zip_source_tell_write —
report current write offset in zip source
#include <zip.h>
zip_int64_t
  
  zip_source_tell_write(zip_source_t
    *source);
The function zip_source_tell_write() returns the current
  write offset for source. The return value can be passed
  to
  zip_source_seek_write(3)
  with whence set to SEEK_SET to
  return to the same location in the source.
Upon successful completion the current write offset is returned. Otherwise, -1
  is returned and the error information in source is set
  to indicate the error.
zip_source_tell_write() was added in libzip 1.0.