v0.40: Create exercises. Challenge the community.

calloc

Your mission

Implement gm_calloc, your own calloc, which allocates memory for an array of nmemb elements of size bytes each, sets every byte to zero, and returns the pointer (or NULL on failure).

See it in action

gm_calloc(4, sizeof(int))  // read back the 4 ints

Expected result

0 0 0 0

Your checklist

For your own tracking. Tests grade your code.

Compute the total number of bytes as nmemb * size, then hand that to malloc.
Before multiplying, guard against overflow: when both nmemb and size are non-zero, the product overflows if nmemb > SIZE_MAX / size (SIZE_MAX comes from <stdint.h>). In that case return NULL instead of wrapping around to a tiny allocation, that wrap is a real-world security bug.
After malloc succeeds, write a zero into every one of the allocated bytes. The caller is promised clean memory, so never leave it uninitialized.
Return NULL if malloc fails.
Return the pointer to the zeroed block on success.

More examples

gm_calloc(SIZE_MAX, SIZE_MAX)  // would overflow, print "null" on NULL

Expected result

null

Constraints

Expected files

calloc.c

Allowed functions

malloc

Allowed headers

<stdlib.h>declares malloc

<stddef.h>defines size_t

<stdint.h>defines SIZE_MAX

libc

Standard C library

Implements the contract of a function from the standard C library.

calloc

Dynamic memory

Your mission

Implement gm_calloc, your own calloc, which allocates memory for an array of nmemb elements of size bytes each, sets every byte to zero, and returns the pointer (or NULL on failure).

See it in action

gm_calloc(4, sizeof(int))  // read back the 4 ints

Expected result

0 0 0 0

Your checklist

For your own tracking. Tests grade your code.

Compute the total number of bytes as nmemb * size, then hand that to malloc.
Before multiplying, guard against overflow: when both nmemb and size are non-zero, the product overflows if nmemb > SIZE_MAX / size (SIZE_MAX comes from <stdint.h>). In that case return NULL instead of wrapping around to a tiny allocation, that wrap is a real-world security bug.
After malloc succeeds, write a zero into every one of the allocated bytes. The caller is promised clean memory, so never leave it uninitialized.
Return NULL if malloc fails.
Return the pointer to the zeroed block on success.

More examples

gm_calloc(SIZE_MAX, SIZE_MAX)  // would overflow, print "null" on NULL

Expected result

null

Constraints

Expected files

calloc.c

Allowed functions

malloc

Allowed headers

<stdlib.h>declares malloc

<stddef.h>defines size_t

<stdint.h>defines SIZE_MAX

libc

Standard C library

Implements the contract of a function from the standard C library.

Tests

Run the tests to grade your code

Build your code, then check each test here. Open a result to inspect its output.

Build

01Four ints all initialized to zeroWaiting

02All eight bytes are NULWaiting

03Multiplication overflow returns NULLWaiting

04Overflow that wraps to a tiny size still returns NULLWaiting

05Allocated block is usable and NUL-terminatedWaiting

06Every byte except the marker is zeroWaiting

07First and last byte of a larger block are zeroWaiting

08Recycled memory still reads back zeroedWaiting

calloc

Your mission

Implement gm_calloc, your own calloc, which allocates memory for an array of nmemb elements of size bytes each, sets every byte to zero, and returns the pointer (or NULL on failure).

See it in action

gm_calloc(4, sizeof(int))  // read back the 4 ints

Expected result

0 0 0 0

Your checklist

For your own tracking. Tests grade your code.

Compute the total number of bytes as nmemb * size, then hand that to malloc.
Before multiplying, guard against overflow: when both nmemb and size are non-zero, the product overflows if nmemb > SIZE_MAX / size (SIZE_MAX comes from <stdint.h>). In that case return NULL instead of wrapping around to a tiny allocation, that wrap is a real-world security bug.
After malloc succeeds, write a zero into every one of the allocated bytes. The caller is promised clean memory, so never leave it uninitialized.
Return NULL if malloc fails.
Return the pointer to the zeroed block on success.

More examples

gm_calloc(SIZE_MAX, SIZE_MAX)  // would overflow, print "null" on NULL

Expected result

null

Constraints

Expected files

calloc.c

Allowed functions

malloc

Allowed headers

<stdlib.h>declares malloc

<stddef.h>defines size_t

<stdint.h>defines SIZE_MAX

libc

Standard C library

Implements the contract of a function from the standard C library.

v0.40: Create exercises. Challenge the community.

Meet Studio →

calloc

Dynamic memory

Your mission

Implement gm_calloc, your own calloc, which allocates memory for an array of nmemb elements of size bytes each, sets every byte to zero, and returns the pointer (or NULL on failure).

See it in action

gm_calloc(4, sizeof(int))  // read back the 4 ints

Expected result

0 0 0 0

Your checklist

For your own tracking. Tests grade your code.

Compute the total number of bytes as nmemb * size, then hand that to malloc.
Before multiplying, guard against overflow: when both nmemb and size are non-zero, the product overflows if nmemb > SIZE_MAX / size (SIZE_MAX comes from <stdint.h>). In that case return NULL instead of wrapping around to a tiny allocation, that wrap is a real-world security bug.
After malloc succeeds, write a zero into every one of the allocated bytes. The caller is promised clean memory, so never leave it uninitialized.
Return NULL if malloc fails.
Return the pointer to the zeroed block on success.

More examples

gm_calloc(SIZE_MAX, SIZE_MAX)  // would overflow, print "null" on NULL

Expected result

null

Constraints

Expected files

calloc.c

Allowed functions

malloc

Allowed headers

<stdlib.h>declares malloc

<stddef.h>defines size_t

<stdint.h>defines SIZE_MAX

libc

Standard C library

Implements the contract of a function from the standard C library.

Tests

Run the tests to grade your code

Build your code, then check each test here. Open a result to inspect its output.

Build

01Four ints all initialized to zeroWaiting

02All eight bytes are NULWaiting

03Multiplication overflow returns NULLWaiting

04Overflow that wraps to a tiny size still returns NULLWaiting

05Allocated block is usable and NUL-terminatedWaiting

06Every byte except the marker is zeroWaiting

07First and last byte of a larger block are zeroWaiting

08Recycled memory still reads back zeroedWaiting