I'm trying to allocate memory only if i need it for the next while.
char *str = malloc(sizeof(char));
int i = 0;
while(something == true){
str[i] = fgetc(fp);
str = realloc(str, strlen(str)+1);
i++;
}
free(str);
But for some reason the code above give me an "Invalid read of size 1" at strlen().
strlen will not determine the size of the allocated char array even if it contains a null terminated string. See proposed fix although I do not like the code structure overall: You will always end up with an extra allocated character.
char *str = malloc(sizeof(char));
int i = 0;
while(something == true){
str[i] = fgetc(fp);
str = realloc(str, (i+2)*sizeof(char));
i++;
}
// str[i*sizeof(char)]='\0'; <-- Add this if you want a null terminated string
free(str);
I would propose the following code that would avoid allocating the extra character:
char *str = NULL;
int i = 0;
while(something == true){
str = realloc(str, (i+1)*sizeof(char));
str[i] = fgetc(fp);
i++;
}
free(str);
As per documentation, "In case that ptr is a null pointer, the function behaves like malloc, assigning a new block of size bytes and returning a pointer to its beginning."
This is in case you are not reading text and not planning to use such functions as strlen, strcat...
Chunk at a time allocation:
char *str = malloc(sizeof(char));
int i = 0;
const int chunk_size = 100;
while(something == true){
str[i] = fgetc(fp);
if (i % chunk_size == 0)
str = realloc(str, (i+1+chunk_size)*sizeof(char));
i++;
}
// str[i*sizeof(char)]='\0'; <-- Add this if you want a null terminated string
free(str);
Related
char* scanString()
{
char* str = NULL;
char* temp = NULL;
int numOfChars = 0;
char c = '0';
while (c != '\n')
{
scanf(" %c", &c);
if (c != '\n')
{
if (numOfChars == 0)
{
char* str = (char*)malloc(sizeof(char));
char* temp = str;
if (str == NULL)
return str;
str[0] = c;
numOfChars++;
}
else
{
str = (char*)realloc(str, sizeof(char) * (numOfChars + 1));
str[numOfChars] = c;
if (str == NULL)
return temp;
else
{
temp = str;
numOfChars++;
}
}
}
}
str = (char*)realloc(str, sizeof(char) * (numOfChars+1));
if (str == NULL)
{
str = temp;
return str;
}
str[numOfChars] = '\0';
return str;
}
int main()
{
char* m;
printf("write:\n");
m = scanString();
printf("%s\n", m);
}
I tried to create a function for scanning a string of unknown size char by char and i don't know what'ss the problem here. Btw please don't approach me to any other code or try to use different libraries.
There is one big error and some inconsistencies.
The big error is here:
if (numOfChars == 0)
{
char* str = (char*)malloc(sizeof(char)); // Oops a new var!
char* temp = str; // and another one!
if (str == NULL)
return str;
str[0] = c;
numOfChars++;
}
You declare 2 new variables in that bloc that will hide the variables of outer scope. As a result, the first character will be lost and you will get a random value.
The inconsistencies:
temp is useless and should be removed
you read with a format " %c". The format will skip any blank character including \n. It should be "%c"
you fail to test the return value of scanf. On end of file (of any other read error) you will enter an endless loop. It should be:
if (1 != scanf("%c", &c)) break;
Once this is fixed, you should get the expected output, but other improvements are still possible:
the idiomatic way to read one character is getc or getchar
allocating one character at a time is an anti-pattern because (re-)allocation is a rather expensive operation. For a real world program, you should always allocate a bunch or memory and keep track of the available part
sizeof(char) is 1 per standard
the distinction for numOfChars == 0 is useless. realloc on a NULL pointer is the same of malloc.
Your code is overly complicated and wrong and as stated in a comment you need to replace the format string " %c" with "%c".
The main problem is here:
if (numOfChars == 0)
{
char* str = (char*)malloc(sizeof(char));
You declare a new str variable which shadows the str variable you've declared at the beginning of the function.
Just replace char* str = (char*)malloc(sizeof(char)) with str = malloc(sizeof(char));. BTW the cast is not necessary.
