Related
I'm a newbie to C. I had extended the question from the previous question: Strange behavior when returning "string" with C (Thanks for all who answered or commented that question, by the way.)
Pretty straight forward:
Why can this work:
#include <stdio.h>
int func() {
int i = 5;
return i;
}
int main() {
printf("%d",func());
}
But not this:
#include <stdio.h>
char * func() {
char c[] = "Hey there!";
return c;
}
int main() {
printf("%s",func());
}
From the previous question, logically the int i should not exist too because the function has returned, but why can it still be returned while char c[] cannot?
(It seems to be duplicated from "Pointers and memory scope" but I would like to know more about what is the difference between returning an int and a char *.)
Problem is not returning char *, it is returning something that is allocated on stack.
If you allocate memory for your string rather than pointing to function stack, there will be no problem. Something like this:
char * func() {
char c[] = "Hey there!";
return strdup(c);
}
int main() {
char* str = func();
printf("%s", str);
free(str);
}
It is important to mention that in both cases, you are copying a value and in both cases copied value is correct, but the meaning of copied value differs.
In first case, your are copying an int value and after your return from function, you are using that int value which will be valid. But in 2nd case, even though you have a valid pointer value, it refers to an invalid address of memory which is stack of called function.
Based on suggestions in comment, I decided to add another better practice in memory allocating for this code:
#define NULL (void*)0
int func(char *buf, int len) {
char c[] = "Hey there!";
int size = strlen(c) + 1;
if (len >= size) {
strcpy(buf, c);
}
return size;
}
int main() {
int size = func(NULL, 0);
char *buf = calloc(size, sizeof(*buf));
func(buf, size);
printf("%s", buf);
free(buf);
return 0;
}
Similar approach is used in a lot of windows API functions. This approach is better, because owner of pointer is more obvious (main in here).
In the first example the return value is copied. In your second example you're returning a pointer, which will point to a memory location which no longer exists.
In the first case, you return the int value 5 from the function. You can then print that value.
In the second case however, you return a value of type char *. That value points to an array that is local to the function func. After that function returns the array goes out of scope, so the pointer points to invalid memory.
The difference between these two cases is a value that you use directly, versus a pointer value that no longer points to valid memory. Had you returned a pointer to memory allocated by malloc, then the pointer would point to valid memory.
You are trying to return pointer to local array, which is very bad. If you want to return a pointer to array, allocate it dynamically using malloc inside your func();
Then you must call free() on caller side to free up memory you allocated when you no longer need it
In the first example, you return an int, and the second you return a pointer to a char. They both return in exactly the same manner, it is just a matter of understanding the stack and how values are returned.
Even though i was declared in the function and is allocated on the stack, when the function returns it returns the value of i (which is basically copied, so when i falls off the stack the value of i is still returned.)
This is the exact same thing that happens to the char * in the second example. It will still be a pointer to a char, and it returns the 'copied' value of c. However, since it was allocated on the stack, the address it points to is effectively invalid. The pointer value itself has not changed, but what it points to has.
You would have to dynamically allocate this to avoid this situation.
The return value of function is returned by copy. In the first example, you get a copy of the integer variable from the function. In the second you get a copy of the char pointer, not a copy of the string.
The pointer references the string data that has automatic storage, so is no longer valid after the function returns. The space becomes available for use by other code and many be modified - any attempt to access it has undefined behaviour.
The point is, it is a pointer that is returned, not a string; in C a strings (and more generally arrays) are not a first-class data types.
Depending on your needs there are a number of valid ways of returning the string data; for example the following is valid:
char* func()
{
static char c[] = "Hey there!";
return c;
}
because here although the local variable goes out of scope the static data is not destroyed or de-allocated, and any reference to it remains valid.
Another alternative is to embed the string in a struct which is a first-class data type:
typedef struct
{
char content[256] ;
} sString ;
sString func()
{
sString c = {"Hey there!"};
return c;
}
Or more conventionally to copy the data to a caller buffer:
char* func( char* buffer )
{
char c[] = "Hey there!";
strcpy( buffer, c ) ;
return buffer ;
}
I have omitted code to mitigate the possibility of buffer overrun above for clarity in this last example, such code is advised.
