5.5 Files opened without an explicit ACTION= specifier

The Fortran standard says that if an OPEN statement is executed without an explicit ACTION= specifier, the default value is processor dependent. GNU Fortran behaves as follows:

  1. Attempt to open the file with ACTION='READWRITE'
  2. If that fails, try to open with ACTION='READ'
  3. If that fails, try to open with ACTION='WRITE'
  4. If that fails, generate an error

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Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-8.4.0/gfortran/Files-opened-without-an-explicit-ACTION_003d-specifier.html