malloc

From cppreference.com
C
Dynamic memory management
Defined in header <stdlib.h>
void * malloc ( size_t size ) ;

Allocates size bytes of uninitialized storage.

If allocation succeeds, returns a pointer that is suitably aligned for any object type with fundamental alignment .

If size is zero, the behavior of malloc is implementation-defined. For example, a null pointer may be returned. Alternatively, a non-null pointer may be returned; but such a pointer should not be dereferenced , and should be passed to free to avoid memory leaks.

malloc is thread-safe: it behaves as though only accessing the memory locations visible through its argument, and not any static storage.

A previous call to free , free_sized , and free_aligned_sized (since C23) or realloc that deallocates a region of memory synchronizes-with a call to malloc that allocates the same or a part of the same region of memory. This synchronization occurs after any access to the memory by the deallocating function and before any access to the memory by malloc . There is a single total order of all allocation and deallocation functions operating on each particular region of memory.

(since C11)

Parameters

size - number of bytes to allocate

Return value

On success, returns the pointer to the beginning of newly allocated memory. To avoid a memory leak, the returned pointer must be deallocated with free() or realloc() .

On failure, returns a null pointer.

Example

Run this code 
#include <stdio.h>   
#include <stdlib.h> 
 
int main(void) 
{
    int *p1 = malloc(4*sizeof(int));  // allocates enough for an array of 4 int
    int *p2 = malloc(sizeof(int[4])); // same, naming the type directly
    int *p3 = malloc(4*sizeof *p3);   // same, without repeating the type name
 
    if(p1) {
        for(int n=0; n<4; ++n) // populate the array
            p1[n] = n*n;
        for(int n=0; n<4; ++n) // print it back out
            printf("p1[%d] == %d\n", n, p1[n]);
    }
 
    free(p1);
    free(p2);
    free(p3);
}

Output: 

p1[0] == 0
p1[1] == 1
p1[2] == 4
p1[3] == 9

References

  • C17 standard (ISO/IEC 9899:2018):
  • 7.22.3.4 The malloc function (p: 254)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.22.3.4 The malloc function (p: 349)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.20.3.3 The malloc function (p: 314)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.10.3.3 The malloc function

See also

deallocates previously allocated memory
(function)
C++ documentation for malloc
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