5.6 String conversion and formatting

Functions for number conversion and formatted string output.

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 snprintf(2).

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 vsnprintf(2).

PyOS_snprintf and PyOS_vsnprintf wrap the Standard C library functions snprintf() and 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 and format != NULL.

If the platform doesn't have vsnprintf() and the buffer size needed to avoid truncation exceeds size by more than 512 bytes, Python aborts with a Py_FatalError.

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.

double PyOS_ascii_strtod( const char *nptr, char **endptr)

Convert a string to a double. This function behaves like the Standard C function strtod() does in the C locale. It does this without changing the current locale, since that would not be thread-safe.

PyOS_ascii_strtod should typically be used for reading configuration files or other non-user input that should be locale independent. New in version 2.4.

See the Unix man page strtod(2) for details.

char * PyOS_ascii_formatd( char *buffer, size_t buf_len, const char *format, double d): Convert a double to a string using the "." as the decimal separator. format is a printf()-style format string specifying the number format. Allowed conversion characters are "e", "E", "f", "F", "g" and "G".
The return value is a pointer to buffer with the converted string or NULL if the conversion failed. New in version 2.4.

double PyOS_ascii_atof( const char *nptr): Convert a string to a double in a locale-independent way. New in version 2.4.
See the Unix man page atof(2) for details.

See About this document... for information on suggesting changes.