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.
Related
I am writing a piece of code that needs to copy values from one char pointer to another. The primary requirements are that the code should work as fast as possible and across as many platforms as possible.
I see that I can do this string copy using either of these two functions:
char * strncpy ( char * destination, const char * source, size_t num );
and
void * memcpy ( void * destination, const void * source, size_t num );
How can I determine what will be best for my requirements?
In general, how can I find out relative speeds of two different functions (across platforms)?
They don't do the same thing. If you're certain the source and destination buffers are the same size, go with memcpy. If not, strncpy does bound checking and it's the way to go.
But unless you do millions of operations, this shouldn't be a factor in your decision. Do some profiling first, see how much time (if any) you save by using memcpy.
From performance POV, memcpy is faster, as it doesn't check the content itself, just copying it.
And regarding compatibility - memcpy is part of the C standard.
If you have to copy strings (unless you know their size in advance) you should always use strncpy.
Even if underlying implementation is not so different memcpy is much faster because it does not have to check what it's copying, strncpy will stop when it'll copy the end of string character (NULL).
EDIT
Some considerations about performance of strncpy compared to memcpy.
We can suppose to have two - for example - 512 bytes buffers and we want to copy the content (a NULL terminatd string of - say - 32 characters) from the first buffer to the second one.
What strncpy will do: will copy, byte per byte, from the source to the target. It'll stop when it'll copy the NULL character and then it'll set everything else to 0. This is a possible implementation of this function:
char *strncpy(char *dest, const char *src, size_t n)
{
char *ret = dest;
do {
if (!n--)
return ret;
} while (*dest++ = *src++);
while (n--)
*dest++ = 0;
return ret;
}
What memcpy will do: will bulk copy everything from the source to the target. This is a possible implementation of this function:
void *memcpy(void *dest, const void *src, size_t n)
{
char *dp = dest;
const char *sp = src;
while (n--)
*dp++ = *sp++;
return dest;
}
Even if both versions are not optimized you can see that it performs much less operations but it can be slower only when the string length is much shorter than the total buffer length (it'll save the *sp++ in the second while). So even testing for performance isn't reliable because it depends on the real inputs (so you should have some sort of statistical data from real world).
Considerations
Moreover we should remember that true implementations are not so simple!!! Look this implementation of strncpy:
http://sourceware.org/git/?p=glibc.git;a=blob;f=string/strncpy.c;h=f6ee27832da95d9da9aef8a6fcf73f53f997c796;hb=HEAD
Now look at memcpy implementation:
http://sourceware.org/git/?p=glibc.git;a=blob;f=string/memcpy.c;h=3080fcb4de4cec83a57b65bf07995a1e41abb1f6;hb=HEAD
memcpy will always be faster than strncpy for any real world situation even in the corner case I spoke before (look PAGE_COPY_FWD_MAYBE).
Note
It's not your case because you will use it massively but in general I think that if you have to copy a string then use strncpy. Always. Probably you do not need any performance boost there, it'll handle NULL terminator for you and it'll be always clear you're copying a string to whom will read your code.
Your case is different because you really do a lot of computation then if the buffer size is known you should use memcpy because it could be much much much more faster (specially if the buffer isn't tiny).
References
Demo implementations from: http://clc-wiki.net/wiki/
Performance test for memcpy: http://eetimes.com/design/embedded/4024961/Optimizing-Memcpy-improves-speed
The functions you select here have different functionality. Chose the one that you need.
memcpy will copy everything and both buffers need to be of the same size
strncpy will copy from a smaller c-string and fill the rest of the target buffer with /0
strcpy will copy only the string from target to destination (but you should use one of the safe variants).
Regarding performance: memcpy vs. strncpy - memcpy is faster but you need to know the size of the input string. strcpy may be the fastest if your string sizes vary much because it determines the required size while copying.
In C I have a path in one of my strings
/home/frankv/
I now want to add the name of files that are contained in this folder - e.g. file1.txt file123.txt etc.
Having declared my variable either like this
char pathToFile[strlen("/home/frankv/")+1]
or
char *pathToFile = malloc(strlen("/home/frankv/")+1)
My problem is that I cannot simply add more characters because it would cause a buffer overflow. Also, what do I do in case I do not know how long the filenames will be?
I've really gotten used to PHP lazy $string1.$string2 .. What is the easiest way to do this in C?
If you've allocated a buffer with malloc(), you can use realloc() to expand it:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
char *buf;
const char s1[] = "hello";
const char s2[] = ", world";
buf = malloc(sizeof s1);
strcpy(buf, s1);
buf = realloc(buf, sizeof s1 + sizeof s2 - 1);
strcat(buf, s2);
puts(buf);
return 0;
}
NOTE: I have omitted error checking. You shouldn't. Always check whether malloc() returns a null pointer; if it does, take some corrective action, even if it's just terminating the program. Likewise for realloc(). And if you want to be able to recover from a realloc() failure, store the result in a temporary so you don't clobber your original pointer.
Use std::string, if possible. Else, reallocate another block of memory and use strcpy and strcat.
