Loading files under UEFI: Difference between revisions

 

== Volume Handle ==

 

There's no common "Open File" function in UEFI. Instead each volume has its own. So the first thing you need to do is locate a volume handle. You probably want to use the same file system that your application was loaded from, so you need to use the image handle provided to your '''efi_main'''. Then you need to use the EFI_LOADED_IMAGE_PROTOCOL to figure out the device your application resides on.

EFI_FILE_HANDLE GetVolume(EFI_HANDLE image)

{

}

 

Here [[GNU-EFI]]'s libefi.a provides a '''LibOpenRoot''' function, which can be used instead of IOVolume:

/* get the volume handle */

return LibOpenRoot(loaded_image->DeviceHandle);

 

== Read Data from File ==

 

=== Open ===

CHAR16 *FileName = L"DATA.TXT";

EFI_FILE_HANDLE FileHandle;

/* open the file */

uefi_call_wrapper(Volume->Open, 5, Volume, &FileHandle, FileName, EFI_FILE_MODE_READ, EFI_FILE_READ_ONLY | EFI_FILE_HIDDEN | EFI_FILE_SYSTEM);

Note that the file name passed to '''Volume->Open''' is a UNICODE16 string, meaning all characters are 2 bytes long. To get the L"" string literal correctly, you'll need to pass the "-fshort-wchar" command line option to gcc.

 

=== Read ===

/* read from the file */

UINT64 ReadSize = FileSize(FileHandle);

 

uefi_call_wrapper(FileHandle->Read, 3, FileHandle, &ReadSize, Buffer);

 

=== Close ===

/* close the file */

uefi_call_wrapper(FileHandle->Close, 1, FileHandle);

 

=== Get File Size ===

 

In order to know how much to read ('''ReadSize''' variable above), you'll need to know the file's size. For that, you should use '''FileHandle->GetInfo'''. The problem is, there's no way of knowing how big buffer the information structure requires, so you'll have to grow the buffer dynamically if GetInfo fails. GNU-EFI provides a pretty handy wrapper for that, [https://sourceforge.net/p/gnu-efi/code/ci/master/tree/lib/hand.c LibFileInfo].

UINT64 FileSize(EFI_FILE_HANDLE FileHandle)

{

return ret;

}

 

== See also ==

* [https://sourceforge.net/p/gnu-efi/code/ci/master/tree/ GNU-EFI source]

 

[[Category:UEFI]]