program to get and set subsystem in PE headers

pesubsys.c

#include <windows.h>
#include <stddef.h>
#include <stdio.h>

int main(int argc, char ** argv)
{
	FILE * fh;
	IMAGE_DOS_HEADER idh = {0};
	DWORD signature = 0;
	long subsys_offset = 0;
	WORD magic = 0, subsystem = 0, newsubsystem = MAXWORD;

	if (argc < 2)
	{
		fprintf(stderr, "Usage: %s pefile [subsystem]\n", argv[0]);
		fprintf(stderr, "specifying a subsystem will change the subsystem in the image\n");
		return 1;
	}

	if (argc > 2)
	{
		long l = strtol(argv[2], NULL, 0);
		if (l >= 0 && l <= MAXWORD)
			newsubsystem = (WORD)l;
	}

	fh = fopen(argv[1], (newsubsystem != MAXWORD) ? "r+b" : "rb");
	if (!fh)
	{
		perror("Error opening file");
		return 2;
	}

	if (fread(&idh, sizeof(idh), 1, fh) != 1)
	{
		perror("Error reading dos header");
		fclose(fh);
		return 2;
	}
	
	if (idh.e_magic != IMAGE_DOS_SIGNATURE)
	{
		fprintf(stderr, "Bad DOS magic\n");
		fclose(fh);
		return 2;
	}


	if (fseek(fh, idh.e_lfanew, SEEK_SET) != 0)
	{
		perror("Error seeking to nt headers");
		fclose(fh);
		return 2;
	}

	if (fread(&signature, sizeof(signature), 1, fh) != 1)
	{
		perror("Error reading NT signature");
		fclose(fh);
		return 2;
	}

	if (signature != IMAGE_NT_SIGNATURE)
	{
		fprintf(stderr, "Bad NT signature\n");
		fclose(fh);
		return 0;
	}

	if (fseek(fh, idh.e_lfanew + offsetof(IMAGE_NT_HEADERS, OptionalHeader.Magic), SEEK_SET) != 0)
	{
		perror("Error seeking to optional header magic");
		fclose(fh);
		return 2;
	}
	if (fread(&magic, sizeof(magic), 1, fh) != 1)
	{
		perror("Error reading optional header magic");
		fclose(fh);
		return 2;
	}

	switch (magic)
	{
	case IMAGE_NT_OPTIONAL_HDR64_MAGIC:
		subsys_offset = idh.e_lfanew + offsetof(IMAGE_NT_HEADERS64, OptionalHeader.Subsystem);
		break;

	case IMAGE_NT_OPTIONAL_HDR32_MAGIC:
		subsys_offset = idh.e_lfanew + offsetof(IMAGE_NT_HEADERS32, OptionalHeader.Subsystem);
		break;

	default:
		fprintf(stderr, "Bad NT Optional Header magic\n");
		fclose(fh);
		return 2;
	}

	if (fseek(fh, subsys_offset, SEEK_SET) != 0)
	{
		perror("Error seeking to subsystem");
		fclose(fh);
		return 2;
	}
	
	if (fread(&subsystem, sizeof(subsystem), 1, fh) != 1)
	{
		perror("Error reading subsystem");
		fclose(fh);
		return 2;
	}

	printf("subsystem %hu\n", subsystem);

	if (newsubsystem != MAXWORD)
	{
		if (fseek(fh, subsys_offset, SEEK_SET) != 0)
		{
			perror("Error seeking to subsystem");
			fclose(fh);
			return 2;
		}

		if (fwrite(&newsubsystem, sizeof(newsubsystem), 1, fh) != 1)
		{
			perror("Error writing new subsystem");
			fclose(fh);
			return 2;
		}
	}

	fclose(fh);
	return 0;
}