Related
this is what I have so far but I can't figure out what is wrong with it
void newCopy(char *s)
{
char newString = malloc(sizeof(int * strlen(s)));
newString = s;
return &newString;
}
void newCopy(char *s)
{
char newString = malloc(sizeof(int * strlen(s)));
First and second problems are here.
First is, You're assigning the return of malloc, which is a pointer, to a variable declared as char. The variable should be declared as char*.
Second is, your input to sizeof is wrong.
int * strlen(s) is nonsense and won't compile, because you're trying to multiply a type and an integer. You meant sizeof(int) (which is an integer) * strlen(s) (also an integer) which will compile.
You should use sizeof(char) instead of sizeof(int), since it is a string.
You should add 1 to the size, since strings in C need to be null terminated by an extra \0 that strlen does not report being part of the string length.
Putting it all together, sizeof(char)*(strlen(s)+1)
newString = s;
Third problem is here. = is not a magic operator - it assigns the value in the variable s (which is a pointer) to the value in the variable newString (which after fixing the above mistake, will also be a pointer). It does nothing beside that.
What you want to do instead is use strcpy, which is a function that copies the contents of one string (by following its pointer) to the contents of another string (by following its pointer). http://www.cplusplus.com/reference/cstring/strcpy/
return &newString;
Fourth and fifth problems are here.
Fourth is, You have declared the function as void and here you are trying to return a char*.
Fifth is, you are trying to return a pointer to something that was declared on the stack (a local variable). As soon as the function returns, anything on the stack for that function is trash and cannot be referenced anymore.
However, if you correctly make newString of type char*, all you need to do is return newString; And you correctly return a pointer by value which points into the heap (thanks to the earlier malloc).
}
Finally, judging by this code, I should inform you that C is not a newbie friendly language, where you can just type things that 'look like' what you want to happen and pray it works. If you're even slightly wrong your code will crash and you will have zero idea why, because you don't know the right way to do things. Either read a really good book on C and teach yourself everything from basic to advance step by step so you know how it all works, or pick up a more user friendly language.
I should start by pointing out that in my opinion, given the number and (especially) nature of the mistakes in this code, you probably need to get a good book on C.
Your newString = s; overwrites the pointer instead of copying the string into the space you just allocated. Thus, you lose the pointer to what you just allocating (leaking the memory) without making a copy. You probably want to use strcpy instead of direct assignment.
Your computation of the size you allocate isn't what you really want either. Typically, for a string of length N, you want to allocate N+1 bytes. You're currently attempting to allocat sizeof(int * strlen(s)) bytes, which shouldn't even compile.
A corrected version should be like:
char *newCopy(char *s)
{
if (s == NULL)
return NULL;
char *newString = malloc(strlen(s) + 1);
if (newString == NULL)
return NULL;
strcpy(newString, s);
return newString;
}
Some people seem to think that C's strcpy() function is bad or evil. While I admit that it's usually better to use strncpy() in order to avoid buffer overflows, the following (an implementation of the strdup() function for those not lucky enough to have it) safely uses strcpy() and should never overflow:
char *strdup(const char *s1)
{
char *s2 = malloc(strlen(s1)+1);
if(s2 == NULL)
{
return NULL;
}
strcpy(s2, s1);
return s2;
}
*s2 is guaranteed to have enough space to store *s1, and using strcpy() saves us from having to store the strlen() result in another function to use later as the unnecessary (in this case) length parameter to strncpy(). Yet some people write this function with strncpy(), or even memcpy(), which both require a length parameter. I would like to know what people think about this. If you think strcpy() is safe in certain situations, say so. If you have a good reason not to use strcpy() in this situation, please give it - I'd like to know why it might be better to use strncpy() or memcpy() in situations like this. If you think strcpy() is okay, but not here, please explain.
