cpython/Doc/c-api/conversion.rst

.. highlight:: c

.. _string-conversion:

String conversion and formatting
================================

Functions for number conversion and formatted string output.


.. c:function:: int PyOS_snprintf(char *str, size_t size,  const char *format, ...)

   Output not more than *size* bytes to *str* according to the format string
   *format* and the extra arguments. See the Unix man page :manpage:`snprintf(3)`.


.. c:function:: int PyOS_vsnprintf(char *str, size_t size, const char *format, va_list va)

   Output not more than *size* bytes to *str* according to the format string
   *format* and the variable argument list *va*. Unix man page
   :manpage:`vsnprintf(3)`.

:c:func:`PyOS_snprintf` and :c:func:`PyOS_vsnprintf` wrap the Standard C library
functions :c:func:`snprintf` and :c:func:`vsnprintf`. Their purpose is to
guarantee consistent behavior in corner cases, which the Standard C functions do
not.

The wrappers ensure that ``str[size-1]`` is always ``'\0'`` upon return. They
never write more than *size* bytes (including the trailing ``'\0'``) into str.
Both functions require that ``str != NULL``, ``size > 0``, ``format != NULL``
and ``size < INT_MAX``. Note that this means there is no equivalent to the C99
``n = snprintf(NULL, 0, ...)`` which would determine the necessary buffer size.

The return value (*rv*) for these functions should be interpreted as follows:

* When ``0 <= rv < size``, the output conversion was successful and *rv*
  characters were written to *str* (excluding the trailing ``'\0'`` byte at
  ``str[rv]``).

* When ``rv >= size``, the output conversion was truncated and a buffer with
  ``rv + 1`` bytes would have been needed to succeed. ``str[size-1]`` is ``'\0'``
  in this case.

* When ``rv < 0``, "something bad happened." ``str[size-1]`` is ``'\0'`` in
  this case too, but the rest of *str* is undefined. The exact cause of the error
  depends on the underlying platform.


The following functions provide locale-independent string to number conversions.

.. c:function:: unsigned long PyOS_strtoul(const char *str, char **ptr, int base)

   Convert the initial part of the string in ``str`` to an :c:expr:`unsigned
   long` value according to the given ``base``, which must be between ``2`` and
   ``36`` inclusive, or be the special value ``0``.

   Leading white space and case of characters are ignored.  If ``base`` is zero
   it looks for a leading ``0b``, ``0o`` or ``0x`` to tell which base.  If
   these are absent it defaults to ``10``.  Base must be 0 or between 2 and 36
   (inclusive).  If ``ptr`` is non-``NULL`` it will contain a pointer to the
   end of the scan.

   If the converted value falls out of range of corresponding return type,
   range error occurs (:c:data:`errno` is set to :c:macro:`!ERANGE`) and
   :c:macro:`!ULONG_MAX` is returned.  If no conversion can be performed, ``0``
   is returned.

   See also the Unix man page :manpage:`strtoul(3)`.

   .. versionadded:: 3.2


.. c:function:: long PyOS_strtol(const char *str, char **ptr, int base)

   Convert the initial part of the string in ``str`` to an :c:expr:`long` value
   according to the given ``base``, which must be between ``2`` and ``36``
   inclusive, or be the special value ``0``.

   Same as :c:func:`PyOS_strtoul`, but return a :c:expr:`long` value instead
   and :c:macro:`LONG_MAX` on overflows.

   See also the Unix man page :manpage:`strtol(3)`.

   .. versionadded:: 3.2


.. c:function:: double PyOS_string_to_double(const char *s, char **endptr, PyObject *overflow_exception)

   Convert a string ``s`` to a :c:expr:`double`, raising a Python
   exception on failure.  The set of accepted strings corresponds to
   the set of strings accepted by Python's :func:`float` constructor,
   except that ``s`` must not have leading or trailing whitespace.
   The conversion is independent of the current locale.

   If ``endptr`` is ``NULL``, convert the whole string.  Raise
   :exc:`ValueError` and return ``-1.0`` if the string is not a valid
   representation of a floating-point number.

   If endptr is not ``NULL``, convert as much of the string as
   possible and set ``*endptr`` to point to the first unconverted
   character.  If no initial segment of the string is the valid
   representation of a floating-point number, set ``*endptr`` to point
   to the beginning of the string, raise ValueError, and return
   ``-1.0``.

   If ``s`` represents a value that is too large to store in a float
   (for example, ``"1e500"`` is such a string on many platforms) then
   if ``overflow_exception`` is ``NULL`` return ``Py_INFINITY`` (with
   an appropriate sign) and don't set any exception.  Otherwise,
   ``overflow_exception`` must point to a Python exception object;
   raise that exception and return ``-1.0``.  In both cases, set
   ``*endptr`` to point to the first character after the converted value.

   If any other error occurs during the conversion (for example an
   out-of-memory error), set the appropriate Python exception and
   return ``-1.0``.

   .. versionadded:: 3.1


.. c:function:: char* PyOS_double_to_string(double val, char format_code, int precision, int flags, int *ptype)

   Convert a :c:expr:`double` *val* to a string using supplied
   *format_code*, *precision*, and *flags*.

   *format_code* must be one of ``'e'``, ``'E'``, ``'f'``, ``'F'``,
   ``'g'``, ``'G'`` or ``'r'``.  For ``'r'``, the supplied *precision*
   must be 0 and is ignored.  The ``'r'`` format code specifies the
   standard :func:`repr` format.

   *flags* can be zero or more of the values ``Py_DTSF_SIGN``,
   ``Py_DTSF_ADD_DOT_0``, or ``Py_DTSF_ALT``, or-ed together:

   * ``Py_DTSF_SIGN`` means to always precede the returned string with a sign
     character, even if *val* is non-negative.

   * ``Py_DTSF_ADD_DOT_0`` means to ensure that the returned string will not look
     like an integer.

   * ``Py_DTSF_ALT`` means to apply "alternate" formatting rules.  See the
     documentation for the :c:func:`PyOS_snprintf` ``'#'`` specifier for
     details.

   If *ptype* is non-``NULL``, then the value it points to will be set to one of
   ``Py_DTST_FINITE``, ``Py_DTST_INFINITE``, or ``Py_DTST_NAN``, signifying that
   *val* is a finite number, an infinite number, or not a number, respectively.

   The return value is a pointer to *buffer* with the converted string or
   ``NULL`` if the conversion failed. The caller is responsible for freeing the
   returned string by calling :c:func:`PyMem_Free`.

   .. versionadded:: 3.1


.. c:function:: int PyOS_stricmp(const char *s1, const char *s2)

   Case insensitive comparison of strings. The function works almost
   identically to :c:func:`!strcmp` except that it ignores the case.


.. c:function:: int PyOS_strnicmp(const char *s1, const char *s2, Py_ssize_t  size)

   Case insensitive comparison of strings. The function works almost
   identically to :c:func:`!strncmp` except that it ignores the case.