— Macro: int MAXSYMLINKS

The macro MAXSYMLINKS specifies how many symlinks some function will follow before returning ELOOP. Not all functions behave the same and this value is not the same a that returned for _SC_SYMLOOP by sysconf. In fact, the sysconf result can indicate that there is no fixed limit although MAXSYMLINKS exists and has a finite value.

— Function: int symlink (const char *oldname, const char *newname)

The symlink function makes a symbolic link to oldname named newname.

The normal return value from symlink is 0. A return value of -1 indicates an error. In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST

There is already an existing file named newname.

EROFS

The file newname would exist on a read-only file system.

ENOSPC

The directory or file system cannot be extended to make the new link.

EIO

A hardware error occurred while reading or writing data on the disk.

— Function: int readlink (const char *filename, char *buffer, size_t size)

The readlink function gets the value of the symbolic link filename. The file name that the link points to is copied into buffer. This file name string is not null-terminated; readlink normally returns the number of characters copied. The size argument specifies the maximum number of characters to copy, usually the allocation size of buffer.

If the return value equals size, you cannot tell whether or not there was room to return the entire name. So make a bigger buffer and call readlink again. Here is an example:

          char *
          readlink_malloc (const char *filename)
          {
            int size = 100;
            char *buffer = NULL;
          
            while (1)
              {
                buffer = (char *) xrealloc (buffer, size);
                int nchars = readlink (filename, buffer, size);
                if (nchars < 0)
                  {
                    free (buffer);
                    return NULL;
                  }
                if (nchars < size)
                  return buffer;
                size *= 2;
              }
          }

A value of -1 is returned in case of error. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EINVAL

The named file is not a symbolic link.

EIO

A hardware error occurred while reading or writing data on the disk.

— Function: char * canonicalize_file_name (const char *name)

The canonicalize_file_name function returns the absolute name of the file named by name which contains no ., .. components nor any repeated path separators (/) or symlinks. The result is passed back as the return value of the function in a block of memory allocated with malloc. If the result is not used anymore the memory should be freed with a call to free.

If any of the path components is missing the function returns a NULL pointer. This is also what is returned if the length of the path reaches or exceeds PATH_MAX characters. In any case errno is set accordingly.

ENAMETOOLONG

The resulting path is too long. This error only occurs on systems which have a limit on the file name length.

EACCES

At least one of the path components is not readable.

ENOENT

The input file name is empty.

ENOENT

At least one of the path components does not exist.

ELOOP

More than MAXSYMLINKS many symlinks have been followed.

This function is a GNU extension and is declared in stdlib.h.

— Function: char * realpath (const char *restrict name, char *restrict resolved)

A call to realpath where the resolved parameter is NULL behaves exactly like canonicalize_file_name. The function allocates a buffer for the file name and returns a pointer to it. If resolved is not NULL it points to a buffer into which the result is copied. It is the callers responsibility to allocate a buffer which is large enough. On systems which define PATH_MAX this means the buffer must be large enough for a pathname of this size. For systems without limitations on the pathname length the requirement cannot be met and programs should not call realpath with anything but NULL for the second parameter.

One other difference is that the buffer resolved (if nonzero) will contain the part of the path component which does not exist or is not readable if the function returns NULL and errno is set to EACCES or ENOENT.

This function is declared in stdlib.h.