I am trying to return and print a function in C. Printing it out works in the function just fine, but when I try to print it after returning it from the function, I get nonsense.
I have already tried a lot and I think I have seen at least 6 stack overflow posts similar to this and this is the closest thing I can get to working that is not a segmentation fault or an error.
Code:
char* getBitstring(unsigned short int instr) {
//this is what the code below is going to convert into. It is set to default
//as a 16 bit string full of zeros to act as a safety default.
char binaryNumber[] = "0000000000000000";
//....
//doing things to binaryNumber
//.....
printf("don't get excited yet %s\n", binaryNumber); //note, this works
return binaryNumber;
}
int main(int argc, char *argv[]) {
char *a = getBitstring(0x1234);
printf("%s", a); //this fails
return 0;
}
Here is the output:
don't get excited yet 0001001000110100
������e��r�S�����$�r�#�t�$�r�����ͅS�������t����
This is because you are returning a pointer to an object allocated in automatic memory - an undefined behavior.
If you want to return a string from a function, you need to return either a dynamically-allocated block, or a statically allocated block.
Another choice is to pass the buffer into the function, and provide the length as the return value of the function, in the way the file reading functions do it:
size_t getBitstring(unsigned short int instr, char* buf, size_t buf_size) {
... // Fill in the buffer without going over buf_size
printf("don't get excited yet %s\n", binaryNumber);
return strlen(buf);
}
Here is how you call this function:
char binaryNumber[] = "0000000000000000";
size_t len = getBitstring(instr, binaryNumber, sizeof(binaryNumber));
Now binaryNumber is an automatic variable in the context of the caller, so the memory would be around while the caller needs it.
This is a good example of a problem people hit when they're learning C that they probably wouldn't hit if they were operating in Java or another more modern language that doesn't expose the details of memory layout to the user.
While everyone's answer here is probably technically correct, I'm going to try a different tack to see if I can answer your question without just giving you a line of code that will fix it.
First you need to understand what's going on when returning a
variable defined inside a function, like this:
void f(void) {
int x = 0;
/* do some crazy stuff with x */
return x;
}
What happens when you call return x;? I'm probably omitting some
details here, but what is essentially going on is that the calling
context gets the value stored inside the variable named x at
that time. The storage that is allocated to store that value is
no longer guaranteed to contain that value after the function is
over.
Second, we need to understand what happens when we refer to an array
by its name. Say we have a 'string':
char A[] = "12345";
In C, this is actually equivalent to declaring an array of
characters that ends in a \0:
char A[6] = { '1' , '2' , '3' , '4' , '5' , '\0' };
Then this is sort of like declaring six chars A[0], A[1], ...
, A[5]. The variable A is actually of type char * i.e. it is a
pointer containing the memory address storing A[0] (the beginning
of the array).
Finally, we need to understand what happens when you call printf to
print a string like this:
printf("%s", A);
What you're saying here is "print all the bytes starting at memory
address A until you hit a byte that contains \0".
So, let's put it all together. When you run your code, the variable binaryNumber is a pointer containing the memory address of the first character of the array: binaryNumber[0], and this address is what's returned by the function. BUT, you've declared the array inside of getBitString, so as we know that the memory allocated for the array is no longer guaranteed to store the same values after getBitString is over.
When you run printf("%s", a) you're telling C to "print all the bytes starting at memory address a until you get to a byte containing \0 -- but since that memory address is only guaranteed to contain valid values inside getBitString, what you get is whatever garbage it happens to contain at the time when you call it outside of getBitString.
So what can you do to resolve the problem? Well you have several options, here are is a (non-exhaustive) list:
You declare binaryString outside of getBitString so that it's still valid when you try to access it in main
You declare binaryString as static as some others have suggested, which is effectively the same thing as above, except that the actual variable name binaryString is only valid inside the function, but the memory allocated to store the array is still valid outside the function.
You make a copy of the string using the strdup() function before you return it in your function. Remember that if you do this, you have to free() the pointer returned by strdup() after you're done with it, otherwise what you've got is a memory leak.
