Properties of strcpy() - c

I have a global definition as following:
#define globalstring "example1"
typedef struct
{
char key[100];
char trail[10][100];
bson_value_t value;
} ObjectInfo;
typedef struct
{
ObjectInfo CurrentOrderInfoSet[5];
} DataPackage;
DataPackage GlobalDataPackage[10];
And I would like to use the strcpy() function in some of my functions as following:
strcpy(GlobalDataPackage[2].CurrentOrderInfoSet[0].key, "example2");
char string[100] = "example3";
strcpy(GlobalDataPackage[2].CurrentOrderInfoSet[0].key, string);
strcpy(GlobalDataPackage[2].CurrentOrderInfoSet[0].key, globalstring);
First question: Are the global defined strings all initiated with 100 times '\0'?
Second qestion: I am a bit confused as to how exactly strcpy() works. Does it only overwrite the characters necessary to place the source string into the destination string plus a \0 at the end and leave the rest as it is or does it fully delete any content of the destination string prior to that?
Third question: All my strings are fixed length of 100. If I use the 3 examples of strcpy() above, with my strings not exceeding 99 characters, does strcpy() properly overwrite the destination string and NULL terminate it? Meaning do I run into problems when using functions like strlen(), printf() later?
Fourth question: What happens when I strcpy() empty strings?
I plan to overwrite these strings in loops various times and would like to know if it would be safer to use memset() to fully "empty" the strings prior to strcpy() on every iteration.
Thx.

Are the global defined strings all initiated with 100 times '\0'?
Yes. Global char arrays will be initilizated to all zeros.
I am a bit confused as to how exactly strcpy() works. Does it only overwrite the characters necessary to place the source string into the destination string plus a \0 at the end and leave the rest as it
Exactly. It copies the characters up until and including '\0' and does not care about the rest.
If I use ... my strings not exceeding 99 characters, does strcpy() properly overwrite the destination string and NULL terminate it?
Yes, but NULL is a pointer, it's terminated with zero byte, sometimes called NUL. You might want to see What is the difference between NUL and NULL? .
Meaning do I run into problems when using functions like strlen(), printf() later?
Not if your string lengths are less than or equal to 99.
What happens when I strcpy() empty strings?
It just copies one zero byte.
would like to know if it would be safer to use memset() to fully "empty" the strings prior to strcpy() on every iteration.
Safety is a broad concept. As far as safety as in if the program will execute properly, there is no point in caring about anything after zero byte, so just strcpy it.
But you should check if your strings are less than 99 characters and handle what to do it they are longer. You might be interested in strnlen, but the interface is confusing - I recommend to use memcpy + explicitly manually set zero byte.

Related

A little query, String in C

Recently I was programming in my Code Blocks and I did a little program only for hobby in C.
char littleString[1];
fflush( stdin );
scanf( "%s", littleString );
printf( "\n%s", littleString);
If I created a string of one character, why does the CodeBlocks allow me to save 13 characters?
C have no bounds-checking, writing out of bounds of arrays or dynamically allocated memory can't be checked by the compiler. Instead it will lead to undefined behavior.
To prevent buffer overflow with scanf you can tell it to only read a specific number of characters, and nothing more. So to tell it to read only one character you use the format "%1s".
As a small side-note: Remember that strings in C have an extra character in them, the terminator (character '\0'). So if you have a string that should contain one character, the size actually needs to be two characters.
LittleString is not a string. It is a char array of length one. In order for a char array to be a string, it must be null terminated with an \0. You are writing past the memory you have allotted for littleString. This is undefined behavior.Scanf just reads user input from the console and assigns it to the variable specified, in this case littleString. If you would like to control the length of user input which is assigned to the variable, I would suggest using scanf_s. Please note that scanf_s is not a C99 standard
Many functions in C is implemented without any checks for correctness of use. In other words, it is the callers responsibility that the arguments fulfill some rules set by the function.
Example: For strcpy the Linux man page says
The strcpy() function copies the string pointed to by src,
including the terminating null byte ('\0'), to the buffer
pointed to by dest. The strings may not overlap, and the
destination string dest must be large enough to receive the copy.
If you as a caller break that contract by passing a too small buffer, you'll have undefined behavior and anything can happen.
The program may crash or even do exactly what you expected in 99 out of 100 times and do something strange in 1 out of 100 times.