You have the same problem with temp.
The code below is entirely based on your code, but it is simpler and correct. Basically the case where numOfChars is 0 should not be treated specially. You can just use realloc, because realloc(NULL, foo) is equivalent to malloc(foo) .
char* scanString()
{
char* str = NULL;
char* temp = NULL;
int numOfChars = 0;
char c = '0';
while (c != '\n')
{
scanf("%c", &c);
if (c != '\n')
{
str = realloc(str, sizeof(char) * (numOfChars + 1));
if (str == NULL)
return temp;
str[numOfChars] = c;
temp = str;
numOfChars++;
}
}
str = realloc(str, sizeof(char) * (numOfChars + 1));
if (str == NULL)
{
str = temp;
return str;
}
str[numOfChars] = '\0';
return str;
}
Or even simpler:
char* scanString()
{
char* str = NULL;
char* temp = NULL;
int numOfChars = 0;
char c = '0';
while (c != '\n')
{
scanf("%c", &c);
str = realloc(str, sizeof(char) * (numOfChars + 1));
if (str == NULL)
return temp;
str[numOfChars] = c;
temp = str;
if (c == '\n')
{
str[numOfChars] = '\0';
}
numOfChars++;
}
return str;
}
I wrote this code, but inserts garbage in the start of string:
void append(char *s, char c) {
int len = strlen(s);
s[len] = c;
s[len + 1] = '\0';
}
int main(void) {
char c, *s;
int i = 0;
s = malloc(sizeof(char));
while ((c = getchar()) != '\n') {
i++;
s = realloc(s, i * sizeof(char));
append(s, c);
}
printf("\n%s",s);
}
How can I do it?
There are multiple problems in your code:
you iterate until you read a newline ('\n') from the standard input stream. This will cause an endless loop if the end of file occurs before you read a newline, which would happen if you redirect standard input from an empty file.
c should be defined as int so you can test for EOF properly.
s should be null terminated at all times, you must set the first byte to '\0' after malloc() as this function does not initialize the memory it allocates.
i should be initialized to 1 so the first realloc() extends the array by 1 etc. As coded, your array is one byte too short to accommodate the extra character.
you should check for memory allocation failure.
for good style, you should free the allocated memory before exiting the program
main() should return an int, preferably 0 for success.
Here is a corrected version:
#include <stdio.h>
#include <stdlib.h>
/* append a character to a string, assuming s points to an array with enough space */
void append(char *s, char c) {
size_t len = strlen(s);
s[len] = c;
s[len + 1] = '\0';
}
int main(void) {
int c;
char *s;
size_t i = 1;
s = malloc(i * sizeof(char));
if (s == NULL) {
printf("memory allocation failure\n");
return 1;
}
*s = '\0';
while ((c = getchar()) != EOF && c != '\n') {
i++;
s = realloc(s, i * sizeof(char));
if (s == NULL) {
printf("memory allocation failure\n");
return 1;
}
append(s, c);
}
printf("%s\n", s);
free(s);
return 0;
}
when you call strlen it searches for a '\0' char to end the string. You don't have this char inside your string to the behavior of strlen is unpredictable.
Your append function is acually good.
Also, a minor thing, you need to add return 0; to your main function. And i should start from 1 instead if 0.
Here is how it should look:
int main(void){
char *s;
size_t i = 1;
s = malloc (i * sizeof(char));//Just for fun. The i is not needed.
if(s == NULL) {
fprintf(stderr, "Coul'd not allocate enough memory");
return 1;
}
s[0] = '\0';
for(char c = getchar(); c != '\n' && c != EOF; c = getchar()) {//it is not needed in this case to store the result as an int.
i++;
s = realloc (s,i * sizeof(char) );
if(s == NULL) {
fprintf(stderr, "Coul'd not allocate enough memory");
return 1;
}
append (s,c);
}
printf("%s\n",s);
return 0;
}
Thanks for the comments that helped me improve the code (and for my english). I am not perfect :)
The inner realloc needs to allocate one element more (for the trailing \0) and you have to initialize s[0] = '\0' before starting the loop.
