Python/mysnprintf.c

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