=='\0' ? '|' : STRINGS[i]);
  }
  puts("");puts("");

  puts("What the strings are named:");
  #define STR_PRINT_NAME(name, str) printf("%s: %s\n", #name, str);
  STR_LIST(STR_PRINT_NAME)
  puts("");

  puts("Where the strings are found:");
  printf("%s %2zu ", "d1", STR_GET_POS(d1));
  printf("%s\n", &STRINGS[STR_GET_POS(d1)]);
  printf("%s %2zu ", "d2", STR_GET_POS(d2));
  printf("%s\n", &STRINGS[STR_GET_POS(d2)]);
  printf("%s %2zu ", "d3", STR_GET_POS(d3));
  printf("%s\n", &STRINGS[STR_GET_POS(d3)]);
}

Output:

Memory dump, | marks null terminators:
first|second|third||

What the strings are named:
d1: first
d2: second
d3: third

Where the strings are found:
d1  0 first
d2  6 second
d3 13 third