Btw, you can replace your append by strcat() or write it like
size_t i = 0;
s = malloc(1);
/* TODO: check for s != NULL */
while ((c = getchar()) != '\n') {
s[i] = c;
i++;
s = realloc(s, i + 1);
/* TODO: check for s != NULL */
}
s[i] = '\0';
I'm trying to declare an array of char dynamically, what I need to do is adding the newest character to the string which works fine, the problem is that when I try to print it out, at the beginning of the string there are some unknown characters.
char add[2];
char str2[200];
char c;
int temp = -1;
int num = 0;
char *str3;
str3 = malloc( (size_t)count ); //str3 = malloc(sizeof(char)) not working
while((c= getch()) !='\r')
{
for (int i = 0;i<200;i++)
{
if (str2[i] =='\0')
{
num = i;
break;
}
}
//printf("Num: %d\n",num);
if ((temp == -32) || (temp == 0))
{
}
else
{
if(isalnum((char)c) == 0)
{
if((c == '\'') || (c == -118) || (c == -115) || (c == -107) || (c == -123) || (c == -105)|| (c == 32))
{
realloc(str3,sizeof(char)+2);
printf("true: %c\n",c);
//realloc(str2,sizeof(char)+1);
add[1] = '\0';
add[0] = c;
strcat(str3,add);
strcat(str2,add);
printf("%s\n",str2);
printf("%s\n",str3);
}
else if (c == 8)
{
printf("Deleting something...\n");
}
}
else
{
realloc(str3,sizeof(char)+2);
printf("true: %c\n",c);
//realloc(str2,sizeof(char)+1);
add[1] = '\0';
add[0] = c;
strcat(str3,add);
strcat(str2,add);
printf("%s\n",str2);
printf("%s\n",str3);
}
}
printf("ASCII Code: %d\n",c);
temp = c;
}
To get some memory to your string, you have to tell malloc how many bytes of memory you want. sizeof(char) returns 1, therefore, you'll only have 1 byte. In C, strings are terminated by the NULL byte (\0), and printf and others will print until they find that NULL terminator.
If you do something like this:
char *str = malloc(1);
*str = 'a';
printf("%s", str);
You will probably get a very strange output, since you have no NULL terminator.
When you use the unsigned x; str = malloc(x);, it's actually undefined how many bytes you have, since that x variable is not initialized.
Since your question is very unclear, what I can tell you (from what I think you're asking) is how to actually get space for a string of 63 characters plus the NULL terminating byte.
char *str = malloc(64);
strcpy(str, "Stack Overflow");
printf("%s", str);
That will do it.
Also note that the memory block returned by malloc will not be zeroed, therefore you can't possibly know what's in it (that could be the reason you're getting garbage when you're printing).
I recommend you read about memory allocation in a good C book or in Wikipedia...
After your edit and "MCVE"
I made some edits to what I think it is you want. The modifications are explained in the comments of the source. Let me know if you have any doubts.
#include <stdio.h> /* printf */
#include <stdlib.h> /* malloc, free, realloc */
#include <string.h> /* strcat */
#include <ctype.h> /* isalnum */
#include <conio.h> /* getch */
int main(void)
{
char add[2];
char str2[200];
char c;
int temp = -1;
int num = 0;
char *str3;
/* I just think 'count' is an int, since you didn't put it in the code,
* I also deduced that #count will be used as the length of #str3
*/
int count;
/* Here, count is not initialized, so you MUST initialize it in order
* to call malloc with it! Since it seems you want to add character by
* character using realloc, then we just malloc() 2 bytes - 1 for a
* character and one for the NULL terminator.
