Why does this sprintf_s throw an exception?

Question

char filePrefix[] = "test";
char fileName[100]; fileName[0] = 0;
sprintf_s(fileName, "%s", filePrefix);

I can't figure out why there's an exception writing into fileName in the sprintf_s

Exception thrown at 0x00007FF885E3F3A9 (ucrtbased.dll) in foo.exe: 0xC0000005: Access violation writing location 0x0000008331F00000.

Read the documentation. You're missing the `buffersize` argument after `fileName`. — Barmar, Nov 24 '21 at 16:12
BTW, there's no need to use this Microsoft-specific function. There's a standard `snprintf()` that's similar. — Barmar, Nov 24 '21 at 16:14
@Barmar int sprintf( char *buffer, const char *format, ... ); — nicomp, Nov 24 '21 at 16:14
@Barmar: Is `sprintf_s` still Microsoft-specific? According to [cppreference](https://en.cppreference.com/w/c/io/fprintf), it appears to be in C11. — Fred Larson, Nov 24 '21 at 16:21
@FredLarson It is part of the *optional* implementation, detailed in Annex K of the specification. — Oka, Nov 24 '21 at 16:39
@FredLarson Further reading on the annex: https://stackoverflow.com/questions/372980/do-you-use-the-tr-24731-safe-functions — Deduplicator, Nov 24 '21 at 17:09
@nicomp Did your compiler not warn you about this error? It could/should have. C used to have a (pretty well-deserved) reputation for being frustrating and difficult to learn, but if everybody could only use modern compilers with decent warnings, things wouldn't be nearly so bad. — Steve Summit, Nov 24 '21 at 17:10

score 2 · Accepted Answer · answered Nov 24 '21 at 16:15

2

From the documentation, the second argument to sprintf_s should be the size of the destination buffer.

char filePrefix[] = "test";
char fileName[100];
fileName[0] = 0;
sprintf_s(fileName, sizeof fileName, "%s", filePrefix);

answered Nov 24 '21 at 16:15

Oka

1 Answers1