Is this a valid way to add two strings together in c?

I have a function taking two strings string_one and string_two both a pointer to a character.
I thought of a way to add them together:
while (*string_one){
string_one++;
}
*string_one = *string_two;
but I can't see the second string in the output!
How do I add two strings toghether? did I go any close?
When you write
*string_one = *string_two;
you are just copying one character since you are de-referencing a char pointer
to add two strings to one another you need to overwrite the \0 on the first string and then append the characters from the second string (provided you have space enough to do so).
so this
while (*string_one) {
string_one++;
}
*string_one = *string_two;
will only overwrite the \0 with the first character from string_two which will result in the first string will not be null terminated any longer.
instead you should so do something similar again like
while (*string_one) {
string_one++;
}
while (*string_two) {
*string_one++ = *string_two++;
}
*string_one = '\0';
again with the premise that string_one originally pointed to a character string large enough to hold both strings.
Did you try your code at all?
First, you need to make sure that there is enough space for both strings. Concatenating strings and not checking that there is enough space for both is probably the cause of 50% of all hacked computers in the world.
Second, a C string is an array of char with a zero byte as the last char. Your code overwrites the zero byte at the end of string_one with the first char of string_two.
Third, there is a function named strcat in the standard C library doing exactly what you want to do. It doesn't check whether there is enough space, you have to do that before the call.
I'm afraid, your code is not valid.
*string_one = *string_two;
will copy the first element of string_two to string_one. This never adds anything. Moreover, you need to append the complete string, not only one element.
What you need is strcat(). You can find more on that here.
The general description :
The strcat() function appends the src string to the dest string, overwriting the terminating null byte (\0) at the end of dest, and then adds a terminating null byte. The strings may not overlap, and the dest string must have enough space for the result. If dest is not large enough, program behavior is unpredictable.

ANSI C strncpy messing up screen output and other variables' values

Using ANSI C, screen is messing up after the strncpy. Also if I try to print any int variable values become incorrect. However if I move the print line before strncpy everything is fine.
Does anybody know why?
#define TICKET_NAME_LEN 40
struct stock_data
{
char ticket_name[TICKET_NAME_LEN+1];
};
struct stock_data user_input;
char tname[TICKET_NAME_LEN+1] = "testing it";
strncpy(user_input.ticket_name, tname, TICKET_NAME_LEN);
The symptoms you are describing are the classic ones for a copy that is out of control. However, the real source of your problem is almost certainly not in the code you show.
The only possible issue with the code you show is that strncpy() does not guarantee that the output (target) string is null terminated. This won't hurt with the code shown (it doesn't do anything untoward), but other code that expects the string to be null terminated that blithely copies it without ensuring that there's space may go trampling other memory because the string is not null terminated.
If the input (source) string is longer than the space specified (in this case more than TICKET_NAME_LEN bytes long), then user_input.ticket_name will not be null terminated except by accident. If it is shorter, then user_input.ticket_name will be null padded to the length TICKET_NAME_LEN bytes.
If this is the problem, a very simple fix is to add the line:
user_input.ticket_name[TICKET_NAME_LEN] = '\0';
after (or even before, but it is more conventional to do it after) the strncpy().
However, to run into this problem, you'd have to be trying to copy a name of 41 or more characters into the ticket name member of the structure.
It is much more likely that something else is the cause of your trouble.
ISO/IEC 9899:2011 §7.24.2.4 The strncpy function
¶2 The strncpy function copies not more than n characters (characters that follow a null
character are not copied) from the array pointed to by s2 to the array pointed to by
s1.308) If copying takes place between objects that overlap, the behavior is undefined.
¶3 If the array pointed to by s2 is a string that is shorter than n characters, null characters
are appended to the copy in the array pointed to by s1, until n characters in all have been
written.
308) Thus, if there is no null character in the first n characters of the array pointed to by s2, the result will not be null-terminated.

Strings without a '\0' char?

