Getting length of /proc/self/exe symlink

Question

As mentioned on SO, readlink on /proc/self/exe can be used to get the executable path on linux. man 2 readlink recommends that one should use lstat to extract the required length of a path. However, when I stat /proc/self/exe, the st_size member is set to 0. How can I get the length for allocating the buffer?

Answer 1

In practice, I would tend to use a reasonable size (e.g. 256 or 1024, or PATH_MAX) for readlink of /proc/*/exe (or /proc/self/exe)

The point is that almost always, executables are supposed to be started by humans, so either the PATH (for execvp(3) or some shell) or the entire file path is human friendly. I don't know any people who explicitly uses very long filenames (not fitting in width in some terminal screen). I never heard of executable programs (or scripts) whose filename exceeds a hundred of bytes.

So just use a local buffer of some reasonable size (and perhaps strdup it on success if so needed). And readlink(2) returns the number of meaningful bytes in its buffer (so if you really care, grow the buffer and make a loop till it fits).

For readlink of /proc/self/exe, I would do it into a 256 bytes buffer at initialization, and abort (with a meaningful error message) if it does not fit (or fail, e.g. because /proc/ is not mounted).

Answer 2

taken from man 2 lstat, under NOTES

For most files under the /proc directory, stat() does not return the file size in the st_size field; instead the field is returned with the value 0.

That's why it does not work