You have a couple options, but, if you want to do this dynamically using no additional libraries, realloc() is the stdlib function you're looking for:
char *pathToFile = malloc(strlen("/home/frankv/")+1);
char *string_to_add = "filename.txt";
char *p = realloc(pathToFile, strlen(pathToFile) + strlen(string_to_add) + 1);
if (!p) abort();
pathToFile = p;
strcat(p, string_to_add);
Note: you should always assign the result of realloc to a new pointer first, as realloc() returns NULL on failure. If you assign to the original pointer, you are begging for a memory leak.
If you're going to be doing much string manipulation, though, you may want to consider using a string library. Two I've found useful are bstring and ustr.
In case you can use C++, use the std::string. In case you must to use pure C, use what's call doubling - i.e. when out of space in the string - double the memory and copy the string into the new memory. And you'll have to use the second syntax:
char *pathToFile = malloc(strlen("/home/frankv/")+1);
You have chosen the wrong language for manipulating strings!
The easy and conventional way out is to do something like:
#define MAX_PATH 260
char pathToFile[MAX_PATH+1] = "/home/frankv/";
strcat(pathToFile, "wibble/");
Of course, this is error prone - if the resulting string exceeds MAX_PATH characters, anything can happen, and it is this sort of programming which is the route many trojans and worms use to penetrate security (by corrupting memory in a carefully defined way). Hence my deliberate choice of 260 for MAX_PATH, which is what it used to be in Windows - you can still make Windows Explorer do strange things to your files with paths over 260 characters, possibly because of code like this!
strncat may be a small help - you can at least tell it the maximum size of the destination, and it won't copy beyond that.
To do it robustly you need a string library which does variable length strings correctly. But I don't know if there is such a thing for C (C++ is a different matter, of course).
Is there a possibility that strcat can ever fail?
If we pass some incorrect buffer or string, then it might lead to memory corruption. But, apart from that is it possible that this function can return failure like strcat returning NULL even if destination string passed is Non-NULL? If no, why strcat has a return type specified at all?
I have just mentioned strcat as an example. But, this question applies to many string and memory related (like memcpy etc) functions. I just want to know the reasoning behind some of these seemingly "always successful" functions having return types.
Returning a pointer to the target string makes it easy to use the output in this sort of (perhaps not-so-clever) way:
int len = strlen(strcat(firstString, secondString));
Most of them go back to a time when C didn't include 'void', so there was no way to specify that it had no return value. As a result, they specified them to return something, even if it was pretty useless.
The implicit contract of these functions is the following: if you pass-in pointers to valid strings, then the functions will perform as advertised. Pass-in a NULL pointer, and the function may do anything (usually, it will raise a SIGSEGV). Given that the arguments are valid (i.e., point to strings) then the algorithms used can not fail.
I always ignored the return types (wondering who uses them) until today I saw this in glibc-2.11 (copied exactly from the source file) and everything became much more clear:
wchar_t *
wcsdup (s)
const wchar_t *s;
{
size_t len = (__wcslen (s) + 1) * sizeof (wchar_t);
void *new = malloc (len);
if (new == NULL)
return NULL;
return (wchar_t *) memcpy (new, (void *) s, len);
}
It makes it easier to write less code ("chain" it?) I guess.
Here's a pretty standard implementation of strcat from OpenBSD:
char *
strcat(char *s, const char *append)
{
char *save = s;
for (; *s; ++s);
while ((*s++ = *append++) != '\0');
return(save);
}
As long as the inputs passed to it are valid (i.e. append is properly terminated and s is large enough to concatenate it), this can't really fail - it's a simple memory manipulation. That memory is entirely under the control of the caller.
The return value here could be used to chain concatenations, for example:
strcat(strcat(s, t1), t2);
Although this is hardly efficient...
I will be coaching an ACM Team next month (go figure), and the time has come to talk about strings in C. Besides a discussion on the standard lib, strcpy, strcmp, etc., I would like to give them some hints (something like str[0] is equivalent to *str, and things like that).
Do you know of any lists (like cheat sheets) or your own experience in the matter?
I'm already aware of the books for the ACM competition (which are good, see particularly this), but I'm after tricks of the trade.
Thank you.
Edit: Thank you very much everybody. I will accept the most voted answer, and have duly upvoted others which I think are relevant. I expect to do a summary here (like I did here, asap). I have enough material now and I'm certain this has improved the session on strings immensely. Once again, thanks.
It's obvious but I think it's important to know that strings are nothing more than an array of bytes, delimited by a zero byte.
C strings aren't all that user-friendly as you probably know.
Writing a zero byte somewhere in the string will truncate it.
Going out of bounds generally ends bad.
Never, ever use strcpy, strcmp, strcat, etc.., instead use their safe variants: strncmp, strncat, strndup,...
Avoid strncpy. strncpy will not always zero delimit your string! If the source string doesn't fit in the destination buffer it truncates the string but it won't write a nul byte at the end of the buffer. Also, even if the source buffer is a lot smaller than the destination, strncpy will still overwrite the whole buffer with zeroes. I personally use strlcpy.
