Understanding memcpy Behavior with Array and Pointer
Have you ever wondered why the two memcpy lines in your code snippet work exactly the same? In this article, we'll dive into the behavior of memcpy when working with arrays and pointers in C.
Address of Array and First Element
When you pass an array as an argument to a function or use it in an expression, it automatically converts to a pointer to its first element. This conversion is what allows the memcpy function to work with both buff and &buff[0] .
Unary & Operator
The unary & operator is an exception to the array-to-pointer conversion rule. It gives the address of its operand instead of converting an array to a pointer. So, when you write &buff , you get a pointer to the whole array.
Same Starting Location
The starting location of the first element of an array and the starting location of the array itself are the same. Therefore, &buff and buff have the same value, but different types. But, in both cases, data is copied to the same location.
Additionally, it's worth noting that the type difference becomes irrelevant in the context of memcpy . The first parameter of memcpy is declared as void * restrict , which means both &buff and buff are converted to the same type before the call.
In summary, the two memcpy lines work the same because the array buff automatically converts to a pointer to its first element, and the unary & operator gives the address of the whole array. The starting location of the first element and the array itself are the same, causing the data to be copied to the same location.
Next time you encounter a similar scenario, you'll understand why the two versions of memcpy have identical behavior!
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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Memcpy, memcpy_s.
| (C11) | ||||
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| Functions | ||||
| Character manipulation | ||||
| atolatoll (C99) | ||||
| strtoll (C99) |
| strtoull (C99) | ||||
| strtodstrtold (C99) | ||||
| strtoumax (C99) |
| strnlen_s (C11) | ||||
| strtok_s (C11) |
| (1) | ||
| * memcpy( void *dest, const void *src, count ); | (until C99) | |
| * memcpy( void *restrict dest, const void *restrict src, count ); | (since C99) | |
| void *restrict dest, rsize_t destsz, const void *restrict src, rsize_t count ); | (2) | (since C11) |
- dest or src is a null pointer
- destsz or count is greater than RSIZE_MAX
- count is greater than destsz (buffer overflow would occur)
- the source and the destination objects overlap
| Parameters Return value Notes Example References See also |
[ edit ] Parameters
| dest | - | pointer to the object to copy to |
| destsz | - | max number of bytes to modify in the destination (typically the size of the destination object) |
| src | - | pointer to the object to copy from |
| count | - | number of bytes to copy |
[ edit ] Return value
[ edit ] notes.
memcpy may be used to set the effective type of an object obtained by an allocation function.
memcpy is the fastest library routine for memory-to-memory copy. It is usually more efficient than strcpy , which must scan the data it copies or memmove , which must take precautions to handle overlapping inputs.
Several C compilers transform suitable memory-copying loops to memcpy calls.
Where strict aliasing prohibits examining the same memory as values of two different types, memcpy may be used to convert the values.
[ edit ] Example
Possible output:
[ edit ] References
- C11 standard (ISO/IEC 9899:2011):
- 7.24.2.1 The memcpy function (p: 362)
- K.3.7.1.1 The memcpy_s function (p: 614)
- C99 standard (ISO/IEC 9899:1999):
- 7.21.2.1 The memcpy function (p: 325)
- C89/C90 standard (ISO/IEC 9899:1990):
- 4.11.2.1 The memcpy function
[ edit ] See also
| memmove_s (C11) | moves one buffer to another (function) |
| wmemcpy_s (C11) | copies a certain amount of wide characters between two non-overlapping arrays (function) |
| for memcpy | |
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AOverflow.com
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 .
A man is valued by his works, not his words!
Structure assignment and its pitfall in c language.
Jan 28 th , 2013 9:47 pm
There is a structure type defined as below:
| 2 3 4 5 | struct __map_t { int code; char name[NAME_SIZE]; char *alias; }map_t; |
If we want to assign map_t type variable struct2 to sturct1 , we usually have below 3 ways:
| 2 3 4 5 6 7 8 9 10 | struct1.code = struct2.code; strncpy(struct1.name, struct2.name, NAME_SIZE); struct1.alias = struct2.alias; /* Way #2: memcpy the whole memory content of struct2 to struct1 */ memcpy(&struct1, &struct2, sizeof(struct1)); /* Way #3: straight assignment with '=' */ struct1 = struct2; |
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:
| (dest_struct, source_struct, sizeof(dest_struct)); |
} 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:
| 2 | (dest_struct, source_struct, sizeof (dest_struct)); dest_struct->strptr = strdup(source_struct->strptr); |
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:
| 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | #include <stdlib.h> #include <string.h> #define NAME_SIZE 16 typedef struct _map_t { int code; char name[NAME_SIZE]; char *alias; } map_t; int main() { map_t a, b, c; /* initialize the a's members value */ a.code = 1024; snprintf(a.name, NAME_SIZE, "Controller SW3"); char *alias = "RNC&IPA"; a.alias = alias; /* assign the value via memcpy */ memcpy(&b, &a, sizeof(b)); /* assign the value via '=' */ c = a; return 0; } |
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
| > Linux > |
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memcpy(3) — Linux manual page
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Pages that refer to this page: bcopy(3) , bstring(3) , cmsg(3) , CPU_SET(3) , memccpy(3) , memmove(3) , mempcpy(3) , size_t(3type) , void(3type) , wmemcpy(3) , feature_test_macros(7) , signal-safety(7) , string_copying(7)
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std:: memcpy
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Note when assigning the struct, the compiler knows at compile time how big the move is going to be, so it can unroll small copies (do a move n-times in row instead of looping) for instance. Note -mno-memcpy: -mmemcpy. -mno-memcpy. Force (do not force) the use of "memcpy()" for non-trivial block moves. The default is -mno-memcpy, which allows ...
