Mmap vs Malloc: Difference and Comparison

In C programming, there is a dynamic memory that points to the allocation of the memory in the C standard library via a set of functions. Malloc is one of these, which is used for allocating memory.

Then there is mmap. That is used as a memory-mapped system that has its unique input and output. These two provide the same functions but more digging into this topic reveals some differences.

Key Takeaways

1. mmap allocates memory by mapping files or devices into the address space, while malloc allocates memory from the heap.
2. mmap provides shared memory capabilities for interprocess communication, whereas malloc does not.
3. Memory allocated by mmap is automatically initialized to zero, while malloc may return uninitialized memory.

Mmap vs Malloc

Mmap is a system call that maps a file or device into memory. It returns a pointer to the allocated memory that can be used for various purposes, like storing data. Malloc is a library function that rations a block of memory of a specified size and returns a pointer to the first byte of the block.

Mmap in computing is a POSIX multi-tasking system call that records all the data, files, and devices in its memory. This is also known as the method that falls under memory-mapped files and implements the copying of data into its physical memory, as the content can’t be directly read from the drive.

Malloc is the short form for the term memory allocation that is used to allocate a huge block of data dynamically, regarding the specified size, and when it succeeds, it returns a pointer that points to the first byte of the memory allocated, otherwise returns NULL.

In the actual memory allocation mechanism, there are many different accomplishments available that are used by Malloc.

Comparison Table

Parameters of Comparison	Mmap	Malloc
Definition	Mmap is referred to a system call that asks the kernel to find a large address for mapping of pages.	Malloc is the main memory allocation interface that gathers all the facilities available in the system.
Function	Mmap uses the context switch and makes it into a kernelland.	Malloc allocates the block of memory in apile.
Also called as	Mmap is also known as a system call.	Malloc is also known as the main memory allocation interface.
Results	Mmap does not result in affecting other system performance.	Malloc disorganizes and creates a poor performance of the system.
Best suited for	Mmap is best suited to speed up the process of responses given by applications.	Malloc is best suited for allocating memory in any application in the system.

What is Mmap?

Mmap is referred to as a Unix system call of POSIX compliant that maps the data, files, or devices into the memory. It is an input, output method of memory mapping.

A new mapping is created in the virtual address space in the calling process. In addr the initial address of the new mapping is found, and the length of the mapping is specified by the length argument (which should be more than 0).

If the addr is found to be NULL, then the address is chosen by the kernel, which makes the mapping, which is considered to be the easiest method of making new mapping.

If the address is not Found as NULL, then the kernel considers it an indication for placing the mapping on a place on Linux.

The kernel finds the nearest page boundary and tries to make a mapping in that area. If the kernel finds a mapping already exists in the area, it selects a new address that might depend on the indication that the kernel assumed.

As a result of this call, the address of the new mapping is sent back. Right after the call is returned, the file descriptor, fd, closes the mapping immediately without invalidation.

What is Malloc?

The malloc () is a memory allocation function that allocates the size bytes and sends back a pointer to the memory that is allocated. The memory is not set to its initial position.

If the size byte name is 0, then either NULL or a unique value pointer is sent by malloc (), which can, later on, be passed to free () successfully.

The memory space that is been pointed by ptr, is released by the free () function, which was supposed to be sent back by malloc (). Else, undefined behaviour is said to occur if the free (ptr) has been called previously.

No operation is done or performed if the ptr is Found NULL. If the ptr is found to be NULL, then the value of a call is equal to malloc (size) for every value of size; if it is equivalent to 0, and the ptr is not found to be NULL, then the call value is equal to free (ptr).

Only when the ptr is NULL it should have been sent back by a previous call to malloc(). And the area that was pointed was changed, a free (ptr) is done.

By default, an optimistic memory allocation strategy is followed by Linux, which means that if malloc () sends a non-NULL back, there are chances that the memory might be lost.

And if it turns out that the system has a shortage of memory, few processes will be destroyed by the OOM killer.

Main Differences Between Mmap and Malloc

1. Mmap is known as a system call, whereas Malloc is a main memory allocation interface.
2. Mmap helps in the mapping of pages while Malloc allocates devices and data in a system.
3. Mmap converts the context switch into kernel land, on the other hand, Malloc allocates memory in a device.
4. Mmap is preferred for speeding up the process of responses, while Malloc is preferred for allocating memory in a system.
5. Mmap after performing does not affect the performance of any other systems application, whereas, Malloc disturbs the performance to prevent it from people who can disorganise the system.

References

1. https://www.usenix.org/publications/library/proceedings/als01/full_papers/ezolt/ezolt_html/
2. https://www.usenix.org/publications/library/proceedings/als01/full_papers/ezolt/ezolt.ps

Last Updated : 11 June, 2023