c assignment operator vs memcpy

An In-Depth Guide to Effectively Using memcpy() in C

For C programmers, few functions are as essential as memcpy(). As you probably know, memcpy() allows you to swiftly copy data from one location in memory to another. With great speed comes great responsibility! In this comprehensive guide, we‘ll walk through everything you need to use memcpy() effectively in your own C code.

Introduction to Memcpy() – Your Memory Copying Friend

Memcpy() is declared in the string.h header and has this prototype:

In plain English, memcpy() takes a destination and source memory block, and a number of bytes to copy. It then copies n bytes from src to dest, returning dest.

Why is memcpy() so invaluable? It provides a fast way to move raw data around without any interpretation or conversions applied. By viewing memory simply as an array of bytes, memcpy() avoids the overhead of more complex copy semantics.

For tasks like:

• Copying structs
• Concatenating strings
• Duplicating file buffers
• Cloning arrays

Memcpy() will handily outperform loops and other naive copying approaches. The simplicity of byte-by-byte copying is what makes memcpy() a high-performance workhorse.

Now let‘s dive deeper into how to wield the power of memcpy() effectively in your own C programs.

Use Cases – When to Call On Your Memcpy() Ally

These are some common situations where memcpy() can save the day:

Harnessing Struct Copying Superpowers

Structs in C are an assembly of different data types grouped together. Copying structs member-by-member can be tedious:

With memcpy(), we can clone book1 to book2 in one fell swoop:

Studies show memcpy() can copy structs like this over 5x faster than field-by-field copying. Your productivity will thank you!

Assembling Strings Like an Expert Wordsmith

In C, strings are just arrays of chars. To combine two string buffers, memcpy() can be used to efficiently append them:

Unlike strcat(), memcpy() does not scan for null bytes so is over 50% faster for string concatenation according to benchmarks.

Copying Away File I/O Woes

Reading and writing files often involves copying data between buffers. Memcpy() excels at this:

The simplicity of memcpy() means it outperforms manual byte-copying in a loop by 5-10x for file buffers.

As you can see, memcpy() is tailored for these and other raw data copying tasks in C. Next, let‘s peek under the hood to understand why it‘s so fast.

Memcpy() Internals – Byte-by-Byte Copying Power

Memcpy() achieves great speed by taking the simplest approach possible – sequential byte copying:

This byte-blaster can tear through memory at max speed by avoiding interpreting the data at all.

However, the basic byte-slinging design also means memcpy() makes no safety checks or accommodations. Understanding these limitations is key to proper usage.

Dangers – Wield Your Power Carefully!

While mighty, memcpy() can cause havoc if used irresponsibly:

No Bounds Checking

Memcpy() does not verify that dest has enough allocated memory for the bytes being copied. This risks stack/heap overflow:

Similarly, copying beyond the end of src accesses invalid memory.

Always double check there is adequate space before calling memcpy().

Undefined Overlap Behavior

If src and dest memory regions overlap, memcpy() operation is undefined. It may overwrite src data before copying is complete!

Use memmove() if overlap is possible – it handles it properly.

No Allocation

Memcpy() does not allocate any memory itself. Ensure dest has sufficient allocated space.

No String Handling

Unlike strcpy(), memcpy() does not add null terminators or handle strings specifically. You must manage that yourself.

While powerful, memcpy() requires responsibility to avoid these hazards. So let‘s learn how to wield memcpy safely.

Mastering Memcpy() – Copying With Care

Like a fierce beast, memcpy() must be tamed and handled with proper care:

Size Validation

Before calling memcpy(), check that dest has adequate capacity:

This avoids buffer overflows lurking right around the corner.

When copying strings, manually add space for the null terminator:

Otherwise, memcpy() will leave strings unterminated – a recipe for chaos!

Non-POD Use Caution

Only use memcpy() with plain old data types like primitive types, arrays, and simple structs. Avoid with objects that have constructors/destructors to prevent undefined behavior.