Overwriting the contents of a pointer variable is no different than overwriting the contents of normal int variable. So yes, doing e.g. memcpy (&p, &pa1, sizeof p) is equivalent of the assignment p = pa1, but might be less efficient. Lets try it a bit differently instead:
After the memcpy(), on the other hand, if the behavior is defined at all (see above) then each pointer points to a separate copy of the same data. Freeing one of them frees only that copy -- in that case it is still ok to dereference the other one.
The first parameter of memcpy is declared as void * restrict, which means both &buff and buff are converted to the same type before the call. Conclusion. In summary, the two memcpy lines work the same because the array buff automatically converts to a pointer to its first element, and the unary & operator gives the address of the whole array ...
Introduction to Memcpy() - Your Memory Copying Friend. Memcpy() is declared in the string.h header and has this prototype: void *memcpy(void *dest, const void *src, size_t n); 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.
memcpy, memcpy_s. 1) Copies count characters from the object pointed to by src to the object pointed to by dest. Both objects are interpreted as arrays of unsigned char. The behavior is undefined if access occurs beyond the end of the dest array. If the objects overlap (which is a violation of the restrict contract) (since C99), the behavior is ...
Return value. dest [] Notestd::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 ...
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: persona f1() { persona p; p.nombre = "juan"; return p; }
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.
The memcpy() function copies n bytes from memory area src to memory area dest. The memory areas must not overlap. Use memmove(3) if the memory areas do overlap. RETURN VALUE top The memcpy() function returns a pointer to dest. ATTRIBUTES top For an explanation of the terms used in this section, see attributes(7)
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.
And don't worry about performance: memcpy () gets inlined by the compiler for small sizes and does generate a single MOV instruction when it's possible (e.g. copying 4 or 8 bytes). For bigger structs, memcpy () and val = *ptr are still identical because, val = *ptr actually emits code just calling memcpy ().
std:: memcpy. std:: memcpy. Copies count bytes from the object pointed to by src to the object pointed to by dest. If the objects overlap, the behavior is undefined. If the objects are not trivially copyable (e.g. scalars, arrays, C-compatible structs), the behavior is undefined.
memcpy may be used to set the effective type of an object obtained by an allocation function. memcpy is the fastest library routine for memory-to-memory copy. It is usually more efficient than strcpy , which must scan the data it copies or memmove , which must take precautions to handle overlapping inputs.
What is memcpy() memcpy() is a standard function used in the C programming language to copy blocks of memory from one place to another. Its prototype is defined in the string.h header file as follows:. void *memcpy(void *dest, const void *src, size_t n); The memcpy() function copies the contents of a source buffer to a destination buffer, starting from the memory location pointed to by src ...
Cats_and_Shit. •. I think you could say that the assignment operator is parametrically polymorphic, while memcpy uses subtyping. Parametric polymorphism is when an operation is defined abstractly such that works on any type (or any type with certain qualities); for assignment in C that is everything except arrays.
Notes. memcpy may be used to set the effective type of an object obtained by an allocation function.. memcpy is the fastest library routine for memory-to-memory copy. It is usually more efficient than strcpy, which must scan the data it copies or memmove, which must take precautions to handle overlapping inputs.. Several C compilers transform suitable memory-copying loops to memcpy calls.
Return value. If the byte (unsigned char) c was found, memccpy returns a pointer to the next byte in dest after (unsigned char) c.Otherwise it returns a null pointer. [] NoteThe function is identical to the POSIX memccpy.. memccpy (dest, src, 0, count) behaves similar to strncpy (dest, src, count), except that the former returns a pointer to the end of the buffer written, and does not zero-pad ...
A straight assignment is clearer to read. Assignment is a teeny bit riskier to get wrong, so that you assign pointers to structs rather than the structs pointed to. If you fear this, you'll make sure your unit tests cover this. I would not care about this risk. (Similarly, memcpy is risky because you might get the struct size wrong.)
The memcpy function is used to copy a block of data from a source address to a destination address. Below is its prototype. ... If a class doesn't contain pointers, then there is no need to write assignment operator and copy constructor. The compiler creates a default copy constructor and assignment operators for every class.