How to use strcat in c?

[piccolo]

Now why would you want to use strcat?? This routine you have now is quite slow reading on a character base is too slow. Use fread and fwrite.

strcat useage is as follows:
...
char dest[20];

dest[0]='\';
strcat(dest,"this is a ");
//now dest="this is a\0"
strcat(dest,"test");
//now dest="this is a test\0"
strcat(dest,"123456789012345678901234567890");
//now you get a crash because your dest string overflows, so this is a buffer overflow.

Easier loop:
char buffer[1024];
...
while !feof(in){
nrbytes=fread(buffer,1,1024,in);
if (nrbytes) fwrite(buffer,1,nrbytes,out);
}
etc..
Backupping files can be done faster using windows functions if yer running windoze.

[leosmi05]

Just some small remarks regarding your program.
It seems that it can not handle well an extension of 1 or 2 characters or more than 3 characters.
You should start from the back of the file name (argv[1]) and search for the last point '.' character.

And BTW, you don't need to use strcat in case you have at least 3 characters for extension. Just find the last '.' in the name and replace the next 4 characters with "BAK" and '\0'.

The other remarks from piccolo also remain.

Don't know if it's useful for you, but you could use the API PathRenameExtension , which is in shlwapi.dll, this API replace any extension with any extension you want:

PathRenameExtension ("file.ext", ".bak");

Hope it works for you, just #include <shlwapi.h>

This only works in windows, off course...

Here is a generalized routine good for your situation
and can be easily tailored or adapted to fit most
any situation:

// EX: OPEN A FILE FOR LOGGING OUTPUT IN ANALYZE MODE.
// Find the last '\\' to obtain a pointer to just the base file name part if
your buffer contains any path type info
// We could just as eaily searched for last '.' to obtain base file name
extension pointer.

char *szBuffer = buffer;
PCSTR pszBaseName = strrchr( szBuffer, '\\' );
if ( pszBaseName )  // We found a '\\', so advance to the base FILE name
{
// Increment 1 byte past our pointer
pszBaseName++;
strncpy(pszBaseName, "Asprlog.txt\0", 12); // we need 12 to include '\0'
null char	
//replace base file name with newname , here you could have appended
bak or BAK extension if you had searched on the '.' char Ex: as so: 
strncpy(pszBaseName, "bak\0", 4);
}
FILE * pFile;
pFile=fopen(szBuffer,"wt");	// open for write

... and so on, I hope you get the idea.

cheers!