Basically, I just want to know why some people use memcpy() when others use strcpy() and still others use plain strncpy(). Is there any logic to preferring one over the three (disregarding the buffer checks of the first two)?
memcpy can be faster than strcpy and strncpy because it does not have to compare each copied byte with '\0', and because it already knows the length of the copied object. It can be implemented in a similar way with the Duff's device, or use assembler instructions that copy several bytes at a time, like movsw and movsd
I'm following the rules in here. Let me quote from it
strncpy was initially introduced into the C library to deal with fixed-length name fields in structures such as directory entries. Such fields are not used in the same way as strings: the trailing null is unnecessary for a maximum-length field, and setting trailing bytes for shorter names to null assures efficient field-wise comparisons. strncpy is not by origin a ``bounded strcpy,'' and the Committee has preferred to recognize existing practice rather than alter the function to better suit it to such use.
For that reason, you will not get a trailing '\0' in a string if you hit the n not finding a '\0' from the source string so far. It's easy to misuse it (of course, if you know about that pitfall, you can avoid it). As the quote says, it wasn't designed as a bounded strcpy. And i would prefer not to use it if not necessary. In your case, clearly its use is not necessary and you proved it. Why then use it?
And generally speaking, programming code is also about reducing redundancy. If you know you have a string containing 'n' characters, why tell the copying function to copy maximal n characters? You do redundant checking. It's little about performance, but much more about consistent code. Readers will ask themselves what strcpy could do that could cross the n characters and which makes it necessary to limit the copying, just to read in manuals that this cannot happen in that case. And there the confusion start happen among readers of the code.
For the rational to use mem-, str- or strn-, i chose among them like in the above linked document:
mem- when i want to copy raw bytes, like bytes of a structure.
str- when copying a null terminated string - only when 100% no overflow could happen.
strn- when copying a null terminated string up to some length, filling the remaining bytes with zero. Probably not what i want in most cases. It's easy to forget the fact with the trailing zero-fill, but it's by design as the above quote explains. So, i would just code my own small loop that copies characters, adding a trailing '\0':
char * sstrcpy(char *dst, char const *src, size_t n) {
char *ret = dst;
while(n-- > 0) {
if((*dst++ = *src++) == '\0')
return ret;
}
*dst++ = '\0';
return ret;
}
Just a few lines that do exactly what i want. If i wanted "raw speed" i can still look out for a portable and optimized implementation that does exactly this bounded strcpy job. As always, profile first and then mess with it.
Later, C got functions for working with wide characters, called wcs- and wcsn- (for C99). I would use them likewise.
The reason why people use strncpy not strcpy is because strings are not always null terminated and it's very easy to overflow the buffer (the space you have allocated for the string with strcpy) and overwrite some unrelated bit of memory.
With strcpy this can happen, with strncpy this will never happen. That is why strcpy is considered unsafe. Evil might be a little strong.
Frankly, if you are doing much string handling in C, you should not ask yourself whether you should use strcpy or strncpy or memcpy. You should find or write a string library that provides a higher level abstraction. For example, one that keeps track of the length of each string, allocates memory for you, and provides all the string operations you need.
This will almost certainly guarantee you make very few of the kinds of mistakes usually associated with C string handling, such as buffer overflows, forgetting to terminate a string with a NUL byte, and so on.
The library might have functions such as these:
typedef struct MyString MyString;
MyString *mystring_new(const char *c_str);
MyString *mystring_new_from_buffer(const void *p, size_t len);
void mystring_free(MyString *s);
size_t mystring_len(MyString *s);
int mystring_char_at(MyString *s, size_t offset);
MyString *mystring_cat(MyString *s1, ...); /* NULL terminated list */
MyString *mystring_copy_substring(MyString *s, size_t start, size_t max_chars);
MyString *mystring_find(MyString *s, MyString *pattern);
size_t mystring_find_char(MyString *s, int c);
void mystring_copy_out(void *output, MyString *s, size_t max_chars);
int mystring_write_to_fd(int fd, MyString *s);
int mystring_write_to_file(FILE *f, MyString *s);
I wrote one for the Kannel project, see the gwlib/octstr.h file. It made life much simpler for us. On the other hand, such a library is fairly simple to write, so you might write one for yourself, even if only as an exercise.