If by mistake,I define a char array with no \0 as its last character, what happens then?
I'm asking this because I noticed that if I try to iterate through the array with while(cnt!='\0'), where cnt is an int variable used as an index to the array, and simultaneously print the cnt values to monitor what's happening the iteration stops at the last character +2.The extra characters are of course random but I can't get it why it has to stop after 2.Does the compiler automatically inserts a \0 character? Links to relevant documentation would be appreciated.
To make it clear I give an example. Let's say that the array str contains the word doh(with no '\0'). Printing the cnt variable at every loop would give me this:
doh+
or doh^
and so on.
EDIT (undefined behaviour)
Accessing array elements outside of the array boundaries is undefined behaviour.
Calling string functions with anything other than a C string is undefined behaviour.
Don't do it!
A C string is a sequence of bytes terminated by and including a '\0' (NUL terminator). All the bytes must belong to the same object.
Anyway, what you see is a coincidence!
But it might happen like this
,------------------ garbage
| ,---------------- str[cnt] (when cnt == 4, no bounds-checking)
memory ----> [...|d|o|h|*|0|0|0|4|...]
| | \_____/ -------- cnt (big-endian, properly 4-byte aligned)
\___/ ------------------ str
If you define a char array without the terminating \0 (called a "null terminator"), then your string, well, won't have that terminator. You would do that like so:
char strings[] = {'h', 'e', 'l', 'l', 'o'};
The compiler never automatically inserts a null terminator in this case. The fact that your code stops after "+2" is a coincidence; it could just as easily stopped at +50 or anywhere else, depending on whether there happened to be \0 character in the memory following your string.
If you define a string as:
char strings[] = "hello";
Then that will indeed be null-terminated. When you use quotation marks like that in C, then even though you can't physically see it in the text editor, there is a null terminator at the end of the string.
There are some C string-related functions that will automatically append a null-terminator. This isn't something the compiler does, but part of the function's specification itself. For example, strncat(), which concatenates one string to another, will add the null terminator at the end.
However, if one of the strings you use doesn't already have that terminator, then that function will not know where the string ends and you'll end up with garbage values (or a segmentation fault.)
In C language the term string refers to a zero-terminated array of characters. So, pedantically speaking there's no such thing as "strings without a '\0' char". If it is not zero-terminated, it is not a string.
Now, there's nothing wrong with having a mere array of characters without any zeros in it, as long as you understand that it is not a string. If you ever attempt to work with such character array as if it is a string, the behavior of your program is undefined. Anything can happen. It might appear to "work" for some magical reasons. Or it might crash all the time. It doesn't really matter what such a program will actually do, since if the behavior is undefined, the program is useless.
This would happen if, by coincidence, the byte at *(str + 5) is 0 (as a number, not ASCII)
As far as most string-handling functions are concerned, strings always stop at a '\0' character. If you miss this null-terminator somewhere, one of three things will usually happen:
Your program will continue reading past the end of the string until it finds a '\0' that just happened to be there. There are several ways for such a character to be there, but none of them is usually predictable beforehand: it could be part of another variable, part of the executable code or even part of a larger string that was previously stored in the same buffer. Of course by the time that happens, the program may have processed a significant amount of garbage. If you see lots of garbage produced by a printf(), an unterminated string is a common cause.
Your program will continue reading past the end of the string until it tries to read an address outside its address space, causing a memory error (e.g. the dreaded "Segmentation fault" in Linux systems).
Your program will run out of space when copying over the string and will, again, cause a memory error.
And, no, the C compiler will not normally do anything but what you specify in your program - for example it won't terminate a string on its own. This is what makes C so powerful and also so hard to code for.
I bet that an int is defined just after your string and that this int takes only small values such that at least one byte is 0.