*/
count = 2;
str3 = malloc(count);
/* You will be using #strcat to append strings to #str3, so you need
* to put a NULL terminator in it, because strcat will look for that
* NULL byte to find where it should append
*/
*str3 = 0x0;
while((c = getch()) != '\r') {
for (int i = 0;i < 200; i++) {
if (str2[i] =='\0') {
num = i;
break;
}
}
if ((temp == -32) || (temp == 0)) {
/* empty */
} else {
if(isalnum((char)c) == 0)
{
if((c == '\'') || (c == -118) || (c == -115) || (c == -107) || (c == -123) || (c == -105)|| (c == 32))
{
/* this is not the optimal way of using realloc, because
* you should first check for errors, but will do for
* this example.
* You must assign the returned value of realloc to str3.
*
* Also, since #count contains the length
* of #str3, you need to increment it.
*/
str3 = realloc(str3, ++count);
printf("true: %c\n",c);
add[1] = '\0';
add[0] = c;
strcat(str3,add);
strcat(str2,add);
printf("str2: %s\n",str2);
printf("str3: %s\n",str3);
} else if (c == 8) {
printf("Deleting something...\n");
}
} else {
/* see notes above on realloc */
str3 = realloc(str3, ++count);
printf("true: %c\n",c);
add[1] = '\0';
add[0] = c;
strcat(str3,add);
strcat(str2,add);
printf("str2: %s\n",str2);
printf("str3: %s\n",str3);
}
}
printf("ASCII Code: %d\n",c);
temp = c;
}
return 0;
}
In the first two cases, you are only allocating enough space for a single char. If you attempt to write more than one to that block of memory, you'll write past the end of the memory that was allocated for you. Doing so invokes undefined behavior, which in this case manifests as printing strange characters.
In the third case, you allocate x bytes of memory, however x is uninitialized and has an indeterminate value. Reading an indeterminate value is also undefined behavior. In this case it happens to work because the indeterminate value happens to be a valid value and is large enough to hold the string you want, however you can't depend on that behavior.
You need to allocate a byte for every character that you'll need, plus 1 for the terminating null byte that ends a string in C.
Note that the first allocation, this one
str = malloc(sizeof(char));
is exactly equivalent to1
str = malloc(1);
so you don't have room except for one character which is a problem, because it only represents an empty string.
If you allocate this much space you will very likely access memory out of the allocated space, causing undefined and unpredictable behavior. You need to understand what a string in c is,
A string in c is a sequence of non-null characters followed by a null character, so for a string with N characters you need N + 1 array elements (for ascii this equals bytes)
According to that definition of string if you wanted to store the string "Hello" you would need at least the following code
char *str = malloc(6);
if (str != NULL) {
str[0] = 'H';
str[1] = 'e';
str[2] = 'l';
str[3] = 'l';
str[4] = 'o';
str[5] = '\0'; // Or equivalently str[5] = 0;
}
as you can see, the last character being '\0' or 0 — which is the same — is very important.
All the functions in the standard library of c which expect a string parameter expect that there is the null terminator. For instance strlen() will count characters until it reaches the '\0', if it's not there then you can't predict where it is going to stop counting, this causing undefined behavior.
1sizeof(char) is as defined by the c standard always equal to one.
I am trying to read in from stdin (passing in value from a file). I am reading each character from the string and storing it into a dynamically allocated string pointer. When needed I realloc the memory. I am trying to get as many characters as possible. Though I can limit it to 100,000 chars. But the realloc fails after some iteration. But if I specify a chunk size big, say 1048567 during the first initialization in malloc, I am able to read the string completely. Why is this?
Below is my program:
#include <stdio.h>
#include <stdlib.h>
int display_mem_alloc_error();
enum {
CHUNK_SIZE = 31 //31 fails. But 1048567 passes.
