A tool to fix wrong byte order GBA EEPROM saves as created by some emulators.

gbaEepromSaveFix.c

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>



// Compile with "gcc -std=c17 -O2 -fstrict-aliasing -ffunction-sections -Wall -Wl,--gc-sections gbaEepromSaveFix.c -o gbaEepromSaveFix"
int main(int argc, char *argv[])
{
	int res = 0;
	FILE *f = fopen(argv[1], "rb+");
	if(f != NULL)
	{
		do
		{
			fseek(f, 0, SEEK_END);
			const size_t fSize = ftell(f);
			// Minimum 32 because of the unrolled byteswap loop below.
			// The last check is a power of 2 check.
			if(fSize == 0 || fSize < 32 || (fSize & (fSize - 1)) != 0)
			{
				fputs("Broken save file or incorrect size!\n", stderr);
				res = 2;
				break;
			}
			fseek(f, 0, SEEK_SET);

			uint64_t *buf = (uint64_t*)malloc(fSize);
			if(buf == NULL)
			{
				fputs("Out of memory.\n", stderr);
				res = 3;
				break;
			}

			if(fread(buf, 1, fSize, f) != fSize)
			{
				fputs("Failed to read file.\n", stderr);
				res = 4;
				free(buf);
				break;
			}

			for(size_t i = 0; i < fSize / 8; i += 4)
			{
				buf[i + 0] = __builtin_bswap64(buf[i + 0]);
				buf[i + 1] = __builtin_bswap64(buf[i + 1]);
				buf[i + 2] = __builtin_bswap64(buf[i + 2]);
				buf[i + 3] = __builtin_bswap64(buf[i + 3]);
			}

			fseek(f, 0, SEEK_SET);
			if(fwrite(buf, 1, fSize, f) != fSize)
			{
				fputs("Failed to write to file.\n", stderr);
				res = 5;
			}

			free(buf);
		} while(0);

		fclose(f);
	}
	else
	{
		fputs("Failed to open save file.\n", stderr);
		res = 1;
	}

	if(res == 0) puts("Done.");

	return res;
}

Oh, okay. That explains it. I'm sorry for wasting your time.