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Does memmove use dynamic memory for its temporary array

memcpy() vs memmove()Why is Linux memmove() implemented the way it is?How do I determine the size of my array in C?With arrays, why is it the case that a[5] == 5[a]?How does free know how much to free?Creating a memory leak with JavaWhat does the C ??!??! operator do?Why memory functions such as memset, memchr… are in string.h, but not in stdlib.h with another mem functions?Why is Linux memmove() implemented the way it is?Why is memmove faster than memcpy?Segmentation fault with large, contiguous 2D array (with dynamic memory allocation) in Cusing memmove in dynamic allocation in c

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2

According to C11 N1570 standard draft:

7.24.2.2 "The memmove function":

The memmove function copies n characters from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and then the n characters from the temporary array are copied into the object pointed to by s1

So if I choose to move a buffer of size 32K using (file_size = 32K)

memmove(io_Buffer, io_Buffer+17, file_size);

won't the temp buffer be of size 32K?

Question

Can the program allocate dynamic memory on its own? Does it allocate and free the memory in that one line?

c memory

share|improve this question

edited Mar 27 at 7:07

Antti Haapala

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asked Mar 27 at 5:08

clmnoclmno

89955 silver badges1818 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Try ltrace, read glibc source, read GCC source, step debug the assembly, and pray that it does not go too deep :-)
  
  – Ciro Santilli 新疆改造中心996ICU六四事件
  Mar 28 at 10:52
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @CiroSantilli新疆改造中心六四事件法轮功 This was for an embedded application (and.. that's why I was too concerned about the memory), but will try out ltrace for sure.
  
  – clmno
  Mar 28 at 11:45
  

  
  
  
  

add a comment | 

2

According to C11 N1570 standard draft:

7.24.2.2 "The memmove function":

The memmove function copies n characters from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and then the n characters from the temporary array are copied into the object pointed to by s1

So if I choose to move a buffer of size 32K using (file_size = 32K)

memmove(io_Buffer, io_Buffer+17, file_size);

won't the temp buffer be of size 32K?

Question

Can the program allocate dynamic memory on its own? Does it allocate and free the memory in that one line?

c memory

share|improve this question

edited Mar 27 at 7:07

Antti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

asked Mar 27 at 5:08

clmnoclmno

89955 silver badges1818 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Try ltrace, read glibc source, read GCC source, step debug the assembly, and pray that it does not go too deep :-)
  
  – Ciro Santilli 新疆改造中心996ICU六四事件
  Mar 28 at 10:52
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @CiroSantilli新疆改造中心六四事件法轮功 This was for an embedded application (and.. that's why I was too concerned about the memory), but will try out ltrace for sure.
  
  – clmno
  Mar 28 at 11:45
  

  
  
  
  

add a comment | 

According to C11 N1570 standard draft:

7.24.2.2 "The memmove function":

The memmove function copies n characters from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and then the n characters from the temporary array are copied into the object pointed to by s1

So if I choose to move a buffer of size 32K using (file_size = 32K)

memmove(io_Buffer, io_Buffer+17, file_size);

won't the temp buffer be of size 32K?

Question

Can the program allocate dynamic memory on its own? Does it allocate and free the memory in that one line?

c memory

share|improve this question

edited Mar 27 at 7:07

Antti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

asked Mar 27 at 5:08

clmnoclmno

89955 silver badges1818 bronze badges

memmove(io_Buffer, io_Buffer+17, file_size);

share|improve this question

edited Mar 27 at 7:07

Antti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

asked Mar 27 at 5:08

clmnoclmno

89955 silver badges1818 bronze badges

share|improve this question

edited Mar 27 at 7:07

Antti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

asked Mar 27 at 5:08

clmnoclmno

89955 silver badges1818 bronze badges

edited Mar 27 at 7:07

Antti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

edited Mar 27 at 7:07

Antti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

asked Mar 27 at 5:08

clmnoclmno

89955 silver badges1818 bronze badges

asked Mar 27 at 5:08

clmnoclmno

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2 Answers
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6

I think you missed the "as if" in that sentence. That means the effects will be the same as if it did that, not that it will actually do that. I've never seen an implementation of memmove that actually uses a temporary array.

share|improve this answer

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Thank you. Found this other question which points to a few implementations. Also, cheers on Ripple!
  