No one has mentioned strlcpy, developed by Todd C. Miller and Theo de Raadt. As they say in their paper:
The most common misconception is that
strncpy() NUL-terminates the
destination string. This is only true,
however, if length of the source
string is less than the size
parameter. This can be problematic
when copying user input that may be of
arbitrary length into a fixed size
buffer. The safest way to use
strncpy() in this situation is to pass
it one less than the size of the
destination string, and then terminate
the string by hand. That way you are
guaranteed to always have a
NUL-terminated destination string.
There are counter-arguments for the use of strlcpy; the Wikipedia page makes note that
Drepper argues that strlcpy and
strlcat make truncation errors easier
for a programmer to ignore and thus
can introduce more bugs than they
remove.*
However, I believe that this just forces people that know what they're doing to add a manual NULL termination, in addition to a manual adjustment to the argument to strncpy. Use of strlcpy makes it much easier to avoid buffer overruns because you failed to NULL terminate your buffer.
Also note that the lack of strlcpy in glibc or Microsoft's libraries should not be a barrier to use; you can find the source for strlcpy and friends in any BSD distribution, and the license is likely friendly to your commercial/non-commercial project. See the comment at the top of strlcpy.c.
I personally am of the mindset that if the code can be proven to be valid—and done so quickly—it is perfectly acceptable. That is, if the code is simple and thus obviously correct, then it is fine.
However, your assumption seems to be that while your function is executing, no other thread will modify the string pointed to by s1. What happens if this function is interrupted after successful memory allocation (and thus the call to strlen), the string grows, and bam you have a buffer overflow condition since strcpy copies to the NULL byte.
The following might be better:
char *
strdup(const char *s1) {
int s1_len = strlen(s1);
char *s2 = malloc(s1_len+1);
if(s2 == NULL) {
return NULL;
}
strncpy(s2, s1, s1_len);
return s2;
}
Now, the string can grow through no fault of your own and you're safe. The result will not be a dup, but it won't be any crazy overflows, either.
The probability of the code you provided actually being a bug is pretty low (pretty close to non-existent, if not non-existent, if you are working in an environment that has no support for threading whatsoever). It's just something to think about.
ETA: Here is a slightly better implementation:
char *
strdup(const char *s1, int *retnum) {
int s1_len = strlen(s1);
char *s2 = malloc(s1_len+1);
if(s2 == NULL) {
return NULL;
}
strncpy(s2, s1, s1_len);
retnum = s1_len;
return s2;
}
There the number of characters is being returned. You can also:
char *
strdup(const char *s1) {
int s1_len = strlen(s1);
char *s2 = malloc(s1_len+1);
if(s2 == NULL) {
return NULL;
}
strncpy(s2, s1, s1_len);
s2[s1_len+1] = '\0';
return s2;
}
Which will terminate it with a NUL byte. Either way is better than the one that I quickly put together originally.
I agree. I would recommend against strncpy() though, since it will always pad your output to the indicated length. This is some historical decision, which I think was really unfortunate as it seriously worsens the performance.
Consider code like this:
char buf[128];
strncpy(buf, "foo", sizeof buf);
This will not write the expected four characters to buf, but will instead write "foo" followed by 125 zero characters. If you're for instance collecting a lot of short strings, this will mean your actual performance is far worse than expected.
If available, I prefer to use snprintf(), writing the above like:
snprintf(buf, sizeof buf, "foo");
If instead copying a non-constant string, it's done like this:
snprintf(buf, sizeof buf, "%s", input);
This is important, since if input contains % characters snprintf() would interpret them, opening up whole shelvefuls of cans of worms.
I think strncpy is evil too.
To truly protect yourself from programming errors of this kind, you need to make it impossible to write code that (a) looks OK, and (b) overruns a buffer.