Your binaryNumber character array only exists inside of your getBitstring function. Once that function returns, that memory is no longer allocated to your binaryNumber. To keep that memory allocated for use outside of that function you can do one of two things:
Return a dynamically allocated array
char* getBitstring(unsigned short int instr) {
// dynamically allocate array to hold 16 characters (+1 null terminator)
char* binaryNumber = malloc(17 * sizeof(char));
memset(binaryNumber, '0', 16); // Set the first 16 array elements to '0'
binaryNumber[16] = '\0'; // null terminate the string
//....
//doing things to binaryNumber
//.....
return binaryNumber;
}
int main(int argc, char *argv[]) {
char *a = getBitstring(0x1234);
printf("%s", a);
free(a); // Need to free memory, because you dynamically allocated it
return 0;
}
or pass the array into the function as an argument
void* getBitstring(unsigned short int instr, char* binaryNumber, unsigned int arraySize ) {
//....
//doing things to binaryNumber (use arraySize)
//.....
}
int main(int argc, char *argv[]) {
char binaryNumber[] = "0000000000000000";
getBitstring(0x1234, binaryNumber, 16); // You must pass the size of the array
printf("%s", binaryNumber);
return 0;
}
Others have suggested making your binaryNumber array static for a quick fix. This would work, but I would avoid this solution, as it is unnecessary and has other side effects.
Create your return type in dynamic way with malloc() function. create 16+1 blocks for end of string. this is not safe but easy to understand.
char * binaryNumber = (char*) malloc(17*sizeof(char));//create dynamic char sequence
strcpy(binaryNumber,"0000000000000000");//assign the default value with String copy
The final result will be;
char* getBitstring(unsigned short int instr) {
//this is what the code below is going to convert into. It is set to default
//as a 16 bit string full of zeros to act as a safety default.
char * binaryNumber = (char*) malloc(17*sizeof(char));//create dynamic char sequence
strcpy(binaryNumber,"0000000000000000");//assign the default value with String copy function
//....
//doing things to binaryNumber
//.....
printf("don't get excited yet %s\n", binaryNumber); //note, this works
return binaryNumber;
}
int main(int argc, char *argv[]) {
char *a = getBitstring(0x1234);
printf("%s", a); //this fails
return 0;
}
of course include the <string.h> library.
your char binaryNumber[] is local to the function getBitstring(). you cannot return a local variable to other function.
The scope of binaryNumber is over when getBitstring() finishes execution. So, in your main(), char *a is not initialized.
The workaround:
define the array as static so that it does not go out-of-scope. [Not a good approach, but works]
or
use dynamic memory allocation and return the pointer. [don't forget to free later, to avoid memory leak.]
IMO, the second approach is way better.
You need static:
static char binaryNumber[] = "0000000000000000";
Without static keyword, the value of automatic variable is lost after function returns. Probably you know this.
Is there any problem in doing something like this in C
char* wrap(char *inp) {
char *newstr;
newstr = (char *)malloc( sizeof(char) * 4);
newstr[0] = 'A';
newstr[1] = inp[0];
newstr[2] = 'B';
newstr[3] = '\0';
return newstr;
}
Basically I want to know that is there problem in using malloc inside a function and returning local variable.
You're not returning a local variable; you're returning the value stored in a local variable.
This code is fine (although the cast on malloc is unnecessary); it's a somewhat common pattern to allocate memory in one function and free it in another.
That's perfect, as long as you're super sure the caller is going to call free to avoid memory leaks .. That's not a big issue on small programs, but when the program gets complex, believe me, you'll be concerned about much more things than freeing a pointer from a supposed-to-be self-contained function ..
But (hands down), there's a much more satisfying solution to this that the standard C Library itself uses. Use a Buffer ! (Claps, Claps)
You know, there is a reason the fgets function for example requires you to provide a character pointer as the first argument, so that it can write to it rather than returning a malloc'd pointer ..
For example ..
#include <ctype.h>
#include <string.h>
void toLower(char *buf, const char *s) {
for(int i = 0; s[i]; ++i)
buf[i] = tolower(s[i]);
}
int main(int argc, const char ** argv) {
const char *s = "ThAt'S a BiG sTrIng";
char lower_version[strlen(s)];
toLower(lower_version, s);
printf("Original Version: %s\nLower Version: %s\n\tTada !\n", s, lower_version);
}
That way, you don't have to worry how you're going to handle the variable in later uses ..
You're leaving this problem to the function caller to deal with.
