Moving a file on Linux in C

Question

• Platform: Debian Wheezy 3.2.0-4-686-pae
• Complier: GCC (Debian 4.7.2-5) 4.7.2 (Code::Blocks)

I want to move a file from one location to another

Answer 1

The usual way to move a file in C is to use rename(2), which sometimes fail.

If you cannot use the rename(2) syscall (e.g. because source and target are on different filesystems), you have to query the size, permission and other metadata of the source file with stat(2); copy the data looping on read(2), write(2) (using a buffer of several kilobytes), open(2), close(2) and the metadata using chmod(2), chown(2), utime(2). You might also care about copying attributes using getxattr(2), setxattr(2), listxattr(2). You could also in some cases use sendfile(2), as commented by David C. Rankin.

And if the source and target are on different filesystems, there is no way to make the move atomic and avoid race conditions (So using rename(2) is preferable when possible, because it is atomic according to its man page). The source file can always be modified (by another process) during the move operations...

So a practical way to move files is to first try doing a rename(2), and if that fails with EXDEV (when oldpath and newpath are not on the same mounted filesystem), then you need to copy bytes and metadata. Several libraries provide functions doing that, e.g. Qt QFile::rename.

Read Advanced Linux Programming - and see syscalls(2) - for more (and also try to strace some mv command to understand what it is doing). That book is freely and legally downloadable (so you could find several copies on the Web).

The /bin/mv command (see mv(1)) is part of GNU coreutils which is free software. You could either study its source code, or use strace(1) to understand what that command does (in terms of syscalls(2)). In some open source Unix shells like sash or busybox, mv might be a shell builtin. See also path_resolution(7) and glob(7).

There are subtle corner cases (imagine another process or pthread doing some file operations on the same filesystem, directory, or files). Read some operating system textbook for more.

Using a mix of snprintf(3), system(3), mv(1) could be tricky if the file name contains weird characters such as tab or or newlines, or starts with an initial -. See errno(3).