`malloc`

Article
02/07/2023

Allocates memory blocks.

Syntax

void *malloc(
   size_t size
);

Parameters

size
Bytes to allocate.

Return value

malloc returns a void pointer to the allocated space, or NULL if there's insufficient memory available. To return a pointer to a type other than void, use a type cast on the return value. The storage space pointed to by the return value is suitably aligned for storage of any type of object that has an alignment requirement less than or equal to that of the fundamental alignment. (In Visual C++, the fundamental alignment is the alignment that's required for a double, or 8 bytes. In code that targets 64-bit platforms, it's 16 bytes.) Use _aligned_malloc to allocate storage for objects that have a larger alignment requirement—for example, the SSE types __m128 and __m256, and types that are declared by using __declspec(align( n )) where n is greater than 8. If size is 0, malloc allocates a zero-length item in the heap and returns a valid pointer to that item. Always check the return from malloc, even if the amount of memory requested is small.

Remarks

The malloc function allocates a memory block of at least size bytes. The block may be larger than size bytes because of the space that's required for alignment and maintenance information.

malloc sets errno to ENOMEM if a memory allocation fails or if the amount of memory requested exceeds _HEAP_MAXREQ. For information about this and other error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

The startup code uses malloc to allocate storage for the _environ, envp, and argv variables. The following functions and their wide-character counterparts also call malloc.

The C++ _set_new_mode function sets the new handler mode for malloc. The new handler mode indicates whether, on failure, malloc is to call the new handler routine as set by _set_new_handler. By default, malloc doesn't call the new handler routine on failure to allocate memory. You can override this default behavior so that, when malloc fails to allocate memory, malloc calls the new handler routine in the same way that the new operator does when it fails for the same reason. To override the default, call _set_new_mode(1) early in your program, or link with NEWMODE.OBJ (see Link options).

When the application is linked with a debug version of the C run-time libraries, malloc resolves to _malloc_dbg. For more information about how the heap is managed during the debugging process, see CRT debug heap details.

malloc is marked __declspec(noalias) and __declspec(restrict). These attributes mean that the function is guaranteed not to modify global variables, and that the pointer returned isn't aliased. For more information, see noalias and restrict.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Routine	Required header
`malloc`	`<stdlib.h>` and `<malloc.h>`

For more compatibility information, see Compatibility.

Libraries

All versions of the C run-time libraries.

Example

// crt_malloc.c
// This program allocates memory with
// malloc, then frees the memory with free.

#include <stdlib.h>   // For _MAX_PATH definition
#include <stdio.h>
#include <malloc.h>

int main( void )
{
   char *string;

   // Allocate space for a path name
   string = malloc( _MAX_PATH );

   // In a C++ file, explicitly cast malloc's return.  For example,
   // string = (char *)malloc( _MAX_PATH );

   if( string == NULL )
      printf( "Insufficient memory available\n" );
   else
   {
      printf( "Memory space allocated for path name\n" );
      free( string );
      printf( "Memory freed\n" );
   }
}

Memory space allocated for path name
Memory freed