This is perfectly fine as long as you call free() somewhere to avoid memory leaks. Part of your program design should be defining the "owner" of each pointer. Such ownership can be transferred, so you should keep track of the owner throughout the lifetime of the pointer. The owner at the time the pointer becomes unused should be responsible for calling free().
I am trying to convert an arbitrary buffer to a string of its binary representation. I was looking at some code from here: http://snippets.dzone.com/posts/show/2076 in order to get started. I realize that this code can not convert an arbitrary buffer, but only the specific case of an int; however, I figured that I could adapt it to any case once it was working.
The problem is that it returns some strange symbols (like this: �왿") instead of the binary. Does anyone either know what is wrong with this code specifically or explain how to convert an arbitrary buffer?
Please keep in mind that I am new to c++.
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <string.h>
char *getBufferAsBinaryString(void *in)
{
int pos=0;
char result;
char bitstring[256];
memset(bitstring, 0, 256);
unsigned int *input= (unsigned int *)in;
for(int i=31;i>=0;i--)
{
if (((*input >> i) & 1)) result = '1';
else result = '0';
bitstring[pos] = result;
if ((i>0) && ((i)%4)==0)
{
pos++;
bitstring[pos] = ' ';
}
pos++;
}
return bitstring;
}
int main(int argc, char* argv[])
{
int i=53003;
char buffer[1024];
char *s=getBufferAsBinaryString(&i);
strcpy(buffer, s);
printf("%s\n", buffer);
}
The array bitstring has what is known as automatic duration, which means that it springs into existence when the function is called and disappears when the function returns.
Therefore, the pointer that this version of getBufferAsBinaryString returns is to an array which no longer exists by the time the caller receives the pointer. (Remember that the statement return bitstring; returns a pointer to the first character in bitstring; by the "equivalence of arrays and pointers," the mention of the array bitstring in this context is equivalent to &bitstring[0].)
When the caller tries to use the pointer, the string created by getBufferAsBinaryString might still be there, or the memory might have been re-used by some other function. Therefore, this version of getBufferAsBinaryString is not adequate and not acceptable. Functions must never return pointers to local, automatic-duration arrays.
Since the problem with returning a pointer to a local array is that the array has automatic duration by default, the simplest fix to the above non-functional version of getBufferAsBinaryString is to declare the array static, instead:
char *getBufferAsBinaryString(void *in)
{
int pos=0;
char result;
static char bitstring[256];
memset(bitstring, 0, 256);
unsigned int *input= (unsigned int *)in;
for(int i=31;i>=0;i--)
{
if (((*input >> i) & 1)) result = '1';
else result = '0';
bitstring[pos] = result;
if ((i>0) && ((i)%4)==0)
{
pos++;
bitstring[pos] = ' ';
}
pos++;
}
return bitstring;
}
Now, the bitstring array does not disappear when getBufferAsBinaryString returns, so the pointer is still valid by the time the caller uses it.
Returning a pointer to a static array is a practical and popular solution to the problem of "returning" an array, but it has one drawback. Each time you call the function, it re-uses the same array and returns the same pointer. Therefore, when you call the function a second time, whatever information it "returned" to you last time will be overwritten. (More precisely, the information, that the function returned a pointer to, will be overwritten.)
Although the static return array technique will work, the caller has to be a little bit careful, and must never expect the return pointer from one call to the function to be usable after a later call to the function
But you still have a different problem with passing in void * which I did not cover, in addition not passing in the size of your buffer. Since you shouldn't assume that your array ends in the \0, you should also pass in the size of your buffer.
Ignoring the other issues with memory and safety, you are not returning a valid null terminated string. Nor do you pass in the size of the input buffer, so you really just print out the 32-bit bit representation of the first 32-bits of the input buffer.
So, in addition to passing in a char buffer to write to or simply returning a std::string, you should also pass in the size of the input buffer and loop over that as well.
You can't return locally-allocated arrays from a function. As soon as the function finishes, the array ceases to exist, so the pointer you've returned no longer points to a valid object.
Instead of using a char array to represent your bitstring, you should consider using std::string, which has sensible value semantics (i.e., you can copy it, return it from a function, etc, without having to worry about it except from an efficiency standpoint).
