Depending on the fields in your struct and the alignment that you or your compiler choose for the struct, there may be bits that are allocated as part of the struct that don't actually belong to any fields. That is, structs may have bytes that exist just to pad out the struct to fit alignment. In grad school, I ran into a really nasty bug where I was hashing structs using the raw bytes that made them up. I had initialization functions which set all of the fields in the struct, but since I was just hashing on the raw bytes upto the size of the struct, the default values of the padding bytes actually made a difference and caused otherwise identical structs to hash differently. The big kicker was that this only happened on my university's cluster and not my laptop since the compiler chose different alignment values for the different platforms. I learned a lot from that bug. The real takeaway is not to hash on raw bytes of a struct, but using calloc would have helped just as well. Answer from OrionsChastityBelt_ on reddit.com
TutorialsPoint
tutorialspoint.com › c_standard_library › c_function_calloc.htm
C library - calloc() function
The C stdlib library calloc() function is used to allocates the requested memory and returns a pointer to it. It reserves a block of memory for array of objects and initialize all bytes in the allocated storage to zero.
GeeksforGeeks
geeksforgeeks.org › c language › dynamic-memory-allocation-in-c-using-malloc-calloc-free-and-realloc
Dynamic Memory Allocation in C - GeeksforGeeks
Please refer memory layout of C programs for details · Array size can be increased or decreased as needed. Memory persists even after the function that allocated it finishes, allowing functions to return pointers to it. This is different from stack allocated variables as it is not safe to return address of those variable. The malloc(), calloc(), realloc() and free() functions are the primary tools for dynamic memory management in C, they are part of the C Standard Library and are defined in the <stdlib.h> header file.
Published 2 weeks ago
Real-world use case where calloc() is absolutely necessary over malloc()?
Depending on the fields in your struct and the alignment that you or your compiler choose for the struct, there may be bits that are allocated as part of the struct that don't actually belong to any fields. That is, structs may have bytes that exist just to pad out the struct to fit alignment. In grad school, I ran into a really nasty bug where I was hashing structs using the raw bytes that made them up. I had initialization functions which set all of the fields in the struct, but since I was just hashing on the raw bytes upto the size of the struct, the default values of the padding bytes actually made a difference and caused otherwise identical structs to hash differently. The big kicker was that this only happened on my university's cluster and not my laptop since the compiler chose different alignment values for the different platforms. I learned a lot from that bug. The real takeaway is not to hash on raw bytes of a struct, but using calloc would have helped just as well. More on reddit.com
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October 3, 2025malloc - What does the first "c" stand for in "calloc"? - Stack Overflow
A student asked the question and I didn't know for sure. Guesses include: "counted", "clearing", "chunked", "complete", ... The standard library documentation doesn't say what it stands for and... More on stackoverflow.com
How Do I Know When to Use Malloc or Calloc in C Programming?
calloc() is just a helper function for creating dynamic array. It uses malloc() and initializes the allocated memory contents to zeroes. malloc() simply allocates the memory. More on reddit.com
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July 15, 2017newbie question, malloc vs calloc
calloc adds an extra step in the process, it’s allocation and initialization. Sometimes it’s just unnecessary to initialize a buffer. If I know that in my code after I call malloc I’m going to immediately fill that buffer with something else, there’s no point in ensuring it will be all filled with zero. In practice I’ve only found myself using calloc when initializing structs that contain pointers to ensure they are initialized to NULL when I wrote a library. (yes I know not all architectures use 0 as NULL) and in a project where I was calculating a cipher key of variable size, so it needed to always have the same initial value. More on reddit.com
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May 24, 2021Videos
05:23
Calloc() in C - YouTube
06:48
Calloc in C is easy! 🧹 - YouTube
C Programming - Dynamic Memory Allocation with calloc()
09:05
malloc vs calloc Differences Explained | C Programming Tutorial ...
15:48
Mastering Dynamic Memory Allocation in C || The calloc() & realloc() ...
16:31
[Day 19] - calloc, realloc, and alloca - YouTube
Codecademy
codecademy.com › docs › memory management › calloc()
C | Memory Management | calloc() | Codecademy
August 18, 2022 - Dynamically allocates an array of memory blocks of a specified type.
Programiz
programiz.com › c-programming › c-dynamic-memory-allocation
C Dynamic Memory Allocation Using malloc(), calloc(), free ...
The malloc() function allocates memory and leaves the memory uninitialized, whereas the calloc() function allocates memory and initializes all bits to zero. ... The above statement allocates contiguous space in memory for 25 elements of type float.