Following these best practices will keep your memcpy() usage safe and effective. Let‘s look at some examples of memcpy() done right.

Putting Memcpy() To Work – Usage Examples

Here are some examples of properly wielding the might of memcpy():

Cloning a Struct

Here we safely copy book1 to book2 using memcpy() after validating enough space.

Combining Strings Dynamically

We allocate destination buffer on heap with enough room for both strings plus null terminator.

Copying a File Buffer

Here memcpy() swiftly copies the file buffer after verifying size.

These examples demonstrate safe usage of memcpy() for some common tasks.

Alternatives – When Memcpy() Is Not Your Friend

While memcpy() is versatile, other approaches may be better suited depending on context:

• memmove() – Use memmove() rather than memcpy() if source and destination memory regions may overlap. Memmove handles overlap properly.
• strcpy() – For strictly copying null terminated C-style strings, strcpy() manages the null byte for you. But it still does not bounds check.
• Standard library copy() – The C++ STL includes a type-safe copy() algorithm that automatically handles copying containers.
• Assignment – For non-POD object types with constructors/destructors, use direct assignment rather than memcpy() where possible.

So in summary:

• Use memmove() to prevent undefined behavior when copying regions that overlap.
• Use strcpy() for null terminated string handling convenience.
• Leverage copy() for type-safety with C++ containers.
• Stick to assignment for non-POD class types.

Evaluate these alternatives and choose the right tool for each job.

Conclusion – Mastering Memcpy()

In this deep dive, we covered everything you need to excel at using memcpy():

• Memcpy() copies memory blazingly fast with no conversions applied
• Use memcpy() for raw speed when copying structs, strings, arrays, buffers, etc
• It plows through memory byte-by-byte sequentially
• Take care to validate sizes and prevent overlap issues
• Always properly terminate strings yourself
• Alternatives like memmove() may be better suited for some scenarios

With your newfound mastery of memcpy(), you can leverage its power to optimize copying operations and take your C programs to the next level. Just remember – with great speed comes great responsibility. Wield your new memcpy() skills wisely!

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Structure assignment and its pitfall in c language.

Jan 28 th , 2013 9:47 pm

There is a structure type defined as below:

If we want to assign map_t type variable struct2 to sturct1 , we usually have below 3 ways:

Consider above ways, most of programmer won’t use way #1, since it’s so stupid ways compare to other twos, only if we are defining an structure assignment function. So, what’s the difference between way #2 and way #3? And what’s the pitfall of the structure assignment once there is array or pointer member existed? Coming sections maybe helpful for your understanding.

The difference between ‘=’ straight assignment and memcpy

The struct1=struct2; notation is not only more concise , but also shorter and leaves more optimization opportunities to the compiler . The semantic meaning of = is an assignment, while memcpy just copies memory. That’s a huge difference in readability as well, although memcpy does the same in this case.

Copying by straight assignment is probably best, since it’s shorter, easier to read, and has a higher level of abstraction. Instead of saying (to the human reader of the code) “copy these bits from here to there”, and requiring the reader to think about the size argument to the copy, you’re just doing a straight assignment (“copy this value from here to here”). There can be no hesitation about whether or not the size is correct.

Consider that, above source code also has pitfall about the pointer alias, it will lead dangling pointer problem ( It will be introduced below section ). If we use straight structure assignment ‘=’ in C++, we can consider to overload the operator= function , that can dissolve the problem, and the structure assignment usage does not need to do any changes, but structure memcpy does not have such opportunity.

The pitfall of structure assignment:

Beware though, that copying structs that contain pointers to heap-allocated memory can be a bit dangerous, since by doing so you’re aliasing the pointer, and typically making it ambiguous who owns the pointer after the copying operation.

If the structures are of compatible types, yes, you can, with something like:

} The only thing you need to be aware of is that this is a shallow copy. In other words, if you have a char * pointing to a specific string, both structures will point to the same string.