Questions on C strings

I am new to C and I am very much confused with the C strings. Following are my questions.
Finding last character from a string
How can I find out the last character from a string? I came with something like,
char *str = "hello";
printf("%c", str[strlen(str) - 1]);
return 0;
Is this the way to go? I somehow think that, this is not the correct way because strlen has to iterate over the characters to get the length. So this operation will have a O(n) complexity.
Converting char to char*
I have a string and need to append a char to it. How can i do that? strcat accepts only char*. I tried the following,
char delimiter = ',';
char text[6];
strcpy(text, "hello");
strcat(text, delimiter);
Using strcat with variables that has local scope
Please consider the following code,
void foo(char *output)
{
char *delimiter = ',';
strcpy(output, "hello");
strcat(output, delimiter);
}
In the above code,delimiter is a local variable which gets destroyed after foo returned. Is it OK to append it to variable output?
How strcat handles null terminating character?
If I am concatenating two null terminated strings, will strcat append two null terminating characters to the resultant string?
Is there a good beginner level article which explains how strings work in C and how can I perform the usual string manipulations?
Any help would be great!
Last character: your approach is correct. If you will need to do this a lot on large strings, your data structure containing strings should store lengths with them. If not, it doesn't matter that it's O(n).
Appending a character: you have several bugs. For one thing, your buffer is too small to hold another character. As for how to call strcat, you can either put the character in a string (an array with 2 entries, the second being 0), or you can just manually use the length to write the character to the end.
Your worry about 2 nul terminators is unfounded. While it occupies memory contiguous with the string and is necessary, the nul byte at the end is NOT "part of the string" in the sense of length, etc. It's purely a marker of the end. strcat will overwrite the old nul and put a new one at the very end, after the concatenated string. Again, you need to make sure your buffer is large enough before you call strcat!
O(n) is the best you can do, because of the way C strings work.
char delimiter[] = ",";. This makes delimiter a character array holding a comma and a NUL Also, text needs to have length 7. hello is 5, then you have the comma, and a NUL.
If you define delimiter correctly, that's fine (as is, you're assigning a character to a pointer, which is wrong). The contents of output won't depend on delimiter later on.
It will overwrite the first NUL.
You're on the right track. I highly recommend you read K&R C 2nd Edition. It will help you with strings, pointers, and more. And don't forget man pages and documentation. They will answer questions like the one on strcat quite clearly. Two good sites are The Open Group and cplusplus.com.
A "C string" is in reality a simple array of chars, with str[0] containing the first character, str[1] the second and so on. After the last character, the array contains one more element, which holds a zero. This zero by convention signifies the end of the string. For example, those two lines are equivalent:
char str[] = "foo"; //str is 4 bytes
char str[] = {'f', 'o', 'o', 0};
And now for your questions:
Finding last character from a string
Your way is the right one. There is no faster way to know where the string ends than scanning through it to find the final zero.
Converting char to char*
As said before, a "string" is simply an array of chars, with a zero terminator added to the end. So if you want a string of one character, you declare an array of two chars - your character and the final zero, like this:
char str[2];
str[0] = ',';
str[1] = 0;
Or simply:
char str[2] = {',', 0};
Using strcat with variables that has local scope
strcat() simply copies the contents of the source array to the destination array, at the offset of the null character in the destination array. So it is irrelevant what happens to the source after the operation. But you DO need to worry if the destination array is big enough to hold the data - otherwise strcat() will overwrite whatever data sits in memory right after the array! The needed size is strlen(str1) + strlen(str2) + 1.
How strcat handles null terminating character?
The final zero is expected to terminate both input strings, and is appended to the output string.
Finding last character from a string
I propose a thought experiment: if it were generally possible to find the last character
of a string in better than O(n) time, then could you not also implement strlen
in better than O(n) time?
Converting char to char*
You temporarily can store the char in an array-of-char, and that will decay into
a pointer-to-char:
char delimiterBuf[2] = "";
delimiterBuf[0] = delimiter;
...
strcat(text, delimiterBuf);
If you're just using character literals, though, you can simply use string literals instead.
Using strcat with variables that has local scope
The variable itself isn't referenced outside the scope. When the function returns,
that local variable has already been evaluated and its contents have already been
copied.
How strcat handles null terminating character?
"Strings" in a C are NUL-terminated sequences of characters. Both inputs to
strcat must be NUL-terminated, and the result will be NUL-terminated. It
wouldn't be useful for strcat to write an extra NUL-byte to the result if it
doesn't need to.
(And if you're wondering what if the input strings have multiple trailing
NUL bytes already, I propose another thought experiment: how would strcat know
how many trailing NUL-bytes there are in a string?)
