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Commit805889d

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Make snprintf.c follow the C99 standard for snprintf's result value.
C99 says that the result should be the number of bytes that would havebeen emitted given a large enough buffer, not the number we actuallywere able to put in the buffer. It's time to make our substituteimplementation comply with that. Not doing so results in inefficiencyin buffer-enlargement cases, and also poses a portability hazard forthird-party code that might expect C99-compliant snprintf behaviorwithin Postgres.In passing, remove useless tests for str == NULL; neither C99 norpredecessor standards ever allowed that except when count == 0,so I see no reason to expend cycles on making that a non-crash casefor this implementation. Also, don't waste a byte in pg_vfprintf'slocal I/O buffer; this might have performance benefits by allowingaligned writes during flushbuffer calls.Discussion:https://postgr.es/m/17245.1534289329@sss.pgh.pa.us
1 parent777e6dd commit805889d

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‎src/port/snprintf.c

Lines changed: 63 additions & 31 deletions
Original file line numberDiff line numberDiff line change
@@ -2,6 +2,7 @@
22
* Copyright (c) 1983, 1995, 1996 Eric P. Allman
33
* Copyright (c) 1988, 1993
44
*The Regents of the University of California. All rights reserved.
5+
* Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
56
*
67
* Redistribution and use in source and binary forms, with or without
78
* modification, are permitted provided that the following conditions
@@ -49,8 +50,8 @@
4950
*SNPRINTF, VSNPRINTF and friends
5051
*
5152
* These versions have been grabbed off the net. They have been
52-
* cleaned up to compile properly and support for most of theSingle Unix
53-
*Specification has been added. Remaining unimplemented features are:
53+
* cleaned up to compile properly and support for most of theC99
54+
*specification has been added. Remaining unimplemented features are:
5455
*
5556
* 1. No locale support: the radix character is always '.' and the '
5657
* (single quote) format flag is ignored.
@@ -64,25 +65,24 @@
6465
* 5. Space and '#' flags are not implemented.
6566
*
6667
*
67-
*Theresult values ofthese functions are not the sameacrossdifferent
68-
*platforms.This implementationis compatible withtheSingle Unix Spec:
68+
*Historically theresult values ofsprintf/snprintf variedacrossplatforms.
69+
* This implementationnow followstheC99 standard:
6970
*
70-
* 1. -1 is returned only if processing is abandoned due to an invalid
71-
* parameter, such as incorrect format string. (Although not required by
72-
* the spec, this happens only when no characters have yet been transmitted
73-
* to the destination.)
71+
* 1. -1 is returned if an error is detected in the format string, or if
72+
* a write to the target stream fails (as reported by fwrite). Note that
73+
* overrunning snprintf's target buffer is *not* an error.
7474
*
75-
* 2. Forsnprintf and sprintf, 0 is returned if str == NULL or count == 0;
76-
*no data has been stored.
75+
* 2. Forsuccessful writes to streams, the actual number of bytes written
76+
*to the stream is returned.
7777
*
78-
* 3. Otherwise, the number of bytes actually transmitted to the destination
79-
* is returned (excluding the trailing '\0' for snprintf and sprintf).
78+
* 3. For successful sprintf/snprintf, the number of bytes that would have
79+
* been written to an infinite-size buffer (excluding the trailing '\0')
80+
* is returned. snprintf will truncate its output to fit in the buffer
81+
* (ensuring a trailing '\0' unless count == 0), but this is not reflected
82+
* in the function result.
8083
*
81-
* For snprintf with nonzero count, the result cannot be more than count-1
82-
* (a trailing '\0' is always stored); it is not possible to distinguish
83-
* buffer overrun from exact fit. This is unlike some implementations that
84-
* return the number of bytes that would have been needed for the complete
85-
* result string.
84+
* snprintf buffer overrun can be detected by checking for function result
85+
* greater than or equal to the supplied count.
8686
*/
8787

8888
/**************************************************************
@@ -101,15 +101,27 @@
101101
#undeffprintf
102102
#undefprintf
103103