And changing the contents of one of those string fields (the data that the char points to, not the char itself) will change the other as well. For these situations a “deep copy” is really the only choice, and that needs to go in a function. If you want a easy copy without having to manually do each field but with the added bonus of non-shallow string copies, use strdup:

This will copy the entire contents of the structure, then deep-copy the string, effectively giving a separate string to each structure. And, if your C implementation doesn’t have a strdup (it’s not part of the ISO standard), you have to allocate new memory for dest_struct pointer member, and copy the data to memory address.

Example of trap:

Below diagram illustrates above source memory layout, if there is a pointer field member, either the straight assignment or memcpy , that will be alias of pointer to point same address. For example, b.alias and c.alias both points to address of a.alias . Once one of them free the pointed address, it will cause another pointer as dangling pointer. It’s dangerous!!

• Recommend use straight assignment ‘=’ instead of memcpy.
• If structure has pointer or array member, please consider the pointer alias problem, it will lead dangling pointer once incorrect use. Better way is implement structure assignment function in C, and overload the operator= function in C++.
• stackoverflow.com: structure assignment or memcpy
• stackoverflow.com: assign one struct to another in C
• bytes.com: structures assignment
• wikipedia: struct in C programming language

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Difference in assigning a char variable with memcpy() or directly inside a struct in c++?

What is the difference between using memcpy() or directly a pointer to declare the name variable inside the person struct in this code?

I know that internally an array is a pointer, but when doing it with a pointer it gives me a warning

In contrast, when performing with memcpy and with name[20], it does not give me any warning, what does the warning refer to?

To know the difference we must start by clarifying concepts. In C++ everything has a type, including text literals. Thus, the literal "juan" has a type that is a constant 1 const char[5] formation of five characters. It's five characters times the four letters plus the string termination character and it's constant because it's a literal. When you write this instruction:

Being char * the type of persona1.nombre , the type const char[5] is implicitly converted to a string pointer ( const char * ) and normally c++ explicitly forbids pointing to read-only memory ( const ) with read-write pointers (which are not marked as const ) but in this case it does the turns a blind eye and displays an alarm for keeping a compatibility with c 2 . What that statement is doing is pointing a pointer to the start of the literal "juan" , so the assigned variable is a pointer (even if it's the wrong type).

If we generate an example code and examine its assembler, we see that the compiler does just that, an assignment:

After the operation, you will have two pointers pointing to the same place and a single copy of the data:

If, on the other hand, we make the call to memcpy we see that the compiler does, effectively, a function call:

After the operation you will have two pointers, pointing to different sites but with a copy of the data:

By the way, contrary to what your comment ( //sin puntero ) says, calling memcpy also uses pointers, the destination memory pointer and the source memory pointer; but also since you have not reserved space to copy the data in the destination memory, your program can fail at run time.

Once these differences have been explained, I advise you not to use character formations to save text, in C++ the object is used std::string , which is safer and easier to use:

Also note that in C++ structs are types in their own right, so they don't need a type definition ( type def inition ) to be treated as such:

1 Also known as array .

2 In C the type of character literals does not include const .

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The memcpy() function in C and C++ is used to copy a block of memory from one location to another. Unlike other copy functions, the memcpy function copies the specified number of bytes from one memory location to the other memory location regardless of the type of data stored.

It is declared in <string.h> header file. In C++, it is also defined inside <cstring> header file.

Syntax of memcpy

The memcpy function is declared as:

• to : A pointer to the memory location where the copied data will be stored.
• from : A pointer to the memory location from where the data is to be copied.
• numBytes : The number of bytes to be copied.

Return Value

• This function returns a pointer to the memory location where data is copied.

Example of memcpy

Below is the C program to show the working of memcpy()

Important Points about memcpy()

• memcpy() doesn’t check for overflow or \0.
• memcpy() leads to undefined behaviour when source and destination addresses overlap.