};
int display_mem_alloc_error() {
fprintf(stderr, "\nError allocating memory");
exit(1);
}
int main(int argc, char **argv) {
int numStr; //number of input strings
int curSize = CHUNK_SIZE; //currently allocated chunk size
int i = 0; //counter
int len = 0; //length of the current string
int c; //will contain a character
char *str = NULL; //will contain the input string
char *str_cp = NULL; //will point to str
char *str_tmp = NULL; //used for realloc
str = malloc(sizeof(*str) * CHUNK_SIZE);
if (str == NULL) {
display_mem_alloc_error();
}
str_cp = str; //store the reference to the allocated memory
scanf("%d\n", &numStr); //get the number of input strings
while (i != numStr) {
if (i >= 1) { //reset
str = str_cp;
len = 0;
curSize = CHUNK_SIZE;
}
c = getchar();
while (c != '\n' && c != '\r') {
*str = (char *) c;
//printf("\nlen: %d -> *str: %c", len, *str);
str = str + 1;
len = len + 1;
*str = '\0';
c = getchar();
if (curSize / len == 1) {
curSize = curSize + CHUNK_SIZE;
//printf("\nlen: %d", len);
printf("\n%d \n", curSize); //NB: If I comment this then the program simply exits. No message is displayed.
str_tmp = realloc(str_cp, sizeof(*str_cp) * curSize);
if (str_tmp == NULL) {
display_mem_alloc_error();
}
//printf("\nstr_tmp: %d", str_tmp);
//printf("\nstr: %d", str);
//printf("\nstr_cp: %d\n", str_cp);
str_cp = str_tmp;
str_tmp = NULL;
}
}
i = i + 1;
printf("\nlen: %d", len);
//printf("\nEntered string: %s\n", str_cp);
}
str = str_cp;
free(str_cp);
free(str);
str_cp = NULL;
str = NULL;
return 0;
}
Thanks.
When you realloc
str_tmp = realloc(str_cp, sizeof(*str_cp) * curSize);
if (str_tmp == NULL) {
display_mem_alloc_error();
}
//printf("\nstr_tmp: %d", str_tmp);
//printf("\nstr: %d", str);
//printf("\nstr_cp: %d\n", str_cp);
str_cp = str_tmp;
str_tmp = NULL;
you let str_cp point to the new block of memory, but str still points into the old, now freed block. Thus when you access what str points to in the next iteration, you invoke undefined behaviour.
You need to save the offset of str with respect to str_cp, and after the reallocation, letstr point into the new block at its old offset.
And *str = (char *) c; is wrong, although there is a nonzero chance of it being functionally equivalent to the correct *str = c;.
*str = (char *) c;
This line is wrong.
str is a pointer to char and *str is a char but you are assigning a pointer to char to a char. This cannot be done in C.
Moreover:
scanf("%d\n", &numStr);
The \n in scanf call probably does not what you expect:
http://c-faq.com/stdio/scanfhang.html
And also:
str = str_cp;
free(str_cp);
free(str);
You have a double free here. After the assignment str and str_cp will have the same value so doing:
free(str_cp);
free(str);
is as if you do:
free(str);
free(str);
which is undefined behavior (you cannot free twice).
I want to read input from user using C program. I don't want to use array like,
char names[50];
because if the user gives string of length 10, then the remaining spaces are wasted.
If I use character pointer like,
char *names;
then I need to allocate memory for that in such a way of,
names = (char *)malloc(20 * sizeof(char));
In this case also, there is a possibility of memory wastage.
So, what I need is to dynamically allocate memory for a string which is of exactly same as the length of the string.
Lets assume,
If the user input is "stackoverflow", then the memory allocated should be of 14 (i.e. Length of the string = 13 and 1 additional space for '\0').
How could I achieve this?
Read one character at a time (using getc(stdin)) and grow the string (realloc) as you go.
Here's a function I wrote some time ago. Note it's intended only for text input.
char *getln()
{
char *line = NULL, *tmp = NULL;
size_t size = 0, index = 0;
int ch = EOF;
while (ch) {
ch = getc(stdin);
/* Check if we need to stop. */
if (ch == EOF || ch == '\n')
ch = 0;
/* Check if we need to expand. */
if (size <= index) {
size += CHUNK;
tmp = realloc(line, size);
if (!tmp) {
free(line);
line = NULL;
break;
}
line = tmp;
}
/* Actually store the thing. */
line[index++] = ch;
}
return line;
}
You could have an array that starts out with 10 elements. Read input character by character. If it goes over, realloc another 5 more. Not the best, but then you can free the other space later.
