strlen() of an empty array within a struct is not 0 - c

I'm very new to C, and I'm not understanding this behavior. Upon printing the length of this empty array I get 3 instead of 0.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct entry entry;
struct entry{
char arr[16];
};
int main(){
entry a;
printf("%d\n",strlen(a.arr));
return 0;
}
What am I not understanding here?

The statement entry a; does not initialize the struct, so its value is likely garbage. Therefore, there's no guarantee that strlen on any of its members will return anything sensible. In fact, it might even crash the program, or worse.

There is no such thing as an "empty array" in C. Your array of char[16]; always contains 16 bytes - uninitialized as a local variable each char has an unspecified value. In addition, if none of these unspecified values happen to be 0, strlen will read outside the array and your code will have undefined behaviour.
Additionally strlen returns size_t and using %d to print this has undefined behaviour too; you must use %zu where z says that the corresponding argument is size_t.
(If by happenstance you're using the MSVC++ "C" compiler, do note that it might not support %zu. Get a real C compiler and C standard library instead.)

Here's the source code to strlen():
size_t strlen(const char *str)
{
const char *s;
for (s = str; *s; ++s);
return(s - str);
}
Wait, you mean there's source code to strlen()? Why yes. All the standard functions in C are themselves written in C.
This function starts at the memory address specified by str. It then uses the for function to start at that address, and then it goes forward, byte by byte, until it reaches zero. How does that for function do that? Well first it assigns s to str. Then, it checks the value s points to. If it's zero (i.e. if *s returns zero) then the for loop is done. If that value is not zero, the s pointer is incremented, and the zero check is done, over and over, until it finds a zero.
Finally, the distance that the s pointer has moved, minus the original pointer you passed in, is the result of strlen().
In other words, strlen() just walks through memory until it finds the next zero character, and it returns the number of characters from that point to the original pointer.
But, what if it doesn't find a zero? Does it stop? Nope. It will just trudge on and on until it finds a zero or the program crashes.
That is why strlen() is so confusing, and why it's source of many critical bugs in modern software. This doesn't mean you can't use it, but it does mean you must be very very careful to make sure that whatever you pass in is a null-terminated string (i.e. a set of zero or more non-zero characters, followed by a zero character.)
Remember also that in C, you basically have no idea what memory contains when you allocate it or set it aside. If you want it to be all zeros, then you need to make sure to fill it with zeros yourself!
Anyway, the answer to your question involves the use of the memset() function. You'll have to pass memset() the pointer to the beginning of your array, the length of that array, and the value to fill it with (in your case, zero of course!)

No initialization of a, this leads to undefined behavior.
C "strings" are '\0' terminated arrays of char. So strlen() will browse whole memory from given address until it either finds a '\0' or results in a segmentation fault.

What am I not understanding here?
Perhaps the mis-understanding is that auto variables, such as:
entry a;
are assigned memory from the process' stack. The pre-existing content of that stack memory is not zeroed-out for your benefit. Hence the value(s) of the elements of a, which will also be located on the process stack, will not be initially zeroed-out for your benefit. Rather, the entire content of a and its elements (including .arr) will contain bizarre and perhaps unexpected values.
C programmers learn to initialize auto variables by zeroing them out, or initializing them with a desirable value.
For example, the question code might do this as follows:
int main(){
entry a =
{
.arr[0] = 0
};
...
}
Or:
int main(){
entry a;
memset(&a, 0, sizeof(a));
...
}