Cppreference
cppreference.com › c › memory › calloc
calloc - cppreference.com
September 4, 2023 - #include <stdio.h> #include <stdlib.h> int main(void) { int* p1 = calloc(4, sizeof(int)); // allocate and zero out an array of 4 int int* p2 = calloc(1, sizeof(int[4])); // same, naming the array type directly int* p3 = calloc(4, sizeof *p3); // same, without repeating the type name if (p2) { for (int n = 0; n < 4; ++n) // print the array printf("p2[%d] == %d\n", n, p2[n]); } free(p1); free(p2); free(p3); } Output: p2[0] == 0 p2[1] == 0 p2[2] == 0 p2[3] == 0 · C23 standard (ISO/IEC 9899:2024): 7.22.3.2 The calloc function (p: TBD) C17 standard (ISO/IEC 9899:2018): 7.22.3.2 The calloc function (p: 253) C11 standard (ISO/IEC 9899:2011): 7.22.3.2 The calloc function (p: 348) C99 standard (ISO/IEC 9899:1999): 7.20.3.1 The calloc function (p: 313) C89/C90 standard (ISO/IEC 9899:1990): 4.10.3.1 The calloc function ·
Sololearn
sololearn.com › en › Discuss › 1687173 › what-does-the-c-stand-for-in-calloc-the-c-language-memory-allocation-function
What does the c stand for in calloc, the C language memory allocation function? | Sololearn: Learn to code for FREE!
February 13, 2019 - When SL was working on the malloc/calloc lessons, we once thought that it stood for 'contiguous'. Although it could not be confirmed nor invalidated, both malloc and calloc allocates contiguous memory, so 'contiguous' really isn't that good of a contender for the spot.
Quora
quora.com › What-is-the-difference-between-calloc-and-malloc-in-C-What-is-the-difference-between-calloc-and-realloc-Which-one-should-be-preferred-for-memory-allocation-purposes-Why-so
What is the difference between 'calloc' and 'malloc' in C? What is the difference between 'calloc' and 'realloc'? Which one should be preferred for memory allocation purposes? Why so? - Quora
Answer (1 of 2): The malloc function returns a block of memory from the heap, without initializing the contents of the memory, and returns a pointer to the beginning of that block. If there is not enough memory available to satisfy the request, NULL is returned. The calloc function returns a blo...
W3Schools
w3schools.com › c › ref_stdlib_calloc.php
C stdlib calloc() Function
C Examples C Real-Life Examples C Exercises C Quiz C Code Challenges C Practice Problems C Compiler C Syllabus C Study Plan C Interview Q&A C Certificate ... // Allocate memory for a number of items int numItems = 15; int *myArray = calloc(numItems, sizeof(int)); // Write into the memory for(int i = 0; i < numItems; i++) { myArray[i] = i + 1; } // Display the contents of the memory for(int i = 0; i < numItems; i++) { printf("%d ", myArray[i]); } // Free the memory free(myArray); myArray = NULL; Try it Yourself »
Reddit
reddit.com › r/cprogramming › real-world use case where calloc() is absolutely necessary over malloc()?
r/cprogramming on Reddit: Real-world use case where calloc() is absolutely necessary over malloc()?
October 3, 2025 -
As a CS student, I'm trying to understand the practical trade-offs between calloc() and malloc(). I know calloc() zeroes the memory. But are there specific, real-world C applications where relying on mallOC() + manual zeroing would lead to subtle bugs or be technically incorrect? Trying to move past the textbook difference.
Top answer 1 of 24
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Depending on the fields in your struct and the alignment that you or your compiler choose for the struct, there may be bits that are allocated as part of the struct that don't actually belong to any fields. That is, structs may have bytes that exist just to pad out the struct to fit alignment. In grad school, I ran into a really nasty bug where I was hashing structs using the raw bytes that made them up. I had initialization functions which set all of the fields in the struct, but since I was just hashing on the raw bytes upto the size of the struct, the default values of the padding bytes actually made a difference and caused otherwise identical structs to hash differently. The big kicker was that this only happened on my university's cluster and not my laptop since the compiler chose different alignment values for the different platforms. I learned a lot from that bug. The real takeaway is not to hash on raw bytes of a struct, but using calloc would have helped just as well.
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i believe if the memory allocated is in a fresh page the os doesn't need to clear the memory so sometimes it can be a little faster
Top answer 1 of 6
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According to an excerpt from the book Linux System Programming (by Robert Love), no official sources exist on the etymology of calloc.
Some plausible candidates seem to be:
- Count or counted, because
calloctakes a separate count argument. Clear, because it ensures that the returned memory chunk has been cleared.
- Brian Kernighan is reported to believe that the "c" stands for clear (although he has admitted he's not sure).
- (See comments.) An early
calloc.cseems to contain an explicit reference to the word clear in a source code comment (but no reference to the word count or to any other candidate). In another source code comment in the filemalloc.c, the word clear appears again, in reference to the wordcalloc.
