C pointers: copy strings
1
I am trying to copy the string contained in
char *src
into
char *dst
this is my function but doesn't compile
char *mystrncpy(char *dst, char *src, size_t len)
{
char *temp = dst;
char *p = dst;
size_t i = 0;
for (p = dst; i < len; p += 1)
{
*p = *(src + p);
i += 1;
}
return *temp;
}
c string pointers
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
add a comment |
1
I am trying to copy the string contained in
char *src
into
char *dst
this is my function but doesn't compile
char *mystrncpy(char *dst, char *src, size_t len)
{
char *temp = dst;
char *p = dst;
size_t i = 0;
for (p = dst; i < len; p += 1)
{
*p = *(src + p);
i += 1;
}
return *temp;
}
c string pointers
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
*p = *(src + p);shouldn't that be something among the lines of*p = *(src + i);instead?
– Blaze
Nov 22 '18 at 14:55
yes it does compile but doesn't do what expected
– cucklordv4
Nov 22 '18 at 14:58
1
what is the purpose ofp < src?
– Brice
Nov 22 '18 at 15:03
1
If it does not compile you should include the error message in the question.
– Broman
Nov 22 '18 at 15:03
2
The behavior ofp < srcis not defined by the C standard. You should not compare pointers from different objects for order. (Pointers may be compared for equality. Pointers within one object, such as an array or structure, may be compared for order.) Nor is there any reason to do such a comparison; the function is apparently designed to copylencharacters, so onlylenis needed to control the loop.
– Eric Postpischil
Nov 22 '18 at 15:08
add a comment |
1
1
1
I am trying to copy the string contained in
char *src
into
char *dst
this is my function but doesn't compile
char *mystrncpy(char *dst, char *src, size_t len)
{
char *temp = dst;
char *p = dst;
size_t i = 0;
for (p = dst; i < len; p += 1)
{
*p = *(src + p);
i += 1;
}
return *temp;
}
c string pointers
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
I am trying to copy the string contained in
char *src
into
char *dst
this is my function but doesn't compile
char *mystrncpy(char *dst, char *src, size_t len)
{
char *temp = dst;
char *p = dst;
size_t i = 0;
for (p = dst; i < len; p += 1)
{
*p = *(src + p);
i += 1;
}
return *temp;
}
c string pointers
c string pointers
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
edited Nov 22 '18 at 15:20
cucklordv4
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
asked Nov 22 '18 at 14:53
cucklordv4cucklordv4
113
113
*p = *(src + p);shouldn't that be something among the lines of*p = *(src + i);instead?
– Blaze
Nov 22 '18 at 14:55
yes it does compile but doesn't do what expected
– cucklordv4
Nov 22 '18 at 14:58
1
what is the purpose ofp < src?
– Brice
Nov 22 '18 at 15:03
1
If it does not compile you should include the error message in the question.
– Broman
Nov 22 '18 at 15:03
2
The behavior ofp < srcis not defined by the C standard. You should not compare pointers from different objects for order. (Pointers may be compared for equality. Pointers within one object, such as an array or structure, may be compared for order.) Nor is there any reason to do such a comparison; the function is apparently designed to copylencharacters, so onlylenis needed to control the loop.
– Eric Postpischil
Nov 22 '18 at 15:08
add a comment |
*p = *(src + p);shouldn't that be something among the lines of*p = *(src + i);instead?
– Blaze
Nov 22 '18 at 14:55
yes it does compile but doesn't do what expected
– cucklordv4
Nov 22 '18 at 14:58
1
what is the purpose ofp < src?
– Brice
Nov 22 '18 at 15:03
1
If it does not compile you should include the error message in the question.
– Broman
Nov 22 '18 at 15:03
2
The behavior ofp < srcis not defined by the C standard. You should not compare pointers from different objects for order. (Pointers may be compared for equality. Pointers within one object, such as an array or structure, may be compared for order.) Nor is there any reason to do such a comparison; the function is apparently designed to copylencharacters, so onlylenis needed to control the loop.
– Eric Postpischil
Nov 22 '18 at 15:08
*p = *(src + p); shouldn't that be something among the lines of *p = *(src + i); instead?– Blaze
Nov 22 '18 at 14:55
*p = *(src + p); shouldn't that be something among the lines of *p = *(src + i); instead?– Blaze
Nov 22 '18 at 14:55
yes it does compile but doesn't do what expected
– cucklordv4
Nov 22 '18 at 14:58
yes it does compile but doesn't do what expected
– cucklordv4
Nov 22 '18 at 14:58
1
1
what is the purpose of
p < src ?– Brice
Nov 22 '18 at 15:03
what is the purpose of
p < src ?– Brice
Nov 22 '18 at 15:03
1
1
If it does not compile you should include the error message in the question.
– Broman
Nov 22 '18 at 15:03
If it does not compile you should include the error message in the question.
