/Python/mysnprintf.c

  1#include "Python.h"
  2#include <ctype.h>
  3
  4/* snprintf() wrappers.  If the platform has vsnprintf, we use it, else we
  5   emulate it in a half-hearted way.  Even if the platform has it, we wrap
  6   it because platforms differ in what vsnprintf does in case the buffer
  7   is too small:  C99 behavior is to return the number of characters that
  8   would have been written had the buffer not been too small, and to set
  9   the last byte of the buffer to \0.  At least MS _vsnprintf returns a
 10   negative value instead, and fills the entire buffer with non-\0 data.
 11
 12   The wrappers ensure that str[size-1] is always \0 upon return.
 13
 14   PyOS_snprintf and PyOS_vsnprintf never write more than size bytes
 15   (including the trailing '\0') into str.
 16
 17   If the platform doesn't have vsnprintf, and the buffer size needed to
 18   avoid truncation exceeds size by more than 512, Python aborts with a
 19   Py_FatalError.
 20
 21   Return value (rv):
 22
 23	When 0 <= rv < size, the output conversion was unexceptional, and
 24	rv characters were written to str (excluding a trailing \0 byte at
 25	str[rv]).
 26
 27	When rv >= size, output conversion was truncated, and a buffer of
 28	size rv+1 would have been needed to avoid truncation.  str[size-1]
 29	is \0 in this case.
 30
 31	When rv < 0, "something bad happened".  str[size-1] is \0 in this
 32	case too, but the rest of str is unreliable.  It could be that
 33	an error in format codes was detected by libc, or on platforms
 34	with a non-C99 vsnprintf simply that the buffer wasn't big enough
 35	to avoid truncation, or on platforms without any vsnprintf that
 36	PyMem_Malloc couldn't obtain space for a temp buffer.
 37
 38   CAUTION:  Unlike C99, str != NULL and size > 0 are required.
 39*/
 40
 41int
 42PyOS_snprintf(char *str, size_t size, const  char  *format, ...)
 43{
 44	int rc;
 45	va_list va;
 46
 47	va_start(va, format);
 48	rc = PyOS_vsnprintf(str, size, format, va);
 49	va_end(va);
 50	return rc;
 51}
 52
 53int
 54PyOS_vsnprintf(char *str, size_t size, const char  *format, va_list va)
 55{
 56	int len;  /* # bytes written, excluding \0 */
 57#ifdef HAVE_SNPRINTF
 58#define _PyOS_vsnprintf_EXTRA_SPACE 1
 59#else
 60#define _PyOS_vsnprintf_EXTRA_SPACE 512
 61	char *buffer;
 62#endif
 63	assert(str != NULL);
 64	assert(size > 0);
 65	assert(format != NULL);
 66	/* We take a size_t as input but return an int.  Sanity check
 67	 * our input so that it won't cause an overflow in the
 68         * vsnprintf return value or the buffer malloc size.  */
 69	if (size > INT_MAX - _PyOS_vsnprintf_EXTRA_SPACE) {
 70		len = -666;
 71		goto Done;
 72	}
 73
 74#ifdef HAVE_SNPRINTF
 75	len = vsnprintf(str, size, format, va);
 76#else
 77	/* Emulate it. */
 78	buffer = PyMem_MALLOC(size + _PyOS_vsnprintf_EXTRA_SPACE);
 79	if (buffer == NULL) {
 80		len = -666;
 81		goto Done;
 82	}
 83
 84	len = vsprintf(buffer, format, va);
 85	if (len < 0)
 86		/* ignore the error */;
 87
 88	else if ((size_t)len >= size + _PyOS_vsnprintf_EXTRA_SPACE)
 89		Py_FatalError("Buffer overflow in PyOS_snprintf/PyOS_vsnprintf");
 90
 91	else {
 92		const size_t to_copy = (size_t)len < size ?
 93					(size_t)len : size - 1;
 94		assert(to_copy < size);
 95		memcpy(str, buffer, to_copy);
 96		str[to_copy] = '\0';
 97	}
 98	PyMem_FREE(buffer);
 99#endif
100Done:
101	if (size > 0)
102		str[size-1] = '\0';
103	return len;
104#undef _PyOS_vsnprintf_EXTRA_SPACE
105}