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Working With Mutable Binary Data

This article contains code examples of common tasks that apply specifically to mutable data objects, CFMutableData objects.

Modifying Bytes

The three basic CFMutableData functions are CFDataGetMutableBytePtr, CFDataGetLength and CFDataSetLength. The CFDataGetMutableBytePtr function returns a pointer for writing into the bytes contained in the mutable data object. The CFDataGetLength function returns the number of bytes contained in the data object. The CFDataSetLength function allows you to truncate or extend the length of a mutable data object. The CFDataIncreaseLength function also allows you to change the length of a mutable data object.

In Listing 1, CFDataGetMutableBytePtr is used to return a pointer to the bytes in data2. The bytes in data2 are then overwritten with the contents of data1 using the CFDataGetBytes function.

Listing 1 Modifying bytes

 // Create some strings.

const UInt8 *myString = "string for data1";

const UInt8 *yourString = "string for data2";

// Declare buffers used later.

const UInt8 *data1Bytes, *data2Bytes;

// Create mutable data objects, data1 and data2.

CFMutableDataRef data1 = CFDataCreateMutable(NULL, 0);

CFMutableDataRef data2 = CFDataCreateMutable(NULL, 0);

CFDataAppendBytes(data1, myString, strlen(myString));

CFDataAppendBytes(data2, yourString, strlen(yourString));

// Get and print the data1 bytes.

data1Bytes = CFDataGetBytePtr(data1);

fprintf(stdout, "data1 before: \"%s\"\n", data1Bytes);

// Get and print the data2 bytes.

data2Bytes = CFDataGetMutableBytePtr(data2);

fprintf(stdout, "data2 before: \"%s\"\n", data2Bytes);

// Copy the bytes from data1 into the mutable bytes from data2.

CFDataGetBytes(data1,

               CFRangeMake(0, CFDataGetLength(data1)),

               data2Bytes);

// Get and print the data2 bytes again.

fprintf(stdout, "data2 after: \"%s\"\n", CFDataGetBytePtr(data2));

This is the output from Listing 1:

data1 before: "string for data1"

data2 before: "string for data2"

data2 after: "string for data1"

Appending Bytes

The CFDataAppendBytes function lets you append bytes of the specified length to a mutable data object. For example, Listing 2 copies the bytes in data2 into aBuffer and then appends aBuffer to data1:

Listing 2 Appending bytes

 // Create mutable data objects, data1 and data2.

CFMutableDataRef data1 = CFDataCreateMutable(NULL, 0);

CFDataAppendBytes(data1, "ABCD", strlen("ABCD"));

CFMutableDataRef data2 = CFDataCreateMutable(NULL, 0);

CFDataAppendBytes(data2, "EFGH", strlen("EFGH"));

// Get the data2 bytes.

const UInt8 *aBuffer = CFDataGetBytePtr(data2);

// Append the bytes from data2 to data1.

CFDataAppendBytes(data1, aBuffer, CFDataGetLength(data2));

Replacing Bytes

You can delete a range of bytes in a mutable data object (using the CFDataDeleteBytes function) or replace a range of bytes with other bytes (using the CFDataReplaceBytes function).

In Listing 3, a range of bytes in data1 is replaced by the bytes in data2 using CFDataReplaceBytes (the content of data1 changes from “Liz and John” to “Liz and Larry”):

Listing 3 Replacing bytes

// Create mutable data objects, data1 and data2.

CFMutableDataRef data1 = CFDataCreateMutable(NULL, 0);

CFDataAppendBytes(data1, "Liz and John", strlen("Liz and John"));

CFMutableDataRef data2 = CFDataCreateMutable(NULL, 0);

CFDataAppendBytes(data2, "Larry", strlen("Larry"));

// Allocate a buffer the length of data2.

unsigned len = CFDataGetLength(data2);

unsigned char *aBuffer = malloc(len);

// Put the data2 bytes into the buffer.

CFDataGetBytes(data2, CFRangeMake(0, len), aBuffer);

// Replace John with Larry.

CFDataReplaceBytes(data1, CFRangeMake(8, CFDataGetLength(data1) - 8), aBuffer, len);

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