Don't use printf(string), instead use printf("%s", string). Try thinking of the consequences if the user puts a %d in the string.
You can't compare strings with if( s1 == s2 )
doStuff(s1);
You have to compare every character in the string. Use strcmp or better strncmp.
if( strncmp( s1, s2, BUFFER_SIZE ) == 0 )
doStuff(s1);
Abusing strlen() will dramatically worsen the performance.
for( int i = 0; i < strlen( string ); i++ ) {
processChar( string[i] );
}
will have at least O(n2) time complexity whereas
int length = strlen( string );
for( int i = 0; i < length; i++ ) {
processChar( string[i] );
}
will have at least O(n) time complexity. This is not so obvious for people who haven't taken time to think of it.
The following functions can be used to implement a non-mutating strtok:
strcspn(string, delimiters)
strspn(string, delimiters)
The first one finds the first character in the set of delimiters you pass in. The second one finds the first character not in the set of delimiters you pass in.
I prefer these to strpbrk as they return the length of the string if they can't match.
str[0] is equivalent to 0[str], or more generally str[i] is i[str] and i[str] is *(str + i).
NB
this is not specific to strings but it works also for C arrays
The strn* variants in stdlib do not necessarily null terminate the destination string.
As an example: from MSDN's documentation on strncpy:
The strncpy function copies the
initial count characters of strSource
to strDest and returns strDest. If
count is less than or equal to the
length of strSource, a null character
is not appended automatically to the
copied string. If count is greater
than the length of strSource, the
destination string is padded with null
characters up to length count.
confuse strlen() with sizeof() when using a string:
char *p = "hello!!";
strlen(p) != sizeof(p)
sizeof(p) yield, at compile time, the size of the pointer (4 or 8 bytes) whereas strlen(p) counts, at runtime, the lenght of the null terminated char array (7 in this example).
strtok is not thread safe, since it uses a mutable private buffer to store data between calls; you cannot interleave or annidate strtok calls also.
A more useful alternative is strtok_r, use it whenever you can.
kmm has already a good list. Here are the things I had problems with when I started to code C.
String literals have an own memory section and are always accessible. Hence they can for example be a return value of function.
Memory management of strings, in particular with a high level library (not libc). Who is responsible to free the string if it is returned by function or passed to a function?
When should "const char *" and when "char *" be used. And what does it tell me if a function returns a "const char *".
All these questions are not too difficult to learn, but hard to figure out if you don't get taught them.
I have found that the char buff[0] technique has been incredibly useful.
Consider:
struct foo {
int x;
char * payload;
};
vs
struct foo {
int x;
char payload[0];
};
see https://stackoverflow.com/questions/295027
See the link for implications and variations
I'd point out the performance pitfalls of over-reliance on the built-in string functions.
char* triple(char* source)
{
int n=strlen(source);
char* dest=malloc(n*3+1);
strcpy(dest,src);
strcat(dest,src);
strcat(dest,src);
return dest;
}
I would discuss when and when not to use strcpy and strncpy and what can go wrong:
char *strncpy(char* destination, const char* source, size_t n);
char *strcpy(char* destination, const char* source );
I would also mention return values of the ansi C stdlib string functions. For example ask "does this if statement pass or fail?"
if (stricmp("StrInG 1", "string 1")==0)
{
.
.
.
}
perhaps you could illustrate the value of sentinel '\0' with following example
char* a = "hello \0 world";
char b[100];
strcpy(b,a);
printf(b);
I once had my fingers burnt when in my zeal I used strcpy() to copy binary data. It worked most of the time but failed mysteriously sometimes. Mystery was revealed when I realized that binary input sometimes contained a zero byte and strcpy() would terminate there.
You could mention indexed addressing.
An elements address is the base address + index * sizeof element
A common error is:
char *p;
snprintf(p, 3, "%d", 42);
it works until you use up to sizeof(p) bytes.. then funny things happens (welcome to the jungle).
Explaination
with char *p you are allocating space for holding a pointer (sizeof(void*) bytes) on the stack. The right thing here is to allocate a buffer or just to specify the size of the pointer at compile time:
char buf[12];
char *p = buf;
snprintf(p, sizeof(buf), "%d", 42);
Pointers and arrays, while having the similar syntax, are not at all the same. Given:
char a[100];
char *p = a;
For the array, a, there is no pointer stored anywhere. sizeof(a) != sizeof(p), for the array it is the size of the block of memory, for the pointer it is the size of the pointer. This become important if you use something like: sizeof(a)/sizeof(a[0]). Also, you can't ++a, and you can make the pointer a 'const' pointer to 'const' chars, but the array can only be 'const' chars, in which case you'd be init it first. etc etc etc
If possible, use strlcpy (instead of strncpy) and strlcat.
Even better, to make life a bit safer, you can use a macro such as:
#define strlcpy_sz(dst, src) (strlcpy(dst, src, sizeof(dst)))
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);