Function returning malloc'ed memory.

OK, I wrote a function that receives two number ranges (say 1 to 1 million), and the address of a counter variable. All values are unsigned long long.

Code:

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// prototype
u_long_64 *semiprimegen(u_long_64 first_range, u_long_64 second_range, u_long_64 *semiprimes_count);

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The function will generate semi-prime numbers, allocating the first bunch of memory and reallocating as needed, modifying the counter variable until the ranges are met, sorts the allocated memory, and returns a pointer to that memory, which contains *semiprimes_count elements of type unsigned long long.

So, In my main function, I have this:

Code:

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semiprimes = semiprimegen(first_range, second_range, &semiprimes_count);

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Now, after this statement, I accessed the semiprimes_count variable. It works, info is there like its supposed to. I can see the address the semiprimes pointer has (confirmed by printing to console right before semiprimes function returns). But I CAN'T access the memory! What is wrong with this? The printf I put is:

Code:

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printf("20th semiprime is %I64u\n", *(semiprimes + 19));

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The program crashes right there. In that printf. Anybody knows what's wrong? Thanks.

Fixed it!

What I explained in the post was absolutely ok. It was a problem I had later in the code, but for some reason, it crashed before displaying the printf (no idea why).

I got scared, I suddenly thought that malloc'ed memory would go out of scope or something, apparently its not the case :)

Quote:

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Originally Posted by samus250

Fixed it!

What I explained in the post was absolutely ok. It was a problem I had later in the code, but for some reason, it crashed before displaying the printf (no idea why).

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Using fflush(stdout) may help here - it makes sure the output has gone to the output before continuing.

Quote:

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I got scared, I suddenly thought that malloc'ed memory would go out of scope or something, apparently its not the case :)

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Certainly not the case - it's available as long as you have a pointer to it (technically the memory is available even if you haven't got a pointer, just that it's hard to find the address of it...).

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Mats

Quote:

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Using fflush(stdout) may help here - it makes sure the output has gone to the output before continuing.

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Where do I put the fflush(stdout) [or fpurge(stoud), as learned from C for Dummies]? Before the printf, or after the printf?

Because I've noticed that problem in some programs. For example, one that enters a loop which may take up to 5 minutes to get out of (finds all prime numbers from a given range, which normally is like 1 to 100 million). Before the loop I wrote a printf, and it doesn't show up until the loop finishes.

Thanks!

Edit: I almost forgot, a lot of weird things also happen depending on where I put the newline character. I always try to put it at the end of each string because sometimes, when I put it at the beginning, it doesn't show up until a next printf. (I'll experiment more on that later).

Normally, you would put fflush immediately after the printf (or after a few printf's, if you have several in a row that you don't care to show until the last one is done, if that makes sense to you).

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Mats

Quote:

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Originally Posted by samus250

Edit: I almost forgot, a lot of weird things also happen depending on where I put the newline character. I always try to put it at the end of each string because sometimes, when I put it at the beginning, it doesn't show up until a next printf. (I'll experiment more on that later).

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That's because stdout is usually line-buffered, which means it's flushed automatically when it encounters a newline.