I've been studying C, and I decided to practice using my knowledge by creating some functions to manipulate strings. I wrote a string reverser function, and a main function that asks for user input, sends it through stringreverse(), and prints the results.
Basically I just want to understand how my function works. When I call it with 'tempstr' as the first param, is that to be understood as the address of the first element in the array? Basically like saying &tempstr[0], right?
I guess answering this question would tell me: Would there be any difference if I assigned a char* pointer to my tempstr array and then sent that to stringreverse() as the first param, versus how I'm doing it now? I want to know whether I'm sending a duplicate of the array tempstr, or a memory address.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char* stringreverse(char* tempstr, char* returnptr);
printf("\nEnter a string:\n\t");
char tempstr[1024];
gets(tempstr);
char *revstr = stringreverse(tempstr, revstr); //Assigns revstr the address of the first character of the reversed string.
printf("\nReversed string:\n"
"\t%s\n", revstr);
main();
return 0;
}
char* stringreverse(char* tempstr, char* returnptr)
{
char revstr[1024] = {0};
int i, j = 0;
for (i = strlen(tempstr) - 1; i >= 0; i--, j++)
{
revstr[j] = tempstr[i]; //string reverse algorithm
}
returnptr = &revstr[0];
return returnptr;
}
Thanks for your time. Any other critiques would be helpful . . only a few weeks into programming :P
EDIT: Thanks to all the answers, I figured it out. Here's my solution for anyone wondering:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void stringreverse(char* s);
int main(void)
{
printf("\nEnter a string:\n\t");
char userinput[1024] = {0}; //Need to learn how to use malloc() xD
gets(userinput);
stringreverse(userinput);
printf("\nReversed string:\n"
"\t%s\n", userinput);
main();
return 0;
}
void stringreverse(char* s)
{
int i, j = 0;
char scopy[1024]; //Update to dynamic buffer
strcpy(scopy, s);
for (i = strlen(s) - 1; i >= 0; i--, j++)
{
*(s + j) = scopy[i];
}
}
First, a detail:
int main()
{
char* stringreverse(char* tempstr, char* returnptr);
That prototype should go outside main(), like this:
char* stringreverse(char* tempstr, char* returnptr);
int main()
{
As to your main question: the variable tempstr is a char*, i.e. the address of a character. If you use C's index notation, like tempstr[i], that's essentially the same as *(tempstr + i). The same is true of revstr, except that in that case you're returning the address of a block of memory that's about to be clobbered when the array it points to goes out of scope. You've got the right idea in passing in the address of some memory into which to write the reversed string, but you're not actually copying the data into the memory pointed to by that block. Also, the line:
returnptr = &revstr[0];
Doesn't do what you think. You can't assign a new pointer to returnptr; if you really want to modify returnptr, you'll need to pass in its address, so the parameter would be specified char** returnptr. But don't do that: instead, create a block in your main() that will receive the reversed string, and pass its address in the returnptr parameter. Then, use that block rather than the temporary one you're using now in stringreverse().
Basically I just want to understand how my function works.
One problem you have is that you are using revstr without initializing it or allocating memory for it. This is undefined behavior since you are writing into memory doesn't belong to you. It may appear to work, but in fact what you have is a bug and can produce unexpected results at any time.
When I call it with 'tempstr' as the first param, is that to be understood as the address of the first element in the array? Basically like saying &tempstr[0], right?
Yes. When arrays are passed as arguments to a function, they are treated as regular pointers, pointing to the first element in the array. There is no difference if you assigned &temp[0] to a char* before passing it to stringreverser, because that's what the compiler is doing for you anyway.
The only time you will see a difference between arrays and pointers being passed to functions is in C++ when you start learning about templates and template specialization. But this question is C, so I just thought I'd throw that out there.
When I call it with 'tempstr' as the first param, is that to be understood as the
address of the first element in the array? Basically like saying &tempstr[0],
right?
char tempstr[1024];
tempstr is an array of characters. When passed tempstr to a function, it decays to a pointer pointing to first element of tempstr. So, its basically same as sending &tempstr[0].
Would there be any difference if I assigned a char* pointer to my tempstr array and then sent that to stringreverse() as the first param, versus how I'm doing it now?
