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Use libc's snprintf, not sprintf, for special cases in snprintf.c.

snprintf.c has always fallen back on libc's *printf implementationwhen printing pointers (%p) and floats. When this code originated,we were still supporting some platforms that lacked native snprintf,so we used sprintf for that. That's not actually unsafe in our usage,but nonetheless builds on macOS are starting to complain about sprintfbeing unconditionally deprecated; and I wouldn't be surprised if otherplatforms follow suit. There seems little reason to believe that anyplatform supporting C99 wouldn't have standards-compliant snprintf,so let's just use that instead to suppress such warnings.Back-patch to v12, which is where we started to require C99. It'salso where we started to use our snprintf.c everywhere, so thiswouldn't be enough to suppress the warning in older branches anyway--- that is, in older branches these aren't necessarily all ourusages of libc's sprintf. It is enough in v12+ because anydeprecation annotation attached to libc's sprintf won't apply topg_sprintf. (Whether all our usages of pg_sprintf are adequatelysafe is not a matter I intend to address here, but perhaps it coulddo with some review.)Per report from Andres Freund and local testing.Discussion:https://postgr.es/m/20221015211955.q4cwbsfkyk3c4ty3@awork3.anarazel.de

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src/port
- snprintf.c

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`‎src/port/snprintf.c`

Lines changed: 7 additions & 7 deletions

Original file line number	Diff line number	Diff line change
`@@ -998,8 +998,8 @@ fmtptr(const void value, PrintfTarget target)`
`998`	`998`	`intvallen;`
`999`	`999`	`charconvert[64];`
`1000`	`1000`
`1001`		`-/* we rely on regular C library'ssprintf to do the basic conversion */`
`1002`		`-vallen=sprintf(convert,"%p",value);`
	`1001`	`+/* we rely on regular C library'ssnprintf to do the basic conversion */`
	`1002`	`+vallen=snprintf(convert,sizeof(convert),"%p",value);`
`1003`	`1003`	`if (vallen<0)`
`1004`	`1004`	`target->failed= true;`
`1005`	`1005`	`else`
`@@ -1149,11 +1149,11 @@ fmtfloat(double value, char type, int forcesign, int leftjust,`
`1149`	`1149`	`intpadlen;/* amount to pad with spaces */`
`1150`	`1150`
`1151`	`1151`	`/*`
`1152`		`- * We rely on the regular C library'ssprintf to do the basic conversion,`
	`1152`	`+ * We rely on the regular C library'ssnprintf to do the basic conversion,`
`1153`	`1153`	`* then handle padding considerations here.`
`1154`	`1154`	`*`
`1155`	`1155`	`* The dynamic range of "double" is about 1E+-308 for IEEE math, and not`
`1156`		`- * too wildly more than that with other hardware. In "f" format,sprintf`
	`1156`	`+ * too wildly more than that with other hardware. In "f" format,snprintf`
`1157`	`1157`	`* could therefore generate at most 308 characters to the left of the`
`1158`	`1158`	`* decimal point; while we need to allow the precision to get as high as`
`1159`	`1159`	`* 308+17 to ensure that we don't truncate significant digits from very`
`@@ -1205,14 +1205,14 @@ fmtfloat(double value, char type, int forcesign, int leftjust,`
`1205`	`1205`	`fmt[2]='*';`
`1206`	`1206`	`fmt[3]=type;`
`1207`	`1207`	`fmt[4]='\0';`
`1208`		`-vallen=sprintf(convert,fmt,prec,value);`
	`1208`	`+vallen=snprintf(convert,sizeof(convert),fmt,prec,value);`
`1209`	`1209`	`}`
`1210`	`1210`	`else`
`1211`	`1211`	`{`
`1212`	`1212`	`fmt[0]='%';`
`1213`	`1213`	`fmt[1]=type;`
`1214`	`1214`	`fmt[2]='\0';`
`1215`		`-vallen=sprintf(convert,fmt,value);`
	`1215`	`+vallen=snprintf(convert,sizeof(convert),fmt,value);`
`1216`	`1216`	`}`
`1217`	`1217`	`if (vallen<0)`
`1218`	`1218`	`gotofail;`
`@@ -1341,7 +1341,7 @@ pg_strfromd(char *str, size_t count, int precision, double value)`
`1341`	`1341`	`fmt[2]='*';`
`1342`	`1342`	`fmt[3]='g';`
`1343`	`1343`	`fmt[4]='\0';`
`1344`		`-vallen=sprintf(convert,fmt,precision,value);`
	`1344`	`+vallen=snprintf(convert,sizeof(convert),fmt,precision,value);`
`1345`	`1345`	`if (vallen<0)`
`1346`	`1346`	`{`
`1347`	`1347`	`target.failed= true;`

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`‎src/port/snprintf.c`

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