This means you need a real string abstraction, which stores the buffer and capacity opaquely, binds them together, forever, and checks bounds. Otherwise, you end up passing strings and their capacities all over the shop. Once you get to real string ops, like modifying the middle of a string, it's almost as easy to pass the wrong length into strncpy (and especially strncat), as it is to call strcpy with a too-small destination.
Of course you might still ask whether to use strncpy or strcpy in implementing that abstraction: strncpy is safer there provided you fully grok what it does. But in string-handling application code, relying on strncpy to prevent buffer overflows is like wearing half a condom.
So, your strdup-replacement might look something like this (order of definitions changed to keep you in suspense):
string *string_dup(const string *s1) {
string *s2 = string_alloc(string_len(s1));
if (s2 != NULL) {
string_set(s2,s1);
}
return s2;
}
static inline size_t string_len(const string *s) {
return strlen(s->data);
}
static inline void string_set(string *dest, const string *src) {
// potential (but unlikely) performance issue: strncpy 0-fills dest,
// even if the src is very short. We may wish to optimise
// by switching to memcpy later. But strncpy is better here than
// strcpy, because it means we can use string_set even when
// the length of src is unknown.
strncpy(dest->data, src->data, dest->capacity);
}
string *string_alloc(size_t maxlen) {
if (maxlen > SIZE_MAX - sizeof(string) - 1) return NULL;
string *self = malloc(sizeof(string) + maxlen + 1);
if (self != NULL) {
// empty string
self->data[0] = '\0';
// strncpy doesn't NUL-terminate if it prevents overflow,
// so exclude the NUL-terminator from the capacity, set it now,
// and it can never be overwritten.
self->capacity = maxlen;
self->data[maxlen] = '\0';
}
return self;
}
typedef struct string {
size_t capacity;
char data[0];
} string;
The problem with these string abstractions is that nobody can ever agree on one (for instance whether strncpy's idiosyncrasies mentioned in comments above are good or bad, whether you need immutable and/or copy-on-write strings that share buffers when you create a substring, etc). So although in theory you should just take one off the shelf, you can end up with one per project.
I'd tend to use memcpy if I have already calculated the length, although strcpy is usually optimised to work on machine words, it feels that you should provide the library with as much information as you can, so it can use the most optimal copying mechanism.
But for the example you give, it doesn't matter - if it's going to fail, it will be in the initial strlen, so strncpy doesn't buy you anything in terms of safety (and presumbly strncpy is slower as it has to both check bounds and for nul), and any difference between memcpy and strcpy isn't worth changing code for speculatively.
The evil comes when people use it like this (although the below is super simplified):
void BadFunction(char *input)
{
char buffer[1024]; //surely this will **always** be enough
strcpy(buffer, input);
...
}
Which is a situation that happens suprising often.
But yeah, strcpy is as good as strncpy in any situation where you are allocating memory for the destination buffer and have already used strlen to find the length.
strlen finds upto last null terminating place.
But in reality buffers are not null terminated.
that's why people use different functions.
Well, strcpy() is not as evil as strdup() - at least strcpy() is part of Standard C.
In the situation you describe, strcpy is a good choice. This strdup will only get into trouble if the s1 was not ended with a '\0'.
I would add a comment indicating why there are no problems with strcpy, to prevent others (and yourself one year from now) wondering about its correctness for too long.
strncpy often seems safe, but may get you into trouble. If the source "string" is shorter than count, it pads the target with '\0' until it reaches count. That may be bad for performance. If the source string is longer than count, strncpy does not append a '\0' to the target. That is bound to get you into trouble later on when you expect a '\0' terminated "string". So strncpy should also be used with caution!