104-
/* Info about where the formatted output is going */
104+
/*
105+
* Info about where the formatted output is going.
106+
*
107+
* dopr and subroutines will not write at/past bufend, but snprintf
108+
* reserves one byte, ensuring it may place the trailing '\0' there.
109+
*
110+
* In snprintf, we use nchars to count the number of bytes dropped on the
111+
* floor due to buffer overrun. The correct result of snprintf is thus
112+
* (bufptr - bufstart) + nchars. (This isn't as inconsistent as it might
113+
* seem: nchars is the number of emitted bytes that are not in the buffer now,
114+
* either because we sent them to the stream or because we couldn't fit them
115+
* into the buffer to begin with.)
116+
*/
105117
typedefstruct
106118
{
107119
char*bufptr;/* next buffer output position */
108120
char*bufstart;/* first buffer element */
109-
char*bufend;/* last buffer element, or NULL */
121+
char*bufend;/* last+1 buffer element, or NULL */
110122
/* bufend == NULL is for sprintf, where we assume buf is big enough */
111123
FILE*stream;/* eventual output destination, or NULL */
112-
intnchars;/* # charsalreadysent to stream */
124+
intnchars;/* # chars sent to stream, or dropped */
113125
boolfailed;/* call is a failure; errno is set */
114126
}PrintfTarget;
115127

@@ -147,17 +159,28 @@ int
147159
pg_vsnprintf(char*str,size_tcount,constchar*fmt,va_listargs)
148160
{
149161
PrintfTargettarget;
162+
charonebyte[1];
150163

151-
if (str==NULL||count==0)
152-
return0;
164+
/*
165+
* C99 allows the case str == NULL when count == 0. Rather than
166+
* special-casing this situation further down, we substitute a one-byte
167+
* local buffer. Callers cannot tell, since the function result doesn't
168+
* depend on count.
169+
*/
170+
if (count==0)
171+
{
172+
str=onebyte;
173+
count=1;
174+
}
153175
target.bufstart=target.bufptr=str;
154176
target.bufend=str+count-1;
155177
target.stream=NULL;
156-
/*target.ncharsis unused in this case */
178+
target.nchars=0;
157179
target.failed= false;
158180
dopr(&target,fmt,args);
159181
*(target.bufptr)='\0';
160-
returntarget.failed ?-1 : (target.bufptr-target.bufstart);
182+
returntarget.failed ?-1 : (target.bufptr-target.bufstart
183+
+target.nchars);
161184
}
162185

163186
int
@@ -177,16 +200,15 @@ pg_vsprintf(char *str, const char *fmt, va_list args)
177200
{
178201
PrintfTargettarget;
179202

180-
if (str==NULL)
181-
return0;
182203
target.bufstart=target.bufptr=str;
183204
target.bufend=NULL;
184205
target.stream=NULL;
185-
/*target.ncharsis unused in this case */
206+
target.nchars=0;/* not really used in this case */
186207
target.failed= false;
187208
dopr(&target,fmt,args);
188209
*(target.bufptr)='\0';
189-
returntarget.failed ?-1 : (target.bufptr-target.bufstart);
210+
returntarget.failed ?-1 : (target.bufptr-target.bufstart
211+
+target.nchars);
190212
}
191213

192214
int
@@ -213,7 +235,7 @@ pg_vfprintf(FILE *stream, const char *fmt, va_list args)
213235
return-1;
214236
}
215237
target.bufstart=target.bufptr=buffer;
216-
target.bufend=buffer+sizeof(buffer)-1;
238+
target.bufend=buffer+sizeof(buffer);/* use the whole buffer */
217239
target.stream=stream;
218240
target.nchars=0;
219241
target.failed= false;
@@ -256,6 +278,10 @@ flushbuffer(PrintfTarget *target)
256278
{
257279
size_tnc=target->bufptr-target->bufstart;
258280

281+
/*
282+
* Don't write anything if we already failed; this is to ensure we
283+
* preserve the original failure's errno.
284+
*/
259285
if (!target->failed&&nc>0)
260286
{
261287
size_twritten;
@@ -1035,7 +1061,10 @@ dostr(const char *str, int slen, PrintfTarget *target)
10351061
{
10361062
/* buffer full, can we dump to stream? */
10371063
if (target->stream==NULL)
1038-
return;/* no, lose the data */
1064+
{
1065+
target->nchars+=slen;/* no, lose the data */
1066+
return;
1067+
}
10391068
flushbuffer(target);
10401069
continue;
10411070
}
@@ -1054,7 +1083,10 @@ dopr_outch(int c, PrintfTarget *target)
10541083
{
10551084
/* buffer full, can we dump to stream? */
10561085
if (target->stream==NULL)
1057-
return;/* no, lose the data */
1086+
{
1087+
target->nchars++;/* no, lose the data */
1088+
return;
1089+
}
10581090
flushbuffer(target);
10591091
}
10601092
*(target->bufptr++)=c;

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