Related

pointer in c program

program in c language
void main()
{
char *a,*b;
a[0]='s';
a[1]='a';
a[2]='n';
a[3]='j';
a[4]='i';
a[5]='t';
printf("length of a %d/n", strlen(a));
b[0]='s';
b[1]='a';
b[2]='n';
b[3]='j';
b[4]='i';
b[5]='t';
b[6]='g';
printf("length of b %d\n", strlen(b));
}
here the output is :
length of a 6
length of b 12
Why and please explain it.
thanks in advance.
You are assigning to pointer (which contains garbage) without allocating memory. What you are noticing is Undefined Behavior. Also main should return an int. Also it does not make sense to try and find the length of an array of chars which are not nul terminated.
This is how you can go about:
Sample code
When you declare any variable it comes with whatever it had in memory previously where your application is running, and since pointers are essentially numbers, whatever number it had referenced to some random memory address.
Then, when setting a[i], the compiler interprets that as you want to step sizeof(a) bytes forward, thus, a[i] is equal to the address (a + i*1) (1 because chars use one byte).
Finally, C-strings need to be NULL terminated (\0, also known as sentinel), and methods like strlen go over the length of the string until hitting the sentinel, most likely, your memory had a stray 0 somewhere that caused strlen to stop.
Allocate some memory and terminate the strings then it will work better
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void main(){
char *a=malloc(10);
char *b=malloc(10);
if(a){
a[0]='s';
a[1]='a';
a[2]='n';
a[3]='j';
a[4]='i';
a[5]='t';
a[6]=(char)0;
printf("length of a %d\n", (int)strlen(a));
}else{
printf("Failed to allocate 10 bytes\n" );
}
if(b){
b[0]='s';
b[1]='a';
b[2]='n';
b[3]='j';
b[4]='i';
b[5]='t';
b[6]='g';
b[7]=(char)0;
printf("length of b %d\n", (int)strlen(b));
}else{
printf("Failed to allocate 10 bytes\n" );
}
free(a);
free(b);
}
Undefined behavior. That's all.
You're using an uninitialized pointer. After that, all bets are off as to what will happen.
Of course, we can attempt to explain why your particular implementation acts in a certain way but it'd be quite pointless outside of novelty.
The indexing operator is de-referencing the pointers a and b, but you never initialized those pointers to point at valid memory. Writing to un-initialized memory triggers undefined behavior.
You are simply "lucky" (or unlucky, it depends on your viewpoint) that the program doesn't crash, that the pointer values are such that you succeed in writing at those locations.
Note that you never write the termination character ('\0') to either string, but still get the "right" value from strlen(); this implies that a and b both point at memory that happens to be full of zeros. More luck.
This is a very broken program; that it manages to run "successfully" is because it's behavior is undefined, and undefined clearly includes "working as the programmer intended".
a and b are both char pointers. First of all, you didn't initialise them and secondly didn't terminate them with NULL.

Is it necessary to initialize the char array for accurate length?

In the below example when I define char array uninitialized and want to find the length, it's undefined behavior.
#include<stdio.h>
int main()
{
char a[250];
printf("length=%d\n",strlen(a));
}
I got "0". I don't know how? Explain it.
Luck. Whether it's good or bad luck is a matter of opinion. The contents of your array are whatever happened to already occupy that memory, and is not initialized. In your case, it happened the first byte was a '\0'.
This is, of course, undefined behavior and you can't depend on it happening this way.
You said in your example you were using an uninitialized char array to show undefined behavior, then when you got "0" you want an explanation? It's... undefined behavior.
If you got 0 for the length if just means that there happens to be a 0 as the first element of a[] in your uninitialized array. When it's an uninitialized local that means, as far as the C standards are concerened, anything can be in there, including a 0.
To address the question in your title: "Is it necessary to initialize the char array for accurate length?"
Yes, to be able to deterministically know the length of a string in a char array via the strlen() function, it is required for a null terminator to be present. That means it needs to be initialized or set in some manner or another.
As other answers say the strlen() result is more a matter of luck than defined behaviour
To find the "size" of the memory block use sizeof() instead
Note: I've also included the string.h and used a long conversion for the integers in the printf
#include<stdio.h>
#include<string.h>
int main()
{
char a[250];
printf("length=%ld\n",strlen(a));
printf("sizeof=%ld\n",sizeof(a));
}
when you define
char a[250];
The array will contains garabage contents and random.
strlen(a) count the number of not null charachter ('\0') till it find a null charachter then it stop.
so if your char a[250]; array contains garabage element and the first element is randomly set to null '\0' the strlen(a) will return 0

Why do I need to allocate memory?