C, as in the C language.
- (See alk's answer and comments.) Possibly a naming convention for a set of functions that were introduced at about the same time.
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I did some research and found the following in "UNIX@ TIME-SHARING SYSTEM: UNIX PROGRAMMER'S MANUAL. Seventh Edition, Volume 2", chapter "PROGRAMMING" (Italics by me):
char *malloc(num);allocates
numbytes. The pointer returned is sufficiently well aligned to be usable for any purpose.NULLis returned if no space is available.char *calloc(num, size);allocates space for
numitems each ofsizesize. The space is guaranteed to be set to 0 and the pointer is sufficiently well aligned to be usable for any purpose.NULLis returned if no space is available.cfree(ptr) char *ptr;Space is returned to the pool used by
calloc. Disorder can be expected if the pointer was not obtained fromcalloc.
The last sentence is a clear evidence that
calloc()was definitely (meant to be?) more different frommalloc()then just by clearing out the memory.Interesting enough there is no reference to
free()on any of those some hundred pages ... :-)Moreover UNIX V6 already had
calloc()which callsalloc(). The (linked) source does not show any approach to zero out any memory.
Concluding from the both facts above I strongly object the theory that the leading "c" in calloc() stands for "clear".
O'Reilly
oreilly.com › library › view › c-in-a › 0596006977 › re22.html
calloc - C in a Nutshell [Book]
December 16, 2005 - If successful, calloc() returns a void pointer to the beginning of the memory block obtained. void pointers are converted automatically to another pointer on assignment, so that you do not need to use an explicit cast, although you may want do so for the sake of clarity. If no memory block of the requested size is available, the function returns a null pointer. Unlike malloc(), calloc() initializes every byte of the block allocated with the value 0.
Authors Peter PrinzTony Crawford
Published 2005
Pages 618
Wikipedia
en.wikipedia.org › wiki › C_dynamic_memory_allocation
C dynamic memory allocation - Wikipedia
February 15, 2026 - The C dynamic memory allocation functions are defined in <stdlib.h> header (<cstdlib> header in C++). malloc() takes a single argument (the amount of memory to allocate in bytes), while calloc() takes two arguments — the number of elements and the size of each element.
Reddit
reddit.com › r/learnprogramming › how do i know when to use malloc or calloc in c programming?
r/learnprogramming on Reddit: How Do I Know When to Use Malloc or Calloc in C Programming?
July 15, 2017 -
I am a beginner in programming in general and I am taking a C programming class required for my major currently in the summer. Since it is in the summer, the class is sped up. We have a final exam coming up and we have to write programs out by hand. We will be given tasks like "create a program that does this...".
I know how to use malloc and calloc. I read and watched all the Youtube videos on it but something is still not clicking for me. I know what it does, it reallocates memory or it sets memory aside for me to use. However, I don't know WHEN to use it and that will be a problem if I am taking the final exam.
So if I am given problems on the exam, how do I identify when to use malloc or calloc? When do I use malloc or calloc? Does it matter which one I choose over the other? I know the subtle difference between them, but I just don't know when to use them....
The Open Group
pubs.opengroup.org › onlinepubs › 9699919799 › functions › calloc.html
calloc
Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard. The calloc() function shall allocate unused space for an array of nelem elements each of whose size in bytes is elsize.
Microsoft Learn
learn.microsoft.com › en-us › cpp › c-runtime-library › reference › calloc
calloc | Microsoft Learn
December 5, 2023 - The C runtime library function calloc allocates zero-initialized memory.
ScholarHat
scholarhat.com › home
How to Dynamically Allocate Memory using calloc() in C?
August 2, 2025 - calloc() function is a powerful function in C that is used for dynamic memory allocation in C programs. It is specifically used for arrays. In this C Tutorial, we'll try to understand What is calloc() function in C language?, Syntax of calloc() ...
Linux Man Pages
linux.die.net › man › 3 › calloc
calloc(3): allocate/free dynamic memory - Linux man page
The calloc() function allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory.
W3Schools
w3schools.com › c › c_memory_allocate.php
C Allocate Memory
The malloc() and calloc() functions allocate some memory and return a pointer to its address. int *ptr1 = malloc(size); int *ptr2 = calloc(amount, size);
Educative
educative.io › answers › what-is-calloc-in-c
What is Calloc in C?
The calloc() function in C is used to allocate a specified amount of memory and then initialize it to zero. The function returns a void pointer to this memory location, which can then be cast to the desired type.
TechOnTheNet
techonthenet.com › c_language › standard_library_functions › stdlib_h › calloc.php
C Language: calloc function (Allocate and Clear Memory Block)
In the C Programming Language, the calloc function allocates a block of memory for an array.