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std::malloc(3) C++ Standard Libary std::malloc(3)


std::malloc - std::malloc


Defined in header <cstdlib>
void* malloc( std::size_t size );

Allocates size bytes of uninitialized storage.

If allocation succeeds, returns a pointer to the lowest (first) byte in the
allocated memory block that is suitably aligned for any scalar type (at least as
strictly as std::max_align_t).

If size is zero, the behavior is implementation defined (null pointer may be
returned, or some non-null pointer may be returned that may not be used to access
storage, but has to be passed to std::free)

The following functions are required to be thread-safe:

* The library versions of operator new and operator delete
* User replacement versions of global operator new and operator
* std::calloc, std::malloc, std::realloc, (since C++11)
(since C++17), std::free

Calls to these functions that allocate or deallocate a particular unit
of storage occur in a single total order, and each such deallocation
call happens-before the next allocation (if any) in this order.


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 std::free() or

On failure, returns a null pointer.


This function does not call constructors or initialize memory in any way. There are
no ready-to-use smart pointers that could guarantee that the matching deallocation
function is called. The preferred method of memory allocation in C++ is using
RAII-ready functions std::make_unique, std::make_shared, container constructors,
etc, and, in low-level library code, new-expression.

For loading a large file, file mapping via OS-specific functions, e.g. mmap on POSIX
or CreateFileMapping(A/W) along with MapViewOfFile on Windows, is preferable to
allocating a buffer for file reading.


// Run this code

#include <iostream>
#include <cstdlib>
#include <string>
#include <memory>

int main()
constexpr std::size_t size = 4;
if (auto ptr = reinterpret_cast<std::string*>(
std::malloc(size * sizeof(std::string))))
{ try
{ for (std::size_t i = 0; i < size; ++i)
std::construct_at(ptr + i, 5, 'a' + i);
for (std::size_t i = 0; i < size; ++i)
std::cout << "ptr[" << i << "] == " << ptr[i] << '\n';
std::destroy_n(ptr, size);
catch(...) {}


p[0] == aaaaa
p[1] == bbbbb
p[2] == ccccc
p[3] == ddddd

See also

operator new allocation functions
operator new[] (function)
get_temporary_buffer obtains uninitialized storage
(deprecated in C++17) (function template)
(removed in C++20)