#include<stdio.h>
#include<stdlib.h>
void main()
{
char *arr;
arr=(char *)malloc(sizeof (char)*4);
scanf("%s",arr);
printf("%s",arr);
}
In the above program, do I really need to allocate the arr?
It is giving me the result even without using the malloc.
My second doubt is ' I am expecting an error in 9th line because I think it must be
printf("%s",*arr);
or something.
do I really need to allocate the arr?
Yes, otherwise you're dereferencing an uninitialised pointer (i.e. writing to a random chunk of memory), which is undefined behaviour.
do I really need to allocate the arr?
You need to set arr to point to a block of memory you own, either by calling malloc or by setting it to point to another array. Otherwise it points to a random memory address that may or may not be accessible to you.
In C, casting the result of malloc is discouraged1; it's unnecessary, and in some cases can mask an error if you forget to include stdlib.h or otherwise don't have a prototype for malloc in scope.
I usually recommend malloc calls be written as
T *ptr = malloc(N * sizeof *ptr);
where T is whatever type you're using, and N is the number of elements of that type you want to allocate. sizeof *ptr is equivalent to sizeof (T), so if you ever change T, you won't need to duplicate that change in the malloc call itself. Just one less maintenance headache.
It is giving me the result even without using the malloc
Because you don't explicitly initialize it in the declaration, the initial value of arr is indeterminate2; it contains a random bit string that may or may not correspond to a valid, writable address. The behavior on attempting to read or write through an invalid pointer is undefined, meaning the compiler isn't obligated to warn you that you're doing something dangerous. On of the possible outcomes of undefined behavior is that your code appears to work as intended. In this case, it looks like you're accessing a random segment of memory that just happens to be writable and doesn't contain anything important.
My second doubt is ' I am expecting an error in 9th line because I think it must be printf("%s",*arr); or something.
The %s conversion specifier tells printf that the corresponding argument is of type char *, so printf("%s", arr); is correct. If you had used the %c conversion specifier, then yes, you would need to dereference arr with either the * operator or a subscript, such as printf("%c", *arr); or printf("%c", arr[i]);.
Also, unless your compiler documentation explicitly lists it as a valid signature, you should not define main as void main(); either use int main(void) or int main(int argc, char **argv) instead.
1. The cast is required in C++, since C++ doesn't allow you to assign void * values to other pointer types without an explicit cast
2. This is true for pointers declared at block scope. Pointers declared at file scope (outside of any function) or with the static keyword are implicitly initialized to NULL.
Personally, I think this a very bad example of allocating memory.
A char * will take up, in a modern OS/compiler, at least 4 bytes, and on a 64-bit machine, 8 bytes. So you use four bytes to store the location of the four bytes for your three-character string. Not only that, but malloc will have overheads, that add probably between 16 and 32 bytes to the actual allocated memory. So, we're using something like 20 to 40 bytes to store 4 bytes. That's a 5-10 times more than it actually needs.
The code also casts malloc, which is wrong in C.
And with only four bytes in the buffer, the chances of scanf overflowing is substantial.
Finally, there is no call to free to return the memory to the system.
It would be MUCH better to use:
int len;
char arr[5];
fgets(arr, sizeof(arr), stdin);
len = strlen(arr);
if (arr[len] == '\n') arr[len] = '\0';
This will not overflow the string, and only use 9 bytes of stackspace (not counting any padding...), rather than 4-8 bytes of stackspace and a good deal more on the heap. I added an extra character to the array, so that it allows for the newline. Also added code to remove the newline that fgets adds, as otherwise someone would complain about that, I'm sure.
In the above program, do I really need to allocate the arr?
You bet you do.
It is giving me the result even without using the malloc.
Sure, that's entirely possible... arr is a pointer. It points to a memory location. Before you do anything with it, it's uninitialized... so it's pointing to some random memory location. The key here is wherever it's pointing is a place your program is not guaranteed to own. That means you can just do the scanf() and at that random location that arr is pointing to the value will go, but another program can overwrite that data.
When you say malloc(X) you're telling the computer that you need X bytes of memory for your own usage that no one else can touch. Then when arr captures the data it will be there safely for your usage until you call free() (which you forgot to do in your program BTW)
This is a good example of why you should always initialize your pointers to NULL when you create them... it reminds you that you don't own what they're pointing at and you better point them to something valid before using them.
I am expecting an error in 9th line because I think it must be printf("%s",*arr)
Incorrect. scanf() wants an address, which is what arr is pointing to, that's why you don't need to do: scanf("%s", &arr). And printf's "%s" specificier wants a character array (a pointer to a string of characters) which again is what arr is, so no need to deference.