You can also use a regular expression, for instance the following piece of code:
char *names
scanf("%m[^\n]", &names)
will get the whole line from stdin, allocating dynamically the amount of space that it takes. After that, of course, you have to free names.
If you ought to spare memory, read char by char and realloc each time. Performance will die, but you'll spare this 10 bytes.
Another good tradeoff is to read in a function (using a local variable) then copying. So the big buffer will be function scoped.
Below is the code for creating dynamic string :
void main()
{
char *str, c;
int i = 0, j = 1;
str = (char*)malloc(sizeof(char));
printf("Enter String : ");
while (c != '\n') {
// read the input from keyboard standard input
c = getc(stdin);
// re-allocate (resize) memory for character read to be stored
str = (char*)realloc(str, j * sizeof(char));
// store read character by making pointer point to c
str[i] = c;
i++;
j++;
}
str[i] = '\0'; // at the end append null character to mark end of string
printf("\nThe entered string is : %s", str);
free(str); // important step the pointer declared must be made free
}
First, define a new function to read the input (according to the structure of your input) and store the string, which means the memory in stack used. Set the length of string to be enough for your input.
Second, use strlen to measure the exact used length of string stored before, and malloc to allocate memory in heap, whose length is defined by strlen. The code is shown below.
int strLength = strlen(strInStack);
if (strLength == 0) {
printf("\"strInStack\" is empty.\n");
}
else {
char *strInHeap = (char *)malloc((strLength+1) * sizeof(char));
strcpy(strInHeap, strInStack);
}
return strInHeap;
Finally, copy the value of strInStack to strInHeap using strcpy, and return the pointer to strInHeap. The strInStack will be freed automatically because it only exits in this sub-function.
This is a function snippet I wrote to scan the user input for a string and then store that string on an array of the same size as the user input. Note that I initialize j to the value of 2 to be able to store the '\0' character.
char* dynamicstring() {
char *str = NULL;
int i = 0, j = 2, c;
str = (char*)malloc(sizeof(char));
//error checking
if (str == NULL) {
printf("Error allocating memory\n");
exit(EXIT_FAILURE);
}
while((c = getc(stdin)) && c != '\n')
{
str[i] = c;
str = realloc(str,j*sizeof(char));
//error checking
if (str == NULL) {
printf("Error allocating memory\n");
free(str);
exit(EXIT_FAILURE);
}
i++;
j++;
}
str[i] = '\0';
return str;
}
In main(), you can declare another char* variable to store the return value of dynamicstring() and then free that char* variable when you're done using it.
Here's a snippet which I wrote which performs the same functionality.
This code is similar to the one written by Kunal Wadhwa.
char *dynamicCharString()
{
char *str, c;
int i = 0;
str = (char*)malloc(1*sizeof(char));
while(c = getc(stdin),c!='\n')
{
str[i] = c;
i++;
realloc(str,i*sizeof(char));
}
str[i] = '\0';
return str;
}
char* load_string()
{
char* string = (char*) malloc(sizeof(char));
*string = '\0';
int key;
int sizer = 2;
char sup[2] = {'\0'};
while( (key = getc(stdin)) != '\n')
{
string = realloc(string,sizer * sizeof(char));
sup[0] = (char) key;
strcat(string,sup);
sizer++
}
return string;
}
int main()
{
char* str;
str = load_string();
return 0;
}
realloc is a pretty expensive action...
here's my way of receiving a string, the realloc ratio is not 1:1 :
char* getAString()
{
//define two indexes, one for logical size, other for physical
int logSize = 0, phySize = 1;
char *res, c;
res = (char *)malloc(sizeof(char));
//get a char from user, first time outside the loop
c = getchar();
//define the condition to stop receiving data
while(c != '\n')
{
if(logSize == phySize)
{
phySize *= 2;
res = (char *)realloc(res, sizeof(char) * phySize);
}
res[logSize++] = c;
c = getchar();
}
//here we diminish string to actual logical size, plus one for \0
res = (char *)realloc(res, sizeof(char *) * (logSize + 1));
res[logSize] = '\0';
return res;
}