What is malloc

Static-duration variables are allocated in main memory, usually along with the executable code of the program, and persist for the lifetime of the program; automatic-duration variables are allocated on the stack and come and go as functions are called and return. For static-duration and automatic-duration variables, the size of the allocation must be compile-time constant except for the case of variable-length automatic arrays[5]. If the required size is not known until run-time for example, if data of arbitrary size is being read from the user or from a disk file , then using fixed-size data objects is inadequate.

On the other hand, variables with memory allocated using malloc remain till the time they are manually freed up. The compiler also tries to warn you about this by giving the warning:. In case memory cannot be allocated: the normal way might cause your program to terminate while malloc will return a NULL which can easily be caught and handled within your program.

If you use malloc instead, you can change the contents later on. For more information check this answer. For more details related to variable-sized arrays, have a look at this.

If you been through other programming languages, you might have used the new keyword. Malloc does exactly the same thing in C. It takes a parameter, what size of memory needs to be allocated and it returns a pointer variable that points to the first memory block of the entire memory block, that you have created in the memory.

Example -. You use malloc when you need to allocate objects that must exist beyond the lifetime of execution of the current block where a copy-on-return would be expensive as well , or if you need to allocate memory greater than the size of that stack.

In this example it seems quite useless indeed. But now imagine that you are using sockets or file IO and must read packets from variable length which you can only determent while running. Or when using sockets and each client connection need some storage on the server. You could make an static array but this gives you a client limit which will be determent while compiling.

Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. When and why to use malloc? Ask Question. Asked 9 years, 10 months ago. Active 10 months ago.

Viewed 81k times. Well, I can't understand when and why it is needed to allocate memory using malloc. Improve this question. Read a good C programming book. It will explain heap allocated memory much better and quicker than I can explain in a few minutes.

You must learn about the differences between heap and stack memory, take a look at this question: stackoverflow. Add a comment. Active Oldest Votes. In the world of programming where every space counts, there are numerous times when we only want an array to have a specific amount of space at run time.

That is, we want to create an array occupying a particular amount of space, dynamically. We do this using malloc. We know what malloc returns and we know what it requires as an input, but how does the syntax of the function work.

The illustration below shows that:. Note: malloc will return NULL if the memory specified is not available and hence, the allocation has failed. View all Courses. All rights reserved. Courses Pricing. How to use "malloc" in C. Find roots of a quadratic equation. Print Pyramids and Patterns. Check prime number. Print the Fibonacci series. Reference Materials string. Start Learning C. Explore C Examples. Store Data in Structures Dynamically.

Find the Size of int, float, double and char. C Dynamic Memory Allocation In this tutorial, you'll learn to dynamically allocate memory in your C program using standard library functions: malloc , calloc , free and realloc. C malloc The name "malloc" stands for memory allocation. The expression results in a NULL pointer if the memory cannot be allocated. C calloc. Table of Contents Why dynamic memory allocation? Previous Tutorial:.

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