C char* pointers pointing to same location where they definitely shouldn't

I'm trying to write a simple C program on Ubuntu using Eclipse CDT (yes, I'm more comfortable with an IDE and I'm used to Eclipse from Java development), and I'm stuck with something weird. On one part of my code, I initialize a char array in a function, and it is by default pointing to the same location with one of the inputs, which has nothing to do with that char array. Here is my code:
char* subdir(const char input[], const char dir[]){
[*] int totallen = strlen(input) + strlen(dir) + 2;
char retval[totallen];
strcpy(retval, input);
strcat(retval, dir);
...}
Ok at the part I've marked with [*], there is a checkpoint. Even at that breakpoint, when I check y locals, I see that retval is pointing to the same address with my argument input. It not even possible as input comes from another function and retval is created in this function. Is is me being unexperienced with C and missing something, or is there a bug somewhere with the C compiler?
It seems so obvious to me that they should't point to the same (and a valid, of course, they aren't NULL) location. When the code goes on, it literally messes up everything; I get random characters and shapes in console and the program crashes.
I don't think it makes sense to check the address of retval BEFORE it appears, it being a VLA and all (by definition the compiler and the debugger don't know much about it, it's generated at runtime on the stack).
Try checking its address after its point of definition.
EDIT
I just read the "I get random characters and shapes in console". It's obvious now that you are returning the VLA and expecting things to work.
A VLA is only valid inside the block where it was defined. Using it outside is undefined behavior and thus very dangerous. Even if the size were constant, it still wouldn't be valid to return it from the function. In this case you most definitely want to malloc the memory.
What cnicutar said.
I hate people who do this, so I hate me ... but ... Arrays of non-const size are a C99 extension and not supported by C++. Of course GCC has extensions to make it happen.
Under the covers you are essentially doing an _alloca, so your odds of blowing out the stack are proportional to who has access to abuse the function.
Finally, I hope it doesn't actually get returned, because that would be returning a pointer to a stack allocated array, which would be your real problem since that array is gone as of the point of return.
In C++ you would typically use a string class.
In C you would either pass a pointer and length in as parameters, or a pointer to a pointer (or return a pointer) and specify the calls should call free() on it when done. These solutions all suck because they are error prone to leaks or truncation or overflow. :/
Well, your fundamental problem is that you are returning a pointer to the stack allocated VLA. You can't do that. Pointers to local variables are only valid inside the scope of the function that declares them. Your code results in Undefined Behaviour.
At least I am assuming that somewhere in the ..... in the real code is the line return retval.
You'll need to use heap allocation, or pass a suitably sized buffer to the function.
As well as that, you only need +1 rather than +2 in the length calculation - there is only one null-terminator.
Try changing retval to a character pointer and allocating your buffer using malloc().
Pass the two string arguments as, char * or const char *
Rather than returning char *, you should just pass another parameter with a string pointer that you already malloc'd space for.
Return bool or int describing what happened in the function, and use the parameter you passed to store the result.
Lastly don't forget to free the memory since you're having to malloc space for the string on the heap...
//retstr is not a const like the other two
bool subdir(const char *input, const char *dir,char *retstr){
strcpy(retstr, input);
strcat(retstr, dir);
return 1;
}
int main()
{
char h[]="Hello ";
char w[]="World!";
char *greet=(char*)malloc(strlen(h)+strlen(w)+1); //Size of the result plus room for the terminator!
subdir(h,w,greet);
printf("%s",greet);
return 1;
}
This will print: "Hello World!" added together by your function.
Also when you're creating a string on the fly you must malloc. The compiler doesn't know how long the two other strings are going to be, thus using char greet[totallen]; shouldn't work.

