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# Operating System Utilities

int `Py_FdIsInteractive`(FILE *fp, const char *filename)
  Return true (nonzero) if the standard I/O file fp with name filename is deemed interactive. This is the case for files for which `isatty(fileno(fp))` is true. If the global flag `Py_InteractiveFlag` is true, this function also returns true if the filename pointer is NULL or if the name is equal to one of the strings `'<stdin>'` or `'???'`.

void `PyOS_AfterFork`()
  Function to update some internal state after a process fork; this should be called in the new process if the Python interpreter will continue to be used. If a new executable is loaded into the new process, this function does not need to be called.

int `PyOS_CheckStack`()
  Return true when the interpreter runs out of stack space. This is a reliable check, but is only available when `USE_STACKCHECK` is defined (currently on Windows using the Microsoft Visual C++ compiler). `USE_STACKCHECK` will be defined automatically; you should never change the definition in your own code.

PyOS_sighandler_t `PyOS_getsig`(int i)
  Return the current signal handler for signal i. This is a thin wrapper around either `sigaction()` or `signal()`. Do not call those functions directly! `PyOS_sighandler_t` is a typedef alias for `void (*)(int)`.

PyOS_sighandler_t `PyOS_setsig`(int i, PyOS_sighandler_t h)
  Set the signal handler for signal i to be h; return the old signal handler. This is a thin wrapper around either `sigaction()` or `signal()`. Do not call those functions directly! `PyOS_sighandler_t` is a typedef alias for `void (*)(int)`.

# System Functions

These are utility functions that make functionality from the `sys` module accessible to C code. They all work with the current interpreter thread’s `sys` module’s dict, which is contained in the internal thread state structure.

PyObject *`PySys_GetObject`(char *name)
  Return value: Borrowed reference.

  Return the object name from the `sys` module or NULL if it does not exist, without setting an exception.

FILE *`PySys_GetFile`(char *name, FILE *def)
  Return the `FILE*` associated with the object name in the `sys` module, or def if name is not in the module or is not associated with a `FILE*`.

int `PySys_SetObject`(char *name, PyObject *v)
  Set name in the `sys` module to v unless v is NULL, in which case name is deleted from the sys module. Returns `0` on success, `-1` on error.

void `PySys_ResetWarnOptions`()
  Reset `sys.warnoptions` to an empty list.

void `PySys_AddWarnOption`(char *s)
  Append s to `sys.warnoptions`.

void `PySys_SetPath`(char *path)
  Set `sys.path` to a list object of paths found in path which should be a list of paths separated with the platform’s search path delimiter (`:` on Unix, `;` on Windows).

void `PySys_WriteStdout`(const char *format, ...)

  Write the output string described by format to `sys.stdout`. No exceptions are raised, even if truncation occurs (see below).

  format should limit the total size of the formatted output string to 1000 bytes or less – after 1000 bytes, the output string is truncated. In particular, this means that no unrestricted “%s” formats should occur; these should be limited using “%.<N>s” where <N> is a decimal number calculated so that <N> plus the maximum size of other formatted text does not exceed 1000 bytes. Also watch out for “%f”, which can print hundreds of digits for very large numbers.

  If a problem occurs, or `sys.stdout` is unset, the formatted message is written to the real (C level) stdout.

void `PySys_WriteStderr`(const char *format, ...)
  As above, but write to `sys.stderr` or stderr instead.

# Process Control

void `Py_FatalError`(const char *message)

  Print a fatal error message and kill the process. No cleanup is performed. This function should only be invoked when a condition is detected that would make it dangerous to continue using the Python interpreter; e.g., when the object administration appears to be corrupted. On Unix, the standard C library function `abort()` is called which will attempt to produce a `core` file.

void `Py_Exit`(int status)

  Exit the current process. This calls `Py_Finalize()` and then calls the standard C library function `exit(status)`.

int `Py_AtExit`(void (*func) ())

  Register a cleanup function to be called by `Py_Finalize()`. The cleanup function will be called with no arguments and should return no value. At most 32 cleanup functions can be registered. When the registration is successful, `Py_AtExit()` returns `0`; on failure, it returns `-1`. The cleanup function registered last is called first. Each cleanup function will be called at most once. Since Python’s internal finalization will have completed before the cleanup function, no Python APIs should be called by func.

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