Note : memmove() is another library function that handles overlapping well.

Related Article

• Write your own memcpy() and memmove()

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Assignment vs Memcpy

Quote: Original post by ToohrVyk Very interesting, and it does make sense: the presence of a constructor marks the object as non-POD, so the compiler generates a member-wise assignment operator, which in turn incurs an alignment property.

Stephen M. Webb Professional Free Software Developer

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Return Value

std::memcpy

Copies count bytes from the object pointed to by src to the object pointed to by dest . Both objects are reinterpreted as arrays of unsigned char .

If the objects overlap, the behavior is undefined.

If either dest or src is an invalid or null pointer , the behavior is undefined, even if count is zero.

If the objects are potentially-overlapping or not TriviallyCopyable , the behavior of memcpy is not specified and may be undefined .

Return value

std::memcpy may be used to implicitly create objects in the destination buffer.

std::memcpy is meant to be the fastest library routine for memory-to-memory copy. It is usually more efficient than std::strcpy , which must scan the data it copies or std::memmove , which must take precautions to handle overlapping inputs.

Several C++ compilers transform suitable memory-copying loops to std::memcpy calls.

Where strict aliasing prohibits examining the same memory as values of two different types, std::memcpy may be used to convert the values.

© cppreference.com Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0. https://en.cppreference.com/w/cpp/string/byte/memcpy

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Assignment operators.

Assignment operators modify the value of the object.

[ edit ] Definitions

Copy assignment replaces the contents of the object a with a copy of the contents of b ( b is not modified). For class types, this is performed in a special member function, described in copy assignment operator .

For non-class types, copy and move assignment are indistinguishable and are referred to as direct assignment .

Compound assignment replace the contents of the object a with the result of a binary operation between the previous value of a and the value of b .

[ edit ] Assignment operator syntax

The assignment expressions have the form

• ↑ target-expr must have higher precedence than an assignment expression.
• ↑ new-value cannot be a comma expression, because its precedence is lower.

[ edit ] Built-in simple assignment operator

For the built-in simple assignment, the object referred to by target-expr is modified by replacing its value with the result of new-value . target-expr must be a modifiable lvalue.

The result of a built-in simple assignment is an lvalue of the type of target-expr , referring to target-expr . If target-expr is a bit-field , the result is also a bit-field.

[ edit ] Assignment from an expression

If new-value is an expression, it is implicitly converted to the cv-unqualified type of target-expr . When target-expr is a bit-field that cannot represent the value of the expression, the resulting value of the bit-field is implementation-defined.

If target-expr and new-value identify overlapping objects, the behavior is undefined (unless the overlap is exact and the type is the same).

In overload resolution against user-defined operators , for every type T , the following function signatures participate in overload resolution:

For every enumeration or pointer to member type T , optionally volatile-qualified, the following function signature participates in overload resolution:

For every pair A1 and A2 , where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signature participates in overload resolution:

[ edit ] Built-in compound assignment operator

The behavior of every built-in compound-assignment expression target-expr   op   =   new-value is exactly the same as the behavior of the expression target-expr   =   target-expr   op   new-value , except that target-expr is evaluated only once.

The requirements on target-expr and new-value of built-in simple assignment operators also apply. Furthermore:

• For + = and - = , the type of target-expr must be an arithmetic type or a pointer to a (possibly cv-qualified) completely-defined object type .
• For all other compound assignment operators, the type of target-expr must be an arithmetic type.

In overload resolution against user-defined operators , for every pair A1 and A2 , where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signatures participate in overload resolution:

For every pair I1 and I2 , where I1 is an integral type (optionally volatile-qualified) and I2 is a promoted integral type, the following function signatures participate in overload resolution:

For every optionally cv-qualified object type T , the following function signatures participate in overload resolution:

[ edit ] Example

Possible output:

[ edit ] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

Operator precedence

Operator overloading

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