通常，每个UEFI系统至少有一个 ESP (EFI System Partition)分区，在这个分区上存放启动文件。EFI内置了EFI_SIMPLE_FILE_SYSTEM_PROTOCOL(简称 FileSystemIo)可以用来操作FAT文件系统【参考1】。
相关的介绍：
\MdePkg\Include\Protocol\SimpleFileSystem.h
///
/// The EFI_FILE_PROTOCOL provides file IO access to supported file systems.
/// An EFI_FILE_PROTOCOL provides access to a file's or directory's contents,
/// and is also a reference to a location in the directory tree of the file system
/// in which the file resides. With any given file handle, other files may be opened
/// relative to this file's location, yielding new file handles.
///
struct _EFI_FILE_PROTOCOL {
///
/// The version of the EFI_FILE_PROTOCOL interface. The version specified
/// by this specification is EFI_FILE_PROTOCOL_LATEST_REVISION.
/// Future versions are required to be backward compatible to version 1.0.
///
UINT64 Revision;
EFI_FILE_OPEN Open;
EFI_FILE_CLOSE Close;
EFI_FILE_DELETE Delete;
EFI_FILE_READ Read;
EFI_FILE_WRITE Write;
EFI_FILE_GET_POSITION GetPosition;
EFI_FILE_SET_POSITION SetPosition;
EFI_FILE_GET_INFO GetInfo;
EFI_FILE_SET_INFO SetInfo;
EFI_FILE_FLUSH Flush;
EFI_FILE_OPEN_EX OpenEx;
EFI_FILE_READ_EX ReadEx;
EFI_FILE_WRITE_EX WriteEx;
EFI_FILE_FLUSH_EX FlushEx;
};
\MdePkg\Include\Protocol\SimpleFileSystem.h
#define EFI_SIMPLE_FILE_SYSTEM_PROTOCOL_GUID \
{ \
0x964e5b22, 0x6459, 0x11d2, {0x8e, 0x39, 0x0, 0xa0, 0xc9, 0x69, 0x72, 0x3b } \
}
\EdkCompatibilityPkg\Foundation\Efi\Protocol\SimpleFileSystem\SimpleFileSystem.c
EFI_GUID gEfiSimpleFileSystemProtocolGuid = EFI_SIMPLE_FILE_SYSTEM_PROTOCOL_GUID;
从原理上来说，先通过这个Protocol OpenVolume,即可获得FAT文件系统上的根目录句柄。目录句柄(EFI_FILE_PROTOCOL)包含了操作该目录里文件的文件操作接口。之后我们再用 Open打开这个目录，选定需要操作的文件即可写入。
\MdePkg\Include\Protocol\SimpleFileSystem.h
/**
Writes data to a file.
@param This A pointer to the EFI_FILE_PROTOCOL instance that is the file
handle to write data to.
@param BufferSize On input, the size of the Buffer. On output, the amount of data
actually written. In both cases, the size is measured in bytes.
@param Buffer The buffer of data to write.
@retval EFI_SUCCESS Data was written.
@retval EFI_UNSUPPORTED Writes to open directory files are not supported.
@retval EFI_NO_MEDIA The device has no medium.
@retval EFI_DEVICE_ERROR The device reported an error.
@retval EFI_DEVICE_ERROR An attempt was made to write to a deleted file.
@retval EFI_VOLUME_CORRUPTED The file system structures are corrupted.
@retval EFI_WRITE_PROTECTED The file or medium is write-protected.
@retval EFI_ACCESS_DENIED The file was opened read only.
@retval EFI_VOLUME_FULL The volume is full.
