ref: 17ebe55031ae6945ad1f671b69267a672328e4b1
dir: /sys/src/cmd/python/Python/mysnprintf.c/
#include "Python.h" #include <ctype.h> /* snprintf() wrappers. If the platform has vsnprintf, we use it, else we emulate it in a half-hearted way. Even if the platform has it, we wrap it because platforms differ in what vsnprintf does in case the buffer is too small: C99 behavior is to return the number of characters that would have been written had the buffer not been too small, and to set the last byte of the buffer to \0. At least MS _vsnprintf returns a negative value instead, and fills the entire buffer with non-\0 data. The wrappers ensure that str[size-1] is always \0 upon return. PyOS_snprintf and PyOS_vsnprintf never write more than size bytes (including the trailing '\0') into str. If the platform doesn't have vsnprintf, and the buffer size needed to avoid truncation exceeds size by more than 512, Python aborts with a Py_FatalError. Return value (rv): When 0 <= rv < size, the output conversion was unexceptional, and rv characters were written to str (excluding a trailing \0 byte at str[rv]). When rv >= size, output conversion was truncated, and a buffer of size rv+1 would have been needed to avoid truncation. 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 unreliable. It could be that an error in format codes was detected by libc, or on platforms with a non-C99 vsnprintf simply that the buffer wasn't big enough to avoid truncation, or on platforms without any vsnprintf that PyMem_Malloc couldn't obtain space for a temp buffer. CAUTION: Unlike C99, str != NULL and size > 0 are required. */ int PyOS_snprintf(char *str, size_t size, const char *format, ...) { int rc; va_list va; va_start(va, format); rc = PyOS_vsnprintf(str, size, format, va); va_end(va); return rc; } int PyOS_vsnprintf(char *str, size_t size, const char *format, va_list va) { int len; /* # bytes written, excluding \0 */ #ifndef HAVE_SNPRINTF char *buffer; #endif assert(str != NULL); assert(size > 0); assert(format != NULL); #ifdef HAVE_SNPRINTF len = vsnprintf(str, size, format, va); #else /* Emulate it. */ buffer = PyMem_MALLOC(size + 512); if (buffer == NULL) { len = -666; goto Done; } len = vsprintf(buffer, format, va); if (len < 0) /* ignore the error */; else if ((size_t)len >= size + 512) Py_FatalError("Buffer overflow in PyOS_snprintf/PyOS_vsnprintf"); else { const size_t to_copy = (size_t)len < size ? (size_t)len : size - 1; assert(to_copy < size); memcpy(str, buffer, to_copy); str[to_copy] = '\0'; } PyMem_FREE(buffer); Done: #endif str[size-1] = '\0'; return len; }