Dynamic Memory Allocation

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Dynamic Memory Allocation

• Problem Don't know how much memory to allocate
  to an array
• Why?
• Memory is allocated to the correct size for a
  record to be written to disk
• Size depends upon a particular computer
  configuration
• How? We use the C malloc and free functions

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malloc() and free()

• void malloc(size_t size)
• void is a generic pointer for any type of data
• size_t is a typedef for unsigned integers
• allocates size in bytes
• returns pointer to new memory or NULL
• void free(void data)
• releases memory pointed to by data
• If data is NULL, no harm done
• C has no garbage collection, skipping free
  results in memory leaks
• Prevent dangling references set a pointer to
  NULL after free
• Freeing from a function (pass a pointer to a
  pointer)
• Function freeMyStructure( void data)
  free(data) data NULL)

Dangling reference a pointer to dynamic memory
that was released Alias Two pointers to the same
dynamically allocated memory
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Rules

• Never attempt to free memory twice
• Never use a pointer after its memory was freed
  (dangling pointer)
• Never attempt to free memory that wasn't
  allocated dynamically.
• Set all pointers to null initially, and after
  freeing them

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Example

• Static allocation int nums100
• Dynamic allocationint nums, nprintf("How
  Many?\n")scanf("d", n)nums malloc(n
  sizeof(int))free(nums)nums NULL

Note sizeof is an operator that finds the byte
length of a data type
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Standard Programming Environment

• Editors vi, emacs, pico
• Compiler gcc or cc
• Linker gcc or cc
• Automated system builder make, ant
• Profiler gprof (find inefficient functions)
• Source control system SCCS, CVS
• tracks code changes
• coordinates changes among programmers
• automatically assign release numbers

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Hints for the last lab

• The file syntax is modelyearrate\n
• A sscanf s will read the entire string up to a
  space. If none, it reads everything
• Find the pointer to the colon, and store a NULL
  ('\0')
• How to find the colon? Either a character by
  character search, or by using the C strstr method
• You can then use sscanf on the back portion

'\0'