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Can a pointer be freed more than once in C?

+2 votes
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What happens if do so? Or can a pointer be freed twice in C?

posted Jan 22, 2015 by Balu

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4 Answers

+3 votes

1st scenario: After freeing a pointer in a C program, freed memory might be reallocated by some other or same program luckily. In this scenario, freeing the same pointer twice won’t cause any issue.

2nd scenario: We can free a pointer. Then, we can allocate memory for same pointer variable. Then, we can use it and free it again. This is also not an issue.

3rd scenario: If we free the same pointer second time without reallocating memory to that pointer, then what happens? As per ANSI/ISO C standard, this is undefined behavior. This undefined behavior may cause anything to the program that we do not expect to happen.

answer Jan 23, 2015 by Shivaranjini
+2 votes

It can corrupt the heap.

answer Jan 22, 2015 by Aditya
+2 votes

Deallocating a memory area with free does not make the contents of the pointer NULL. Suppose that you have int *p = malloc (sizeof (int)) and a has 0xabcdef and you execute free(p) then after execution p still contains 0xabcdef but after the free call this memory address is no more reserved for you. Doing a free on an already freed memory will result in double free memory corruption.

answer Jan 22, 2015 by Salil Agrawal
+2 votes

In addition to Salil Agrawal's ans, I would suggest to use "ptr = NULL" after calling the free().

So even if you call 2nd time free() on already free() memory. It won't crash due to double free or corruption. If you assing NULL to that pointer after calling free().

answer Jan 23, 2015 by Arshad Khan
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I'm somewhat confused working with @staticmethods. My logger and configuration methods are called n times, but I have only one call.
n is number of classes which import the loger and configuration class in the subfolder mymodule. What might be my mistake mistake?

### __init__.py ###

from mymodule.MyLogger import MyLogger
from mymodule.MyConfig import MyConfig

##### my_test.py ##########
from mymodule import MyConfig,MyLogger

#Both methods are static
key,logfile,loglevel = MyConfig().get_config('Logging')
log = MyLogger.set_logger(key,logfile,loglevel)
log.critical(time.time())

#Output
2013-05-21 17:20:37,192 - my_test - 17 - CRITICAL - **********.19
2013-05-21 17:20:37,192 - my_test - 17 - CRITICAL - **********.19
2013-05-21 17:20:37,192 - my_test - 17 - CRITICAL - **********.19
2013-05-21 17:20:37,192 - my_test - 17 - CRITICAL - **********.19
...