**/
typedef
EFI_STATUS
(EFIAPI *EFI_FILE_WRITE)(
IN EFI_FILE_PROTOCOL *This,
IN OUT UINTN *BufferSize,
IN VOID *Buffer
);
完整代码：
#include <Uefi.h>
#include <Library/UefiLib.h>
#include <Library/ShellCEntryLib.h>
#include <Protocol/EfiShell.h>
#include <Library/ShellLib.h>
#include <Protocol/SimpleFileSystem.h>
extern EFI_BOOT_SERVICES *gBS;
extern EFI_SYSTEM_TABLE *gST;
int
EFIAPI
main (
IN int Argc,
IN char **Argv
)
{
EFI_STATUS Status;
EFI_FILE_PROTOCOL *Root;
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL *SimpleFileSystem;
UINTN BufSize;
CHAR16 *Textbuf = (CHAR16*) L"www.lab-z.com";
EFI_FILE_PROTOCOL *FileHandle=0;
Status = gBS->LocateProtocol(
&gEfiSimpleFileSystemProtocolGuid,
NULL,
(VOID **)&SimpleFileSystem);
if (EFI_ERROR(Status)) {
Print(L"Cannot find EFI_SIMPLE_FILE_SYSTEM_PROTOCOL \r\n");
return Status;
}
Status = SimpleFileSystem->OpenVolume(SimpleFileSystem,&Root);
if (EFI_ERROR(Status)) {
Print(L"OpenVolume error \r\n");
return Status;
}
Status = Root -> Open(Root,
&FileHandle,
(CHAR16 *) L"atest.txt",
EFI_FILE_MODE_CREATE | EFI_FILE_MODE_READ | EFI_FILE_MODE_WRITE,
0);
if (EFI_ERROR(Status) || (FileHandle==0)) {
Print(L"Open error \r\n");
return Status;
}
BufSize = StrLen (Textbuf) * 2;
Print(L"[%d]\r\n",BufSize);
Print(L"[%s]\r\n",Textbuf);
Status = FileHandle -> Write(FileHandle, &BufSize, Textbuf);
Print(L"Write Done \r\n");
Status = FileHandle -> Close (FileHandle);
return EFI_SUCCESS;
}
运行结果：
比较有意思的是，我们按照上面的程序写入之后，使用 type 文件名 来显示文件内容和我们的预期有差别，原因是Type命令默认使用 ASCII 来显内容，而我们的文件中写入的是 Unicode。使用 type -u 强制使用 Unicode即可看到我们期望的内容。
根据【参考2】的提示，我们还可以通过给TXT文件加一个头来通知当前内容格式的方法克服这个问题。
关键代码：
UINT16 TextHeader =0xFEFF;
BufSize = 2; Status = FileHandle -> Write(FileHandle, &BufSize, &TextHeader); BufSize = StrLen (Textbuf) * 2; Print(L"[%d]\r\n",BufSize); Print(L"[%s]\r\n",Textbuf); Status = FileHandle -> Write(FileHandle, &BufSize, Textbuf);
运行结果：
可以看到经过这样的处理，都可以正常显示内容了。
完整代码下载
ESPTest
参考：
1.UEFI 原理与编程 戴正华 著 P152页
2.http://www.biosren.com/thread-6541-2-1.html UEFI下使用EFI_FILE_PROTOCOL 檔案操作能多次讀寫嗎?
=============================================================
2024年9月20日
下面这个代码段能够实现将指定内存内容保存为文件，方便调试，有需要的朋友可以试试。
#include <Library/BaseMemoryLib.h>
#include <Library/MemoryAllocationLib.h>
#include <Protocol/SimpleFileSystem.h>
extern EFI_HANDLE gImageHandle;
EFI_STATUS
WriteMemoryToFile(
IN EFI_HANDLE ImageHandle,
IN CHAR16 *FileName,
IN VOID *Buffer,
IN UINTN BufferSize
)
{
EFI_STATUS Status;
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL *SimpleFileSystem;
EFI_FILE_PROTOCOL *Root;
EFI_FILE_PROTOCOL *File;
EFI_HANDLE *Handles = NULL;
UINTN HandleCount = 0;
// Locate all handles that support the Simple File System Protocol
Status = gBS->LocateHandleBuffer(ByProtocol, &gEfiSimpleFileSystemProtocolGuid, NULL, &HandleCount, &Handles);
if (EFI_ERROR(Status)) {
Print(L"Failed to locate handles for Simple File System Protocol: %r\n", Status);
return Status;
}
// Open the first Simple File System Protocol
Status = gBS->HandleProtocol(Handles[0], &gEfiSimpleFileSystemProtocolGuid, (VOID **)&SimpleFileSystem);
if (EFI_ERROR(Status)) {
Print(L"Failed to open Simple File System Protocol: %r\n", Status);
return Status;
}
// Open the root directory
Status = SimpleFileSystem->OpenVolume(SimpleFileSystem, &Root);
if (EFI_ERROR(Status)) {
Print(L"Failed to open root directory: %r\n", Status);
return Status;
}
// Create or open the file
Status = Root->Open(Root, &File, FileName, EFI_FILE_MODE_CREATE | EFI_FILE_MODE_WRITE | EFI_FILE_MODE_READ, 0);
if (EFI_ERROR(Status)) {
Print(L"Failed to open or create file: %r\n", Status);
return Status;
}
// Write the buffer to the file
Status = File->Write(File, &BufferSize, Buffer);
if (EFI_ERROR(Status)) {
Print(L"Failed to write to file: %r\n", Status);
File->Close(File);
return Status;
}
// Close the file
Status = File->Close(File);
if (EFI_ERROR(Status)) {
Print(L"Failed to close file: %r\n", Status);
return Status;
}
Print(L"Memory successfully written to file: %s\n", FileName);
return EFI_SUCCESS;
}
