Question

How can I convert a relative path to an absolute path in C on Unix? Is there a convenient system function for this?

On Windows there is a GetFullPathName function that does the job, but I didn’t find something similar on Unix…

Answer 1

Use realpath().

The realpath() function shall derive, from the pathname pointed to by file_name, an absolute pathname that names the same file, whose resolution does not involve ‘.‘, ‘..‘, or symbolic links. The generated pathname shall be stored as a null-terminated string, up to a maximum of {PATH_MAX} bytes, in the buffer pointed to by resolved_name.

If resolved_name is a null pointer, the behavior of realpath() is implementation-defined.

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The following example generates an absolute pathname for the file identified by the symlinkpath argument. The generated pathname is stored in the actualpath array.

#include <stdlib.h> ... char *symlinkpath = '/tmp/symlink/file'; char actualpath [PATH_MAX+1]; char *ptr;   ptr = realpath(symlinkpath, actualpath);