I would only use memcpy if I was not working with '\0' terminated strings, but that seems to be a matter of taste.
char *strdup(const char *s1)
{
char *s2 = malloc(strlen(s1)+1);
if(s2 == NULL)
{
return NULL;
}
strcpy(s2, s1);
return s2;
}
Problems:
s1 is unterminated, strlen causes the access of unallocated memory, program crashes.
s1 is unterminated, strlen while not causing the access of unallocated memory access memory from another part of your application. It's returned to the user (security issue) or parsed by another part of your program (heisenbug appears).
s1 is unterminated, strlen results in a malloc which the system can't satisfy, returns NULL. strcpy is passed NULL, program crashes.
s1 is unterminated, strlen results in a malloc which is very large, system allocs far too much memory to perform the task at hand, becomes unstable.
In the best case the code is inefficient, strlen requires access to every element in the string.
There are probably other problems... Look, null termination isn't always a bad idea. There are situations where, for computational efficiency, or to reduce storage requirements it makes sense.
For writing general purpose code, e.g. business logic does it make sense? No.
char* dupstr(char* str)
{
int full_len; // includes null terminator
char* ret;
char* s = str;
#ifdef _DEBUG
if (! str)
toss("arg 1 null", __WHENCE__);
#endif
full_len = strlen(s) + 1;
if (! (ret = (char*) malloc(full_len)))
toss("out of memory", __WHENCE__);
memcpy(ret, s, full_len); // already know len, so strcpy() would be slower
return ret;
}
This answer uses size_t and memcpy() for a fast and simple strdup().
Best to use type size_t as that is the type returned from strlen() and used by malloc() and memcpy(). int is not the proper type for these operations.
memcpy() is rarely slower than strcpy() or strncpy() and often significantly faster.
// Assumption: `s1` points to a C string.
char *strdup(const char *s1) {
size_t size = strlen(s1) + 1;
char *s2 = malloc(size);
if(s2 != NULL) {
memcpy(s2, s1, size);
}
return s2;
}
§7.1.1 1 "A string is a contiguous sequence of characters terminated by and including the first null character. ..."
Your code is terribly inefficient because it runs through the string twice to copy it.
Once in strlen().
Then again in strcpy().
And you don't check s1 for NULL.
Storing the length in some additional variable costs you about nothing, while running through each and every string twice to copy it is a cardinal sin.
I have a old program in which some library function is used and i dont have that library.
So I am writing that program using libraries of c++.
In that old code some function is there which is called like this
*string = newstrdup("Some string goes here");
the string variable is declared as char **string;
What he may be doing in that function named "newstrdup" ?
I tried many things but i dont know what he is doing ... Can anyone help
The function is used to make a copy of c-strings. That's often needed to get a writable version of a string literal. They (string literals) are itself not writable, so such a function copies them into an allocated writable buffer. You can then pass them to functions that modify their argument given, like strtok which writes into the string it has to tokenize.
I think you can come up with something like this, since it is called newstrdup:
char * newstrdup(char const* str) {
char *c = new char[std::strlen(str) + 1];
std::strcpy(c, str);
return c;
}
You would be supposed to free it once done using the string using
delete[] *string;
An alternative way of writing it is using malloc. If the library is old, it may have used that, which C++ inherited from C:
char * newstrdup(char const* str) {
char *c = (char*) malloc(std::strlen(str) + 1);
if(c != NULL) {
std::strcpy(c, str);
}
return c;
}
Now, you are supposed to free the string using free when done:
free(*string);
Prefer the first version if you are writing with C++. But if the existing code uses free to deallocate the memory again, use the second version. Beware that the second version returns NULL if no memory is available for dup'ing the string, while the first throws an exception in that case. Another note should be taken about behavior when you pass a NULL argument to your newstrdup. Depending on your library that may be allowed or may be not allowed. So insert appropriate checks into the above functions if necessary. There is a function called strdup available in POSIX systems, but that one allows neither NULL arguments nor does it use the C++ operator new to allocate memory.
