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#include <windows.h>
#include <stdio.h>
 
int main(int argc, char *argv[]){
	if(argc < 2){
		printf("Usage: <Win32UnpackableUPX.exe> <file>\n");
	}else{
		FILE *fFile = fopen(argv[1], "rb");
		if(fFile != NULL){
			printf("File (modified): %s\n", argv[1]);
 
			fseek(fFile, 0, SEEK_END);
			long lSize = ftell(fFile);
			rewind(fFile);
 
			char *cBuffer = (char *)malloc(lSize*sizeof(char *));
			if(cBuffer != NULL){
				fread(cBuffer, 1, lSize, fFile);
				fclose(fFile);
 
				BOOL bUPX = FALSE;
 
				for(int i=0; i < lSize; i++){
					if((cBuffer[i] == 0x55) && (cBuffer[i + 1] == 0x50) && (cBuffer[i + 2] == 0x58) && cBuffer[i + 3] == 0x30){
						printf("Found byte pattern at offset: %i\n", i);
						// This is the only value we need to change (UPX0), not (UPX1) or (UPX!)
						cBuffer[i] = 0x41; // Change U (0x55) to A (0x41).
 
						bUPX = TRUE;
					}
				}
 
				if(bUPX == TRUE){
					fFile = fopen(argv[1], "wb");
					if(fFile != NULL){
						fwrite(cBuffer, 1, lSize, fFile);
						free(cBuffer);
						fclose(fFile);
 
						printf("File %s should be successfully patched!", argv[1]);
					}else{
						printf("Unable to open file (write binary).\n");
						return 0;
					}
				}else{
					printf("File is not packed by UPX.\n");
					free(cBuffer);
 
					return 0;
				}
			}else{
				printf("Error at: malloc()\n");
				return 0;
			}
		}else{
			printf("Unable to open file (read binary).\n");
			return 0;
		}
	}
 
	getchar();
}
 

prog.c:1:21: error: windows.h: No such file or directory
prog.c: In function ‘main’:
prog.c:16: warning: implicit declaration of function ‘malloc’
prog.c:16: warning: incompatible implicit declaration of built-in function ‘malloc’
prog.c:21: error: ‘BOOL’ undeclared (first use in this function)
prog.c:21: error: (Each undeclared identifier is reported only once
prog.c:21: error: for each function it appears in.)
prog.c:21: error: expected ‘;’ before ‘bUPX’
prog.c:23: error: ‘for’ loop initial declaration used outside C99 mode
prog.c:29: error: ‘bUPX’ undeclared (first use in this function)
prog.c:29: error: ‘TRUE’ undeclared (first use in this function)
prog.c:37: warning: implicit declaration of function ‘free’
prog.c:37: warning: incompatible implicit declaration of built-in function ‘free’
prog.c:47: warning: incompatible implicit declaration of built-in function ‘free’
prog.c:18: warning: ignoring return value of ‘fread’, declared with attribute warn_unused_result

Standard output is empty