– Broman
Nov 22 '18 at 15:03
2
2
The behavior of
p < src is not defined by the C standard. You should not compare pointers from different objects for order. (Pointers may be compared for equality. Pointers within one object, such as an array or structure, may be compared for order.) Nor is there any reason to do such a comparison; the function is apparently designed to copy len characters, so only len is needed to control the loop.– Eric Postpischil
Nov 22 '18 at 15:08
The behavior of
p < src is not defined by the C standard. You should not compare pointers from different objects for order. (Pointers may be compared for equality. Pointers within one object, such as an array or structure, may be compared for order.) Nor is there any reason to do such a comparison; the function is apparently designed to copy len characters, so only len is needed to control the loop.– Eric Postpischil
Nov 22 '18 at 15:08
add a comment |
2 Answers
2
active
oldest
votes
3
You're overdoing it.
You probably want this:
char *mystrncpy(char *dst, char *src, size_t len)
{
while (len > 0)
{
*dst++ = *src++;
len--;
}
return dst - len;
}
This is totally wrong
return *temp;
*temp is a char an not a pointer so return temp; would be correct.
BTW: strictly speaking we are not copying a string as a string in C is a NUL terminated sequence of chars. Here we are just copying len bytes.
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
1
@cucklordv4 not quite, consider what the++is doing todst.
– Jabberwocky
Nov 22 '18 at 15:11
1
@Jabberwocky Pointer arithmetic withlenwill work as well.
– Govind Parmar
Nov 22 '18 at 15:18
1
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
|
show 6 more comments
2
You are really close; your error is on this line:
*p = *(src + p);
Think about why you are dereferencing *(src + p)... remember, where is the memory location of the character you're trying to copy?
Also your function should either return void since there's no return statement anywhere, or return dst. Also, you don't need p < src in your loop guard.
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
1
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
Rather than changing the type to returnvoid, the intent may have been that the function returns the destination, as the standardstrncpydoes.
– Eric Postpischil
Nov 22 '18 at 15:07
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
draft saved
draft discarded
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53433538%2fc-pointers-copy-strings%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
3
You're overdoing it.
You probably want this:
char *mystrncpy(char *dst, char *src, size_t len)
{
while (len > 0)
{
*dst++ = *src++;
len--;
}
return dst - len;
}
This is totally wrong
return *temp;
*temp is a char an not a pointer so return temp; would be correct.
BTW: strictly speaking we are not copying a string as a string in C is a NUL terminated sequence of chars. Here we are just copying len bytes.
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
1
@cucklordv4 not quite, consider what the++is doing todst.
– Jabberwocky
Nov 22 '18 at 15:11
1
@Jabberwocky Pointer arithmetic withlenwill work as well.
– Govind Parmar
Nov 22 '18 at 15:18
1
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
|
show 6 more comments
3
You're overdoing it.
You probably want this:
char *mystrncpy(char *dst, char *src, size_t len)
{
while (len > 0)
{
*dst++ = *src++;
len--;
}
return dst - len;
}
This is totally wrong
return *temp;
*temp is a char an not a pointer so return temp; would be correct.
BTW: strictly speaking we are not copying a string as a string in C is a NUL terminated sequence of chars. Here we are just copying len bytes.
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
1
@cucklordv4 not quite, consider what the++is doing todst.
– Jabberwocky
Nov 22 '18 at 15:11
1
@Jabberwocky Pointer arithmetic withlenwill work as well.
– Govind Parmar
Nov 22 '18 at 15:18
1
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
|
show 6 more comments
3
3
3
You're overdoing it.
You probably want this:
char *mystrncpy(char *dst, char *src, size_t len)
{
while (len > 0)
{
*dst++ = *src++;
len--;
}
return dst - len;
}
This is totally wrong
return *temp;
*temp is a char an not a pointer so return temp; would be correct.
BTW: strictly speaking we are not copying a string as a string in C is a NUL terminated sequence of chars. Here we are just copying len bytes.
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
You're overdoing it.
You probably want this:
char *mystrncpy(char *dst, char *src, size_t len)
{
while (len > 0)
{
*dst++ = *src++;
len--;
}
return dst - len;
}
This is totally wrong
return *temp;
*temp is a char an not a pointer so return temp; would be correct.
BTW: strictly speaking we are not copying a string as a string in C is a NUL terminated sequence of chars. Here we are just copying len bytes.
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
edited Nov 22 '18 at 15:25
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
answered Nov 22 '18 at 15:09
JabberwockyJabberwocky
27.7k93773
27.7k93773
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
1
@cucklordv4 not quite, consider what the++is doing todst.
– Jabberwocky
Nov 22 '18 at 15:11
1
@Jabberwocky Pointer arithmetic withlenwill work as well.