Anyway, i've looked with google codesearch for newstrdup functions and found quite a few. Maybe your library is among the results:
Google CodeSearch, newstrdup
there has to be a reason that they wrote a "new" version of strdup. So there must be a corner case that it handles differently. like perhaps a null string returns an empty string.
litb's answer is a replacement for strdup, but I would think there is a reason they did what they did.
If you want to use strdup directly, use a define to rename it, rather than write new code.
The line *string = newstrdup("Some string goes here"); is not showing any weirdness to newstrdup. If string has type char ** then newstrdup is just returning char * as expected. Presumably string was already set to point to a variable of type char * in which the result is to be placed. Otherwise the code is writing through an uninitialized pointer..
newstrdup is probably making a new string that is a duplicate of the passed string; it returns a pointer to the string (which is itself a pointier to the characters).
It looks like he's written a strdup() function to operate on an existing pointer, probably to re-allocate it to a new size and then fill its contents. Likely, he's doing this to re-use the same pointer in a loop where *string is going to change frequently while preventing a leak on every subsequent call to strdup().
I'd probably implement that like string = redup(&string, "new contents") .. but that's just me.
Edit:
Here's a snip of my 'redup' function which might be doing something similar to what you posted, just in a different way:
int redup(char **s1, const char *s2)
{
size_t len, size;
if (s2 == NULL)
return -1;
len = strlen(s2);
size = len + 1;
*s1 = realloc(*s1, size);
if (*s1 == NULL)
return -1;
memset(*s1, 0, size);
memcpy(*s1, s2, len);
return len;
}
Of course, I should probably save a copy of *s1 and restore it if realloc() fails, but I didn't need to get that paranoid.
I think you need to look at what is happening with the "string" variable within the code as the prototype for the newstrdup() function would appear to be identical to the library strdup() version.
Are there any free(*string) calls in the code?
It would appear to be a strange thing do to, unless it's internally keeping a copy of the duplicated string and returning a pointer back to the same string again.
Again, I would ask why?
I have the following C code fragment and have to identify the error and suggest a way of writing it more safely:
char somestring[] = "Send money!\n";
char *copy;
copy = (char *) malloc(strlen(somestring));
strcpy(copy, somestring);
printf(copy);
So the error is that strlen ignores the trailing '\0' of a string and therefore it is not going to be allocated enough memory for the copy but I'm not sure what they're getting at about writing it more safely?
I could just use malloc(strlen(somestring)+1)) I assume but I'm thinking there must be a better way than that?
EDIT: OK, I've accepted an answer, I suspect that the strdup solution would not be expected from us as it's not part of ANSI C. It seems to be quite a subjective question so I'm not sure if what I've accepted is actually the best. Thanks anyway for all the answers.
I can't comment on the responses above, but in addition to checking the
return code and using strncpy, you should never do:
printf(string)
But use:
printf("%s", string);
ref: http://en.wikipedia.org/wiki/Format_string_attack
char somestring[] = "Send money!\n";
char *copy = strdup(something);
if (copy == NULL) {
// error
}
or just put this logic in a separate function xstrdup:
char * xstrdup(const char *src)
{
char *copy = strdup(src);
if (copy == NULL) {
abort();
}
return copy;
}
char somestring[] = "Send money!\n";
char *copy;
size_t copysize;
copysize = strlen(somestring)+1;
copy = (char *) malloc(copysize);
if (copy == NULL)
bail("Oh noes!\n");
strncpy(copy, somestring, copysize);
printf("%s", copy);
Noted differences above:
Result of malloc() must be checked!
Compute and store the memory size!
Use strncpy() because strcpy() is naughty. In this contrived example it won't hurt, but don't get into the habit of using it.
EDIT:
To those thinking I should be using strdup()... that only works if you take the very narrowest view of the question. That's not only silly, it's overlooking an even better answer:
char somestring[] = "Send money!\n";
char *copy = somestring;
printf(copy);
If you're going to be obtuse, at least be good at it.
strlen + 1, for the \0 terminator
malloc may fail; always check malloc return value
Ick... use strdup() like everyone else said and write it yourself if you have to. Since you have time to think about this now... check out the 25 Most Dangerous Programming Errors at Mitre, then consider why the phrase printf(copy) should never appear in code. That is right up there with malloc(strlen(str)) in terms of utter badness not to mention the headache of tracking down why it causes lots of grief when copy is something like "%s%n"...
