preallocating logging files for performance purposes

[Alan Kurlansky]

I'm looking for some VS c++ code that 1) preallocates a file 2) opens a preallocated file for sequential writing starting at the beginning of the file 3) writes(appends) ascii message strings to the file. Thanks

[Lost User]

I'm not sure exactly what you mena by point 1, but opening a file for appending is quite straightforward, and writing text is even more so. Take a look at the STL file stream classes[^] or use the Createfile[^] and associated functions.

I must get a clever new signature for 2011.

[Andrew Brock]

You need to seek to what you want to be the end of the pre-allocated space, then set the EOF marker, then seek back to the start so you are writing from the beginning of the file, like so

HANDLE hFile = CreateFile("C:\\test", GENERIC_WRITE, FILE_SHARE_READ, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
if (hFile != INVALID_HANDLE_VALUE) {
SetFilePointer(hFile, 4 * 1024 * 1024, NULL, SEEK_SET); //seek to 4MB (pre-allocate 4MB)
SetEndOfFile(hFile); //set the EOF to pre-allocate the space
SetFilePointer(hFile, 0, NULL, SEEK_SET); //seek back to the start for writing
//write to the file with WriteFile()
CloseHandle(hFile);
}

Edit: I missed part 3... quite simply seek to the end of the data in the file to append to it.

SetFilePointer(hFile, 0, NULL, SEEK_END); //If you want to append to the end of the entire file (including after the pre-allocated space)
SetFilePointer(hFile, nDataLength, NULL, SEEK_SET); //if you are keeping track of the length of the data you have written to the file with the variable nDataLength

Writing to a file is pretty straight forward. You just need a buffer and the length of data you are writing

LPSTR szWrite = "hello world";
DWORD nLength = strlen(szWrite);
DWORD nBytesWritten;
WriteFile(hFile, szWrite, nLength, &nBytesWritten, NULL);
CloseHandle(hFile);

If you are writing strings in unicode you need to multiply the length by sizeof(wchar_t) or sizeof(TCHAR) Finally, All file writes (and reads) are sequential for all APIs that I know of. It moves the file pointer to the end of the data you have just written (or read)

modified on Wednesday, January 12, 2011 11:23 AM

[jschell]

If you are looking for logging, rather than just output, then the following exists. http://log4cpp.sourceforge.net/[^]