Add documentation for PySys_* functions.
Written by Charlie Shepherd for GHOP. Also fixes #1245.
diff --git a/Doc/c-api/utilities.rst b/Doc/c-api/utilities.rst
index bcd84b3..269f23a 100644
--- a/Doc/c-api/utilities.rst
+++ b/Doc/c-api/utilities.rst
@@ -66,6 +66,66 @@
not call those functions directly! :ctype:`PyOS_sighandler_t` is a typedef
alias for :ctype:`void (\*)(int)`.
+.. _systemfunctions:
+
+System Functions
+================
+
+These are utility functions that make functionality from the :mod:`sys` module
+accessible to C code. They all work with the current interpreter thread's
+:mod:`sys` module's dict, which is contained in the internal thread state structure.
+
+.. cfunction:: PyObject *PySys_GetObject(char *name)
+
+ Return the object *name* from the :mod:`sys` module or *NULL* if it does
+ not exist, without setting an exception.
+
+.. cfunction:: FILE *PySys_GetFile(char *name, FILE *def)
+
+ Return the :ctype:`FILE*` associated with the object *name* in the
+ :mod:`sys` module, or *def* if *name* is not in the module or is not associated
+ with a :ctype:`FILE*`.
+
+.. cfunction:: int PySys_SetObject(char *name, PyObject *v)
+
+ Set *name* in the :mod:`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.
+
+.. cfunction:: void PySys_ResetWarnOptions(void)
+
+ Reset :data:`sys.warnoptions` to an empty list.
+
+.. cfunction:: void PySys_AddWarnOption(char *s)
+
+ Append *s* to :data:`sys.warnoptions`.
+
+.. cfunction:: void PySys_SetPath(char *path)
+
+ Set :data:`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).
+
+.. cfunction:: void PySys_WriteStdout(const char *format, ...)
+
+ Write the output string described by *format* to :data:`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 :data:`sys.stdout` is unset, the formatted message
+ is written to the real (C level) *stdout*.
+
+.. cfunction:: void PySys_WriteStderr(const char *format, ...)
+
+ As above, but write to :data:`sys.stderr` or *stderr* instead.
+
.. _processcontrol: