free() does not clear memory space in LINUX

paulsm4 · 08-17-2006, 06:19 PM

xinminhua - You said:

Quote:

My reply:
You are right ... But we want to understand it.

Hopefully, based on all this information, you now understand the issues. And why your program might "appear" to work on one platform ... and fail on another ... despite the fact that it's a bug on all platforms.

Hopefully, too, you've found and corrected the problem (my money is definitely on mhcox's suggestion: double-freeing pointers).

Most of all, please take sundialsvcs's suggestions to heart - maybe even formalize them in coding guidelines for your team, and enforce them in team code reviews:

Quote:

I advise the following self-disciplines with respect to dynamically allocated memory.

1. "A pointer's value shall be NULL or it shall point to a valid block of storage." Therefore, the statement immediately following should be p = NULL;.
2. "All dynamically-allocated blocks of storage shall be immediately initialized to a known value, such as zeroes." Either use a storage allocation function that does this, or do it yourself.
3. "Allocate all buffers four to eight bytes larger than they need to be, make sure all those bytes are known-zero, and don't tell anyone they exist when you quote the size of the buffer."
4. "When manipulating strings, always use functions that require you to give the size of the buffer and that will not write beyond that." Remember the trailing-null that some routines stomp past the end of the buffer.
5. "Always check. Don't assume. When something is wrong, fail noisily."
6. "Insert debugging code into your program until you are certain that it is completely debugged. Then, and only then ... leave it in. "

Definitely good advice there!

Finally - please let us know what happens!

Sincerely .. PSM

jlliagre · 08-18-2006, 01:17 AM

Quote:

Originally Posted by sundialsvcs

"Allocate all buffers four to eight bytes larger than they need to be, make sure all those bytes are known-zero, and don't tell anyone they exist when you quote the size of the buffer."

While I support the other advices, that one may be more dubious, as it basically allows code presenting small overflow bugs to run undetected, including by valgrind, purify or Sun's studio RTC.

mhcox · 08-18-2006, 03:43 PM

Quote:

Originally Posted by paulsm4

xinminhua - You said:

Hopefully, based on all this information, you now understand the issues. And why your program might "appear" to work on one platform ... and fail on another ... despite the fact that it's a bug on all platforms.

Hopefully, too, you've found and corrected the problem (my money is definitely on mhcox's suggestion: double-freeing pointers).

Most of all, please take sundialsvcs's suggestions to heart - maybe even formalize them in coding guidelines for your team, and enforce them in team code reviews

efinitely good advice there!

Finally - please let us know what happens!

Sincerely .. PSM

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7. Regularly compile your code with memory error detection tools like Purify, valgrind, etc. and run the code through a suite of unit-tests.

Mike