Files in Unix environment.

Hi, i'm messing around with files in C and i've got a little question. I've done a little bit of searching but I can't seem to come up with anything pertaining to my question in a Unix/Linux environment.

I want to create a file in a subdirectory, but I'm not exactly sure how to.

I know to create a file in the current directory, I have to do the following:

Code:

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FILE *ofp = fopen("Blah","w");

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But how would I do a subdirectory. Suppose i'm at r00t and I want to write to a file in a subdirectory affectionately named, "a".

I've tried:

Code:

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FILE *ofp = fopen("//a//Blah","w");

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which just yields a segmentation fault error.

This is something I should have been able to find w/ google, but I've been unsuccessful. Thanks for any help.

First off, the double slashes aren't necessary--I'm not sure whether POSIX mandates that multiple slashes are compressed into one as happens on systems I'm familiar with, but there's no point to it in any case.

Next, does the fopen() itself segfault? I would doubt it, but if so, you've done something very fishy beforehand. If trying to write to ofp segfaults, I'd imagine that's because ofp is NULL. While the C standard doesn't mandate that errno be set on fopen() failure, it will be on your unix system. So check it!

I'd guess you're getting ENOENT. Is the directory "a" a subdirectory of your current working directory? If so, you'd want "a/Blah", not "/a/blah". The latter implies that "a" is in the root directory.

The point here is, check your return values! Functions tend to let you know when an error occurred, and it's generally a good idea to check that.

I forgot to mention something important. The directory "a" doesn't exist yet. I want to create it. I'd imagine that's why my code doesn't work. Dunno why I didn't think of that...

*Brain fart*

Is there a way to do this from C?

[edit]

Quote:

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I forgot to mention something important. The directory "a" doesn't exist yet. I want to create it. I'd imagine that's why my code doesn't work. Dunno why I didn't think of that...

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That's okay; that's what all of the posters in this thread had assumed. Use the function mkdir() as suggested, but read my note about its portability below. [/edit]

What cas means is that fopen() returns a NULL pointer on error, and you should check for this.

Quote:

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I'd guess you're getting ENOENT. Is the directory "a" a subdirectory of your current working directory? If so, you'd want "a/Blah", not "/a/blah". The latter implies that "a" is in the root directory.

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> Suppose i'm at r00t and I want to write to a file in a subdirectory affectionately named, "a".

Quote:

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man 2 mkdir

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It should also be mentioned that mkdir() is a non-standard function, and so if your code uses it, it won't be ANSI-compatible. I believe mkdir() is a POSIX function, however, so you'll have that measure of portability at least.