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
  41 int
  42 PyOS_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
  53 int
  54 PyOS_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
 100 Done:
 101         if (size > 0)
 102                 str[size-1] = '\0';
 103         return len;
 104 #undef _PyOS_vsnprintf_EXTRA_SPACE
 105 }