Pointer initialization and string manipulation in C

I have this function which is called about 1000 times from main(). When i initialize a pointer in this function using malloc(), seg fault occurs, possibly because i did not free() it before leaving the function. Now, I tried free()ing the pointer before returning to main, but its of no use, eventually a seg fault occurs.
The above scenario being one thing, how do i initialize double pointers (**ptr) and pointer to array of pointers (*ptr[])?
Is there a way to copy a string ( which is a char array) into an array of char pointers.
char arr[]; (Lets say there are fifty such arrays)
char *ptr_arr[50]; Now i want point each such char arr[] in *ptr_arr[]
How do i initialize char *ptr_arr[] here?
What are the effects of uninitialized pointers in C?
Does strcpy() append the '\0' on its own or do we have to do it manually? How safe is strcpy() compared to strncpy()? Like wise with strcat() and strncat().
Thanks.
Segfault can be caused by many things. Do you check the pointer after the malloc (if it's NULL)? Step through the lines of the code to see exactly where does it happen (and ask a seperate question with more details and code)
You don't seem to understand the relation of pointers and arrays in C. First, a pointer to array of pointers is defined like type*** or type**[]. In practice, only twice-indirected pointers are useful. Still, you can have something like this, just dereference the pointer enough times and do the actual memory allocation.
This is messy. Should be a separate question.
They most likely crash your program, BUT this is undefined, so you can't be sure. They might have the address of an already used memory "slot", so there might be a bug you don't even notice.
From your question, my advice would be to google "pointers in C" and read some tutorials to get an understanding of what pointers are and how to use them - there's a lot that would need to be repeated in an SO answer to get you up to speed.
The top two hits are here and here.
It's hard to answer your first question without seeing some code -- Segmentation Faults are tricky to track down and seeing the code would be more straightforward.
Double pointers are not more special than single pointers as the concepts behind them are the same. For example...
char * c = malloc(4);
char **c = &c;
I'm not quite sure what c) is asking, but to answer your last question, uninitialized pointers have undefined action in C, ie. you shouldn't rely on any specific result happening.
EDIT: You seem to have added a question since I replied...
strcpy(..) will indeed copy the null terminator of the source string to the destination string.
for part 'a', maybe this helps:
void myfunction(void) {
int * p = (int *) malloc (sizeof(int));
free(p);
}
int main () {
int i;
for (i = 0; i < 1000; i++)
myfunction();
return 0;
}
Here's a nice introduction to pointers from Stanford.
A pointer is a special type of variable which holds the address or location of another variable. Pointers point to these locations by keeping a record of the spot at which they were stored. Pointers to variables are found by recording the address at which a variable is stored. It is always possible to find the address of a piece of storage in C using the special & operator. For instance: if location were a float type variable, it would be easy to find a pointer to it called location_ptr
float location;
float *location_ptr,*address;
location_ptr = &(location);
or
address = &(location);
The declarations of pointers look a little strange at first. The star * symbol which stands in front of the variable name is C's way of declaring that variable to be a pointer. The four lines above make two identical pointers to a floating point variable called location, one of them is called location_ptr and the other is called address. The point is that a pointer is just a place to keep a record of the address of a variable, so they are really the same thing.
A pointer is a bundle of information that has two parts. One part is the address of the beginning of the segment of memory that holds whatever is pointed to. The other part is the type of value that the pointer points to the beginning of. This tells the computer how much of the memory after the beginning to read and how to interpret it. Thus, if the pointer is of a type int, the segment of memory returned will be four bytes long (32 bits) and be interpreted as an integer. In the case of a function, the type is the type of value that the function will return, although the address is the address of the beginning of the function executable.
Also get more tutorial on C/C++ Programming on http://www.jnucode.blogspot.com
You've added an additional question about strcpy/strncpy.
strcpy is actually safer.
It copies a nul terminated string, and it adds the nul terminator to the copy. i.e. you get an exact duplicate of the original string.
strncpy on the other hand has two distinct behaviours:
if the source string is fewer than 'n' characters long, it acts just as strcpy, nul terminating the copy
if the source string is greater than or equal to 'n' characters long, then it simply stops copying when it gets to 'n', and leaves the string unterminated. It is therefore necessary to always nul-terminate the resulting string to be sure it's still valid:
char dest[123];
strncpy(dest, source, 123);
dest[122] = '\0';

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