C what does malloc do

But it is already too late. Somewhere in memory, there is corrupted data. In addition, it is not clear what data is corrupted and what consequences it may have! Is the compiler to blame, that it rearranged the assignment operations? The programmer let the dereferencing of a null pointer happen and thereby led the program in the undefined behavior state. In this particular case, the undefined behavior of a program will be that somewhere in the memory data is corrupted.

Based on the axiom, any null pointer dereference is undefined behavior of a program. There is no such thing as a "harmless" undefined behavior. Any undefined behavior is unacceptable.

Do not allow dereferencing of pointers, which the malloc function and its analogs returned, without their prior check. Do not rely on any other ways to catch the dereferencing of a null pointer. You should only use the good old if operator. The thing, which is perceived as not an error by one programmers, is a vulnerability for others. This is the exact situation which happens in case of null pointer dereference. Others believe that dereferencing of a null pointer cause denial-of-service and represents a vulnerability.

Instead of nominal handling the memory lack, a program, or one of the program threads completes its work. This can lead to loss of data, data integrity, and so on. In other words, the CAD system will simply close, if it is not be able to allocate memory for any complex operation without offering the user to even save the results of its work. I wouldn't like to be unfounded, so here are the proofs. So, the application developers consider the absence of a check after calling calloc as a vulnerability CVE All the fixed fragments which could contain null pointer dereference were approximately the same:.

If you are developing not a very significant application for which a crash during its work is not a trouble, then yes, it is not needed to write checks. However, if you are developing a library, the lack of checks is unacceptable! Not only lazy programmers, who write irresponsible applications, like a Tetris game, can use your library. We must take care both about normal programmers, and normal programs.

Therefore, I ideologically disagree with, for example, Carsten Haitzler, that in the library of EFL Core there are no checks see article. This doesn't let programmers build reliable applications based on such libraries. In general, if you are creating a library, please note that in some applications, dereferencing of a null pointer is a vulnerability. You must handle memory allocation errors and nominally return the information about the failure.

Those who feel lazy to write checks, think that dereferencing affects exactly null pointers. Yes, it often happens this way. But can a developer vouch for the entire code application? I'm sure, no. I'm going to show what I mean with practical examples. Check lines: 65, Right after allocation of memory buffer, a record happens in the cell TheTable[NumBuckets].

If the value of the variable NumBuckets is great enough, we'll taint some data with unpredictable consequences. After such damage, it generally makes no sense to speculate how the program will run. There may be the most unexpected consequences. Therefore, this is not a unique case, but quite a typical situation when data is written not exactly by a null pointer, and by a random shift.

I will continue the correspondence discussion with Carsten Haitzler. He argues that they understand what they are doing when they don't check the result of the malloc function call. No, they don't. Let's take a look, for example, at the code fragment from the EFL library:.

Therefore, the code or line numbers may no longer match to what there is now. However, it is not that essential for telling my story. Here we have a typical situation: there is not enough space for data storing in a buffer, it should be increased.

To increase the size of the buffer the realloc function is used, which may return NULL. Let's take a look at these lines:. In memory some data will be corrupted, but the program will run anyway. The effects are unpredictable again and there will be no good for sure. I wasn't carefully studying the old report on EFL Core Libraries, but this is definitely not the only error. I noticed at least two similar places where after realloc data is recorded to an index.

I'm asking the question once again: "Where's the guarantee that dereferencing of exactly a null pointer will occur? No such guarantees. It is impossible, developing or modifying code, to remember about a nuance considered lately. You can easily spoil something in memory, in doing so the program continues executing as nothing happened.

The only way to write reliable and correct code is to always check the result that is returned by the malloc function. Check and live a peaceful life. I understand perfectly about realloc and everything else that is written in the article. But I am a professional and, allocating memory, I just fill it with zeros using memset at once.

Where it is necessary I use checks. But I'm not going to write the extra checks after each malloc. Generally, to fill the memory immediately after buffer allocation is quite a weird idea. It is strange because there is a function calloc. However, people act like this very often. You don't need to look very far in order to get examples, here is the code from the WebRTC library, used in Chromium:. Memory is allocated, then the buffer is filled with zeros. It's a very common practice, although, in fact, two lines can be reduced to one using calloc.

But it doesn't matter. The main thing is that even such code is not secure! And if a large buffer was allocated, some useful data could be cleared. But there is no point to handle them anyway. By that moment, a large fragment of memory will be corrupted and the following work of the program will be unpredictable. The reader might argue that all this is purely theoretical. Yes, the memset function could theoretically fill the buffer starting from the end of the buffer, but in practice, nobody will realize this function this way.

I would agree that this implementation of memset is really exotic, and I even asked a question on StackOverflow about this topic. This is the reply:. The Linux kernel's memset for the SuperH architecture has this property: link. Unfortunately, this is the code on the unfamiliar type of assembler, so I'm not going to speculate about it.

But still there is such an interesting implementation in the C programming language. I will cite the beginning of the function:.

Here we come to the reason N1 "Dereferencing of a null pointer is undefined behavior". There is no guarantee that the compiler will not interchange the assignments. Memory successfully allocated using calloc. Calloc Memory successfully freed. In other words, if the memory previously allocated with the help of malloc or calloc is insufficient, realloc can be used to dynamically re-allocate memory.

The elements of the array are: 1, 2, 3, 4, 5, Enter the new size of the array: 10 Memory successfully re-allocated using realloc. Skip to content. Change Language. Related Articles. Table of Contents. Save Article. Improve Article. Like Article. Output: Enter number of elements: 5 Memory successfully allocated using malloc. The elements of the array are: 1, 2, 3, 4, 5,. Output: Enter number of elements: 5 Memory successfully allocated using calloc. The elements of the array are: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,.

Previous Difference Between malloc and calloc with Examples. Next How to dynamically allocate a 2D array in C? Recommended Articles. You must learn about the differences between heap and stack memory, take a look at this question: stackoverflow. Add a comment.

Active Oldest Votes. Improve this answer. Sibren 10 10 silver badges 10 10 bronze badges. Just because it's unknown how much memory is needed at compile time, doesn't mean the heap has to be involved. Matt Joiner, that was ONE example. If you think that is worth of downvoting then I have nothing to say. Matt Joiner, edited the answer a little bit. May be it sounds better now.

This answer fails to mention the very important use of malloc to allocate memory with a lifetime beyond that of execution of the current block. Eric Postpischil k 11 11 gold badges silver badges bronze badges. Necrolis Necrolis An object can have block scope and static storage duration. Before C99 you could use alloca. That aside, this is the most correct answer, and should be marked the solution.

ArcticPhoenix ArcticPhoenix 2 2 silver badges 4 4 bronze badges. Let's try and tackle this question considering different aspects Size malloc allows you to allocate much larger memory spaces than the one allocated simply using student p; or int x[n];.

The reason being malloc allocates the space on heap while the other allocates it on the stack The C programming language manages memory statically, automatically, or dynamically. For more information check this answer For more details related to variable-sized arrays, have a look at this. Daksh Shah Daksh Shah 2, 4 4 gold badges 34 34 silver badges 64 64 bronze badges.

What is "the normal way"? You should be more specific here. For instance, you can't resize a static or local variable. All the best. Luke 4 4 silver badges 10 10 bronze badges. Acn Acn 2 2 gold badges 9 9 silver badges 22 22 bronze badges. Walid Walid Walid Walid 17 8 8 bronze badges. DipSwitch DipSwitch 4, 2 2 gold badges 18 18 silver badges 24 24 bronze badges. Variable length arrays can be stored on the stack just fine.

Variable length arrays are no reason to use the heap. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.