I would comment to previous solutions but I do not have enough rep.
Using strncpy here is as wrong as using strcpy(As there is absolutely no risk of overflow). There is a function called memcpy in < string.h > and it is meant exactly for this. It is not only significantly faster, but also the correct function to use to copy strings of known length in standard C.
From the accepted answer:
char somestring[] = "Send money!\n";
char *copy;
size_t copysize;
copysize = strlen(somestring)+1;
copy = (char *) malloc(copysize);
if (copy == NULL)
bail("Oh noes!\n");
memcpy(copy, somestring, copysize); /* You don't use str* functions for this! */
printf("%s", copy);
to add more to Adrian McCarthy's ways to make safer code,
Use a static code analyzer, they are very good at finding this kind of errors
Ways to make the code safer (and more correct).
Don't make an unnecessary copy. From the example, there's no apparent requirement that you actually need to copy somestring. You can output it directly.
If you have to make a copy of a string, write a function to do it (or use strdup if you have it). Then you only have to get it right in one place.
Whenever possible, initialize the pointer to the copy immediately when you declare it.
Remember to allocate space for the null terminator.
Remember to check the return value from malloc.
Remember to free the malloc'ed memory.
Don't call printf with an untrusted format string. Use printf("%s", copy) or puts(copy).
Use an object-oriented language with a string class or any language with built-in string support to avoid most of these problems.
The best way to write it more safely, if one were to be truly interested in such a thing, would be to write it in Ada.
somestring : constant string := "Send money!";
declare
copy : constant string := somestring;
begin
put_line (somestring);
end;
Same result, so what are the differences?
The whole thing is done on the stack
(no pointers). Deallocation is
automatic and safe.
Everything is automaticly range-checked so
there's no chance of buffer-overflow
exploits
Both strings are constants,
so there's no chance of screwing up
and modifying them.
It will probably be way faster than the C, not only because of the lack of dynamic allocation, but because there isn't that extra scan through the string required by strlen().
Note that in Ada "string" is not some special dynamic construct. It's the built-in array of characters. However, Ada arrays can be sized at declaration by the array you assign into them.
The safer way would be to use strncpy instead of strcpy. That function takes a third argument: the length of the string to copy. This solution doesn't stretch beyond ANSI C, so this will work under all environments (whereas other methods may only work under POSIX-compliant systems).
char somestring[] = "Send money!\n";
char *copy;
copy = (char *) malloc(strlen(somestring));
strncpy(copy, somestring, strlen(somestring));
printf(copy);
What is the purpose of the strdup() function in C?
Exactly what it sounds like, assuming you're used to the abbreviated way in which C and UNIX assigns words, it duplicates strings :-)
Keeping in mind it's actually not part of the current (C17) ISO C standard itself(a) (it's a POSIX thing), it's effectively doing the same as the following code:
char *strdup(const char *src) {
char *dst = malloc(strlen (src) + 1); // Space for length plus nul
if (dst == NULL) return NULL; // No memory
strcpy(dst, src); // Copy the characters
return dst; // Return the new string
}
In other words:
It tries to allocate enough memory to hold the old string (plus a '\0' character to mark the end of the string).
If the allocation failed, it sets errno to ENOMEM and returns NULL immediately. Setting of errno to ENOMEM is something malloc does in POSIX so we don't need to explicitly do it in our strdup. If you're not POSIX compliant, ISO C doesn't actually mandate the existence of ENOMEM so I haven't included that here(b).
Otherwise the allocation worked so we copy the old string to the new string(c) and return the new address (which the caller is responsible for freeing at some point).
