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SRobertJames
SRobertJames

Reputation: 9263

C - is it necessary to check freopen return value?

The signature for freopen is FILE * freopen ( const char * filename, const char * mode, FILE * stream )

According to the docs, the return value is the same as what stream is set to. So, my code already checks stream != 0, do I need to check the return value? How could stream == 0 but retval != 0? If I don't need to check both, why does gcc give me a warning if I don't check the retval?

To quiet gcc, is it okay to simply do: stream = freopen(filename, mode, stream); if (!stream) {...})?

Upvotes: 0

Views: 492

Answers (2)

R Sahu
R Sahu

Reputation: 206717

You said:

According to the docs, the return value is the same as what stream is set to.

Not according to http://en.cppreference.com/w/c/io/freopen

Return value
1) A copy of the value of stream on success, null pointer on failure.

You asked:

do I need to check the return value?

Yes, you should, always. freopen can fail for all the reasons that fopen can fail -- permissions, non-existent directory, too many open files, etc. It is a good practice to always check the return value of freopen before you use it.

Upvotes: 4

Random832
Random832

Reputation: 39060

If it fails, it's not going to magically set the original stream variable (which it has no reference to, since the parameter is FILE * not FILE **) to NULL, so you do need to check the return value.

Upvotes: 5

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