Memory API in UNIX/C

 

Memory API in UNIX/C 

The memory API in UNIX/C provides functions that allow programs to allocate, use, resize, and free memory during execution. Proper memory management is essential for building reliable and bug-free software.

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1. Types of Memory in C Programs

A. Stack Memory (Automatic Memory)

• Managed automatically by the compiler.

• Used for:

  • Local variables

  • Function parameters

  • Return values

• Allocated when a function is called and freed when it returns.

• Short-lived: data does not persist after the function exits.

Example

void func() {
    int x;   // stored on stack
}

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B. Heap Memory (Dynamic Memory)

• Managed explicitly by the programmer.

• Used when memory must persist beyond function calls.

• Requires manual allocation and deallocation.

Example

int *x = (int *) malloc(sizeof(int));

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2. Core Memory Allocation Functions

malloc(size_t size)

• Allocates size bytes on the heap.

• Returns:

  • Pointer to allocated memory on success

  • NULL on failure

• Include:

#include <stdlib.h>

Best practice

double *d = (double *) malloc(sizeof(double));

• sizeof() ensures correct allocation size.

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*free(void ptr)

• Releases heap memory previously allocated with malloc().

• Prevents memory leaks.

Example

int *x = malloc(10 * sizeof(int));
free(x);

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3. Other Useful Memory Functions

calloc()

• Allocates memory and initializes it to zero.

• Helps avoid uninitialized data bugs.

realloc()

• Resizes previously allocated memory.

• May move data to a new location and copy old contents.

mmap()

• Requests memory directly from the OS.

• Can create anonymous memory regions, uses swap space (not tied to files).

• Used internally by memory allocators or advanced programs.

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4. OS Support Behind the Scenes

• malloc() and free() are library calls, not system calls.

• Internally they may use:

  • brk/sbrk → change heap size

  • mmap() → obtain additional memory from OS

Programmers should not directly call brk/sbrk.

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5. Common Memory Programming Errors

1. Forgetting to Allocate Memory

char *dst;
strcpy(dst, "hello");   // crash (segmentation fault)

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2. Not Allocating Enough Memory (Buffer Overflow)

malloc(strlen(src));   // missing +1 for '\0'

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3. Using Uninitialized Memory

• Allocated memory may contain random data.

• Must initialize before use.

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4. Memory Leak

• Forgetting to call free().

• Causes memory usage to grow over time.

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5. Dangling Pointer

• Using memory after it has been freed.

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6. Double Free

• Freeing the same memory twice.

• Leads to undefined behavior or crashes.

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7. Invalid Free

• Passing a pointer not returned by malloc() to free().

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6. Debugging Tools

Tools exist to detect memory issues:

• Valgrind

• Purify

They help identify leaks, invalid accesses, and misuse.

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7. Key Takeaway

• Stack → automatic, short-lived, compiler managed

• Heap → manual, flexible, programmer managed

• malloc/free → primary memory API functions

• Careless usage leads to crashes, leaks, or security bugs.