No difference. You might do -
char* pointer = tempstr ; // And can pass pointer
char *revstr = stringreverse(tempstr, revstr);
First right side expression's is evaluavated and the return value is assigned to revstr. But what is revstr that is being passed. Program should allocate memory for it.
char revstr[1024] ;
char *retValue = stringreverse(tempstr, revstr) ;
// ^^^^^^ changed to be different.
Now, when passing tempstr and revstr, they decayed to pointers pointing to their respective first indexes. In that case why this would go wrong -
revstr = stringreverse(tempstr, revstr) ;
Just because arrays are not pointers. char* is different from char[]. Hope it helps !
In response to your question about whether the thing passed to the function is an array or a pointer, the relevant part of the C99 standard (6.3.2.1/3) states:
Except when it is the operand of the sizeof operator or the unary & operator, or is a string literal used to initialize an array, an expression that has type ‘‘array of type’’ is converted to an expression with type ‘‘pointer to type’’ that points to the initial element of the array object and is not an lvalue.
So yes, other than the introduction of another explicit variable, the following two lines are equivalent:
char x[] = "abc"; fn (x);
char x[] = "abc"; char *px = &(x[0]); fn (px);
As to a critique, I'd like to raise the following.
While legal, I find it incongruous to have function prototypes (such as stringreverse) anywhere other than at file level. In fact, I tend to order my functions so that they're not usually necessary, making one less place where you have to change it, should the arguments or return type need to be changed. That would entail, in this case, placing stringreverse before main.
Don't ever use gets in a real program.. It's unprotectable against buffer overflows. At a minimum, use fgets which can be protected, or use a decent input function such as the one found here.
You cannot create a local variable within stringreverse and pass back the address of it. That's undefined behaviour. Once that function returns, that variable is gone and you're most likely pointing to whatever happens to replace it on the stack the next time you call a function.
There's no need to pass in the revstr variable either. If it were a pointer with backing memory (i.e., had space allocated for it), that would be fine but then there would be no need to return it. In that case you would allocate both in the caller:
char tempstr[1024];
char revstr[1024];
stringreverse (tempstr, revstr); // Note no return value needed
// since you're manipulating revstr directly.
You should also try to avoid magic numbers like 1024. Better to have lines like:
#define BUFFSZ 1024
char tempstr[BUFFSZ];
so that you only need to change it in one place if you ever need a new value (that becomes particularly important if you have lots of 1024 numbers with different meanings - global search and replace will be your enemy in that case rather than your friend).
In order to make you function more adaptable, you may want to consider allowing it to handle any length. You can do that by passing both buffers in, or by using malloc to dynamically allocate a buffer for you, something like:
char *reversestring (char *src) {
char *dst = malloc (strlen (src) + 1);
if (dst != NULL) {
// copy characters in reverse order.
}
return dst;
}
This puts the responsibility for freeing that memory on the caller but that's a well-worn way of doing things.
You should probably use one of the two canonical forms for main:
int main (int argc, char *argv[]);
int main (void);
It's also a particularly bad idea to call main from anywhere. While that may look like a nifty way to get an infinite loop, it almost certainly will end up chewing up your stack space :-)
All in all, this is probably the function I'd initially write. It allows the user to populate their own buffer if they want, or to specify they don't have one, in which case one will be created for them:
char *revstr (char *src, char *dst) {
// Cache size in case compiler not smart enough to do so.
// Then create destination buffer if none provided.
size_t sz = strlen (src);
if (dst == NULL) dst = malloc (sz + 1);
// Assuming buffer available, copy string.
if (dst != NULL) {
// Run dst end to start, null terminator first.
dst += sz; *dst = '\0';
// Copy character by character until null terminator in src.
// We end up with dst set to original correct value.
while (*src != '\0')
*--dst = *src++;
}
// Return reversed string (possibly NULL if malloc failed).
return dst;
}
In your stringreverse() function, you are returning the address of a local variable (revstr). This is undefined behaviour and is very bad. Your program may appear to work right now, but it will suddenly fail sometime in the future for reasons that are not obvious.
You have two general choices:
Have stringreverse() allocate memory for the returned string, and leave it up to the caller to free it.
Have the caller preallocate space for the returned string, and tell stringreverse() where it is and how big it is.