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
PyOS_snprintf(char * str,size_t size,const char * format,...)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
PyOS_vsnprintf(char * str,size_t size,const char * format,va_list va)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 }
106