  – clmno
  Mar 27 at 5:20
  

  
  
  
  

add a comment | 

1

The memmove is not a single implementation in modern compilers; it is considered an intrinsic instead. It is easiest to show with an example how the "as if" works:

#include <string.h>

void test_memmove(void * restrict dst, const void * restrict src, size_t n) 
 memmove(dst, src, n);

the restrict in parameters tell that the memory accessed through the pointers do not overlap. So GCC knows to compile this to

test_memmove:
 jmp memcpy

Because the compiler was able to take the restrict into account and "prove" that the memory areas pointed to by these 2 do not overlap, the call to memmove was immediately changed to a (tail) call to memcpy!

share|improve this answer

edited Mar 27 at 7:13

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

add a comment | 

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6

I think you missed the "as if" in that sentence. That means the effects will be the same as if it did that, not that it will actually do that. I've never seen an implementation of memmove that actually uses a temporary array.

share|improve this answer

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Thank you. Found this other question which points to a few implementations. Also, cheers on Ripple!
  
  – clmno
  Mar 27 at 5:20
  

  
  
  
  

add a comment | 

6

I think you missed the "as if" in that sentence. That means the effects will be the same as if it did that, not that it will actually do that. I've never seen an implementation of memmove that actually uses a temporary array.

share|improve this answer

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Thank you. Found this other question which points to a few implementations. Also, cheers on Ripple!
  
  – clmno
  Mar 27 at 5:20
  

  
  
  
  

add a comment | 

I think you missed the "as if" in that sentence. That means the effects will be the same as if it did that, not that it will actually do that. I've never seen an implementation of memmove that actually uses a temporary array.

share|improve this answer

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

share|improve this answer

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

answered Mar 27 at 5:09

David SchwartzDavid Schwartz

144k1414 gold badges153153 silver badges238238 bronze badges

1

The memmove is not a single implementation in modern compilers; it is considered an intrinsic instead. It is easiest to show with an example how the "as if" works:

#include <string.h>

void test_memmove(void * restrict dst, const void * restrict src, size_t n) 
 memmove(dst, src, n);

the restrict in parameters tell that the memory accessed through the pointers do not overlap. So GCC knows to compile this to

test_memmove:
 jmp memcpy

Because the compiler was able to take the restrict into account and "prove" that the memory areas pointed to by these 2 do not overlap, the call to memmove was immediately changed to a (tail) call to memcpy!

share|improve this answer

edited Mar 27 at 7:13

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

add a comment | 

1

The memmove is not a single implementation in modern compilers; it is considered an intrinsic instead. It is easiest to show with an example how the "as if" works:

#include <string.h>

void test_memmove(void * restrict dst, const void * restrict src, size_t n) 
 memmove(dst, src, n);

the restrict in parameters tell that the memory accessed through the pointers do not overlap. So GCC knows to compile this to

test_memmove:
 jmp memcpy

Because the compiler was able to take the restrict into account and "prove" that the memory areas pointed to by these 2 do not overlap, the call to memmove was immediately changed to a (tail) call to memcpy!

share|improve this answer

edited Mar 27 at 7:13

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

add a comment | 

The memmove is not a single implementation in modern compilers; it is considered an intrinsic instead. It is easiest to show with an example how the "as if" works:

#include <string.h>

void test_memmove(void * restrict dst, const void * restrict src, size_t n) 
 memmove(dst, src, n);

the restrict in parameters tell that the memory accessed through the pointers do not overlap. So GCC knows to compile this to

test_memmove:
 jmp memcpy

Because the compiler was able to take the restrict into account and "prove" that the memory areas pointed to by these 2 do not overlap, the call to memmove was immediately changed to a (tail) call to memcpy!

share|improve this answer

edited Mar 27 at 7:13

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

#include <string.h>

void test_memmove(void * restrict dst, const void * restrict src, size_t n) 
 memmove(dst, src, n);

test_memmove:
 jmp memcpy

share|improve this answer

edited Mar 27 at 7:13

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges

answered Mar 27 at 7:06

Antti HaapalaAntti Haapala

91.4k1717 gold badges177177 silver badges215215 bronze badges