BTW, since you tagged this with "best-practices", I'll also recommend that you take care not to write past the end of your destination buffers. Typically this means avoiding strcat and strcpy (unless you've already checked that the input strings won't overflow the destination) and using safer versions (e.g. strncat. Note that strncpy has its own pitfalls, so that's a poor substitute. There also are safer versions that are non-standard, such as strlcpy/strlcat and strcpy_s/strcat_s.)
Similarly, functions like your foo function always should take an additional argument specifying what the size of the destination buffer is (and documentation should make it explicitly clear whether that size accounts for a NUL terminator or not).
How can I find out the last character
from a string?
Your technique with str[strlen(str) - 1] is fine. As pointed out, you should avoid repeated, unnecessary calls to strlen and store the results.
I somehow think that, this is not the
correct way because strlen has to
iterate over the characters to get the
length. So this operation will have a
O(n) complexity.
Repeated calls to strlen can be a bane of C programs. However, you should avoid premature optimization. If a profiler actually demonstrates a hotspot where strlen is expensive, then you can do something like this for your literal string case:
const char test[] = "foo";
sizeof test // 4
Of course if you create 'test' on the stack, it incurs a little overhead (incrementing/decrementing stack pointer), but no linear time operation involved.
Literal strings are generally not going to be so gigantic. For other cases like reading a large string from a file, you can store the length of the string in advance as but one example to avoid recomputing the length of the string. This can also be helpful as it'll tell you in advance how much memory to allocate for your character buffer.
I have a string and need to append a
char to it. How can i do that? strcat
accepts only char*.
If you have a char and cannot make a string out of it (char* c = "a"), then I believe you can use strncat (need verification on this):
char ch = 'a';
strncat(str, &ch, 1);
In the above code,delimiter is a local
variable which gets destroyed after
foo returned. Is it OK to append it to
variable output?
Yes: functions like strcat and strcpy make deep copies of the source string. They don't leave shallow pointers behind, so it's fine for the local data to be destroyed after these operations are performed.
If I am concatenating two null
terminated strings, will strcat
append two null terminating characters
to the resultant string?
No, strcat will basically overwrite the null terminator on the dest string and write past it, then append a new null terminator when it's finished.
How can I find out the last character from a string?
Your approach is almost correct. The only way to find the end of a C string is to iterate throught the characters, looking for the nul.
There is a bug in your answer though (in the general case). If strlen(str) is zero, you access the character before the start of the string.
I have a string and need to append a char to it. How can i do that?
Your approach is wrong. A C string is just an array of C characters with the last one being '\0'. So in theory, you can append a character like this:
char delimiter = ',';
char text[7];
strcpy(text, "hello");
int textSize = strlen(text);
text[textSize] = delimiter;
text[textSize + 1] = '\0';
However, if I leave it like that I'll get zillions of down votes because there are three places where I have a potential buffer overflow (if I didn't know that my initial string was "hello"). Before doing the copy, you need to put in a check that text is big enough to contain all the characters from the string plus one for the delimiter plus one for the terminating nul.
... delimiter is a local variable which gets destroyed after foo returned. Is it OK to append it to variable output?
Yes that's fine. strcat copies characters. But your code sample does no checks that output is big enough for all the stuff you are putting into it.
If I am concatenating two null terminated strings, will strcat append two null terminating characters to the resultant string?
No.
I somehow think that, this is not the correct way because strlen has to iterate over the characters to get the length. So this operation will have a O(n) complexity.
You are right read Joel Spolsky on why C-strings suck. There are few ways around it. The ways include either not using C strings (for example use Pascal strings and create your own library to handle them), or not use C (use say C++ which has a string class - which is slow for different reasons, but you could also write your own to handle Pascal strings more easily than in C for example)
Regarding adding a char to a C string; a C string is simply a char array with a nul terminator, so long as you preserve the terminator it is a string, there's no magic.
char* straddch( char* str, char ch )
{
char* end = &str[strlen(str)] ;
*end = ch ;
end++ ;
*end = 0 ;
return str ;
}
Just like strcat(), you have to know that the array that str is created in is long enough to accommodate the longer string, the compiler will not help you. It is both inelegant and unsafe.
If I am concatenating two null
terminated strings, will strcat append
two null terminating characters to the
resultant string?
No, just one, but what ever follows that may just happen to be nul, or whatever happened to be in memory. Consider the following equivalent:
char* my_strcat( char* s1, const char* s2 )
{
strcpy( &str[strlen(str)], s2 ) ;
}
the first character of s2 overwrites the terminator in s1.
In the above code,delimiter is a local
variable which gets destroyed after
foo returned. Is it OK to append it to
variable output?
In your example delimiter is not a string, and initialising a pointer with a char makes no sense. However if it were a string, the code would be fine, strcat() copies the data from the second string, so the lifetime of the second argument is irrelevant. Of course you could in your example use a char (not a char*) and the straddch() function suggested above.

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