Keep in mind that's the conceptual definition. Any library writer worth their salary may have provided heavily optimised code targeting the particular processor being used.
One other thing to keep in mind, it looks like this is currently slated to be in the C2x iteration of the standard, along with strndup, as per draft N2912 of the document.
(a) However, functions starting with str and a lower case letter are reserved by the standard for future directions. From C11 7.1.3 Reserved identifiers:
Each header declares or defines all identifiers listed in its associated sub-clause, and optionally declares or defines identifiers listed in its associated future library directions sub-clause.*
The future directions for string.h can be found in C11 7.31.13 String handling <string.h>:
Function names that begin with str, mem, or wcs and a lowercase letter may be added to the declarations in the <string.h> header.
So you should probably call it something else if you want to be safe.
(b) The change would basically be replacing if (d == NULL) return NULL; with:
if (d == NULL) {
errno = ENOMEM;
return NULL;
}
(c) Note that I use strcpy for that since that clearly shows the intent. In some implementations, it may be faster (since you already know the length) to use memcpy, as they may allow for transferring the data in larger chunks, or in parallel. Or it may not :-) Optimisation mantra #1: "measure, don't guess".
In any case, should you decide to go that route, you would do something like:
char *strdup(const char *src) {
size_t len = strlen(src) + 1; // String plus '\0'
char *dst = malloc(len); // Allocate space
if (dst == NULL) return NULL; // No memory
memcpy (dst, src, len); // Copy the block
return dst; // Return the new string
}
char * strdup(const char * s)
{
size_t len = 1+strlen(s);
char *p = malloc(len);
return p ? memcpy(p, s, len) : NULL;
}
Maybe the code is a bit faster than with strcpy() as the \0 char doesn't need to be searched again (It already was with strlen()).
No point repeating the other answers, but please note that strdup() can do anything it wants from a C perspective, since it is not part of any C standard. It is however defined by POSIX.1-2001.
From strdup man:
The strdup() function shall return a pointer to a new string, which is a duplicate of the string pointed to by s1. The returned pointer can be passed to free(). A null pointer is returned if the new string cannot be created.
strdup() does dynamic memory allocation for the character array including the end character '\0' and returns the address of the heap memory:
char *strdup (const char *s)
{
char *p = malloc (strlen (s) + 1); // allocate memory
if (p != NULL)
strcpy (p,s); // copy string
return p; // return the memory
}
So, what it does is give us another string identical to the string given by its argument, without requiring us to allocate memory. But we still need to free it, later.
strdup and strndup are defined in POSIX compliant systems as:
char *strdup(const char *str);
char *strndup(const char *str, size_t len);
The strdup() function allocates sufficient memory for a copy of the
string str, does the copy, and returns a pointer to it.
The pointer may subsequently be used as an argument to the function free.
If insufficient memory is available, NULL is returned and errno is set to
ENOMEM.
The strndup() function copies at most len characters from the string str always null terminating the copied string.
It makes a duplicate copy of the string passed in by running a malloc and strcpy of the string passed in. The malloc'ed buffer is returned to the caller, hence the need to run free on the return value.
The most valuable thing it does is give you another string identical to the first, without requiring you to allocate memory (location and size) yourself. But, as noted, you still need to free it (but which doesn't require a quantity calculation, either.)
The statement:
strcpy(ptr2, ptr1);
is equivalent to (other than the fact this changes the pointers):
while(*ptr2++ = *ptr1++);
Whereas:
ptr2 = strdup(ptr1);
is equivalent to:
ptr2 = malloc(strlen(ptr1) + 1);
if (ptr2 != NULL) strcpy(ptr2, ptr1);
So, if you want the string which you have copied to be used in another function (as it is created in heap section), you can use strdup, else strcpy is enough,
The strdup() function is a shorthand for string duplicate, it takes in a parameter as a string constant or a string literal and allocates just enough space for the string and writes the corresponding characters in the space allocated and finally returns the address of the allocated space to the calling routine.