– Govind Parmar
Nov 22 '18 at 15:18
1
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
|
show 6 more comments
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
1
@cucklordv4 not quite, consider what the++is doing todst.
– Jabberwocky
Nov 22 '18 at 15:11
1
@Jabberwocky Pointer arithmetic withlenwill work as well.
– Govind Parmar
Nov 22 '18 at 15:18
1
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
my function needs to return the dst string
– cucklordv4
Nov 22 '18 at 15:10
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
I think i should do return dst; (is it correct?)
– cucklordv4
Nov 22 '18 at 15:11
1
1
@cucklordv4 not quite, consider what the
++ is doing to dst.– Jabberwocky
Nov 22 '18 at 15:11
@cucklordv4 not quite, consider what the
++ is doing to dst.– Jabberwocky
Nov 22 '18 at 15:11
1
1
@Jabberwocky Pointer arithmetic with
len will work as well.– Govind Parmar
Nov 22 '18 at 15:18
@Jabberwocky Pointer arithmetic with
len will work as well.– Govind Parmar
Nov 22 '18 at 15:18
1
1
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
@GovindParmar sure, there are different possibilities.
– Jabberwocky
Nov 22 '18 at 15:18
|
show 6 more comments
2
You are really close; your error is on this line:
*p = *(src + p);
Think about why you are dereferencing *(src + p)... remember, where is the memory location of the character you're trying to copy?
Also your function should either return void since there's no return statement anywhere, or return dst. Also, you don't need p < src in your loop guard.
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
1
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
Rather than changing the type to returnvoid, the intent may have been that the function returns the destination, as the standardstrncpydoes.
– Eric Postpischil
Nov 22 '18 at 15:07
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
add a comment |
2
You are really close; your error is on this line:
*p = *(src + p);
Think about why you are dereferencing *(src + p)... remember, where is the memory location of the character you're trying to copy?
Also your function should either return void since there's no return statement anywhere, or return dst. Also, you don't need p < src in your loop guard.
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
1
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
Rather than changing the type to returnvoid, the intent may have been that the function returns the destination, as the standardstrncpydoes.
– Eric Postpischil
Nov 22 '18 at 15:07
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
add a comment |
2
2
2
You are really close; your error is on this line:
*p = *(src + p);
Think about why you are dereferencing *(src + p)... remember, where is the memory location of the character you're trying to copy?
Also your function should either return void since there's no return statement anywhere, or return dst. Also, you don't need p < src in your loop guard.
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
You are really close; your error is on this line:
*p = *(src + p);
Think about why you are dereferencing *(src + p)... remember, where is the memory location of the character you're trying to copy?
Also your function should either return void since there's no return statement anywhere, or return dst. Also, you don't need p < src in your loop guard.
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
edited Nov 22 '18 at 15:08
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
answered Nov 22 '18 at 15:01
Govind ParmarGovind Parmar
12.2k53462
12.2k53462
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
1
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
Rather than changing the type to returnvoid, the intent may have been that the function returns the destination, as the standardstrncpydoes.
– Eric Postpischil
Nov 22 '18 at 15:07
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
add a comment |
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
1
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
Rather than changing the type to returnvoid, the intent may have been that the function returns the destination, as the standardstrncpydoes.
– Eric Postpischil
Nov 22 '18 at 15:07
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
why I don't need p < src ?
– cucklordv4
Nov 22 '18 at 15:06
1
1
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
@cucklordv4 other question: why do you think you need it?
– Jabberwocky
Nov 22 '18 at 15:07
Rather than changing the type to return
void, the intent may have been that the function returns the destination, as the standard strncpy does.– Eric Postpischil
Nov 22 '18 at 15:07
Rather than changing the type to return
void, the intent may have been that the function returns the destination, as the standard strncpy does.– Eric Postpischil
Nov 22 '18 at 15:07
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
@EricPostpischil Updated my answer.
– Govind Parmar
Nov 22 '18 at 15:08
add a comment |
draft saved
draft discarded
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
draft saved
draft discarded
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53433538%2fc-pointers-copy-strings%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
*p = *(src + p);shouldn't that be something among the lines of*p = *(src + i);instead?– Blaze
Nov 22 '18 at 14:55
yes it does compile but doesn't do what expected
– cucklordv4
Nov 22 '18 at 14:58
1
what is the purpose of
p < src?– Brice
Nov 22 '18 at 15:03
1
If it does not compile you should include the error message in the question.
– Broman
Nov 22 '18 at 15:03
2
The behavior of
p < srcis not defined by the C standard. You should not compare pointers from different objects for order. (Pointers may be compared for equality. Pointers within one object, such as an array or structure, may be compared for order.) Nor is there any reason to do such a comparison; the function is apparently designed to copylencharacters, so onlylenis needed to control the loop.– Eric Postpischil
Nov 22 '18 at 15:08