I am trying to create a small c program that will read a string with arbitrary size, without having any memory leaks.
According to my research, the function malloc can be used to allocate a number of bytes for whatever data we want to store.
In my program, I start by allocating space for 0 characters, and I make the pointer word point to it. Then whenever I read a single character, I make a pointer oldWord that points to word, which frees the old memory location once I allocate a larger memory location for the new character.
My research shows that the function free can be used to free an old memory location that is no longer needed. However, I am not sure where I am going wrong. Below you can see my code.
#include <stdio.h>
#include <stdlib.h>
int main(void){
char *word = malloc(0);
printf("Enter name: ");
readWord(word);
printf("Your name is: %s\n", word);
free(word);
word = realloc(0);
printf("Enter name: ");
readWord(word);
printf("Your name is: %s\n", word);
free(word);
return 0;
}
void readWord(char *word){
int i = 0;
char *oldWord, c = getchar();
while(c != ' ' && c != '\n'){
oldWord = word;
word = realloc(word, i + 1);
free(oldWord);
word[i++] = c;
c = getchar();
}
oldWord = word;
word = realloc(word, i + 1);
free(oldWord);
word[i] = '\0';
}
The problem as I see it here is with
free(oldWord);
without checking the failure of realloc(). In case realloc() is success, passing the same pointer to free() causes undefined behavior.
That said, some more notes
a syntax like
word = realloc(word, i + 1);
is dangerous, in case realloc() fails, you'll lose the actual pointer, too. You should use a temporary pointer to hold the return value of realloc(), check for success and only then, assign it back to the original pointer, if you need.
In your code, c is of char type, which may not be sufficient to hold all the possible values returned by getchar(), for example, EOF. You should use an int type, that is what getchar() returns.
There are multiple problems in your code:
you free the pointer you passed to realloc(). This is incorrect as realloc() will have freed the memory already if the block was moved.
Otherwise the pointer is freed twice.
The pointer reallocated bu readWord() is never passed back to the caller.
Allocating a 0 sized block has unspecified behavior: it may return NULL or a valid pointer that should not be dereferenced but can be passed to free() or realloc().
You do not test for end of file: there is an infinite loop if the file does not have a space nor a linefeed in it, for example if the file is empty.
you do not have a prototype for readWord() before it is called.
Here is an improved yet simplistic version:
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
char *readWord(void);
int main(void) {
char *word;
printf("Enter name: ");
word = readWord();
if (word == NULL) {
printf("Unexpected end of file\n");
else
printf("Your name is: %s\n", word);
free(word);
return 0;
}
char *readWord(void) {
int c;
size_t i = 0;
char *word = NULL;
while ((c = getchar()) != EOF && !isspace(c)) {
word = realloc(word, i + 2);
if (word == NULL)
break;
word[i++] = c;
word[i] = '\0';
}
return word;
}
Related
Given program is working for string that is fixed in code for example
char str[100] = "With fixed string this code works"
// Output of the program is "fixed string this code works"
But as soon as I take the str input using
scanf("%s", &str);
it seems error is found with memory allocation because after input is given code returns error value.
The full code is as following
int main (void) {
char str[100];
char *p = (char *)malloc(sizeof(char) * str[100]);
printf("Enter something: ");
scanf("%s", &str);
*p = str;
p = strchr(str, ' ');
puts(p + 1);
// Check for the first space in given input string if found then
while (*p++)
if (*p == ' ' && *++p)
printf("%s", *p);
printf ("\n\n");
return 0;
}
Not sure if for dynamic memory allocation while using scanf function to input string any other allocation process is required
Your malloc has a bug:
char *p = (char *)malloc(sizeof(char)*str[100]);
Let's simplify a bit first.
Don't cast malloc (See: Do I cast the result of malloc?):
char *p = malloc(sizeof(char)*str[100]);
sizeof(char) is (by definition) always 1 on all architectures, regardless of how many bits it occupies, so we can eliminate it:
char *p = malloc(str[100]);
Now we have:
char str[100];
char *p = malloc(str[100]);
You have undefined behavior. str has no values (i.e. unitialized) and you are passing the element that is one past the end of the array, so you have undefined behavior.
So, the length parameter passed to malloc is, thus, random.
You have four problems:
First: with scanf, it should be like that:
scanf("%s",str);
because str is an address. Also make sure that scanf will scan for a string from stdin until it finds space or new line and that what you don't want. So you'd better use fgets(str, 100, stdin);.
Second: with malloc, it should be like that:
malloc(sizeof(char)*100)
because str[100] has not a specific value.
Third: You shouldn't change the address of memory allocated using malloc
malloc function allocates memory in heap and return the address of memory allocated, so you shouldn't do this p = strchr(str, ' '); since strchr will return the address of the first occurrence of the space (the address returned by malloc will be lost) .
Fourth: freeing the memory allocated using malloc
You should free the memory allocated using malloc using free function which take the address of the memory allocated.
Your code should be like that:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (void) {
char str[100];
char *p = malloc(sizeof(char)*100); //Do NOT cast the return of malloc
printf("Enter something: ");
fgets(str, 100, stdin); //Use fgets to include the spaces between words
strcpy(p, str);//use stcpy to Not change the address of memory allocated
//p = strchr(str, ' '); -->This step will change the address
//of memory allocated using malloc
//puts (p + 1);
/*Instead you can do this*/
char *c = strchr(str, ' ');
puts(c+1);
// Check for the first space in given input string if found then
char *mem =p;
while (*p++)
if (*p == ' ' && *++p)
printf ("%s", p);
printf ("\n\n");
free(mem);
return 0;
}
Scanf %s only reads in a string up to the first whitespace and then stops, which is probably what is causing you problems. I would avoid scanf anyway, it's dangerous and can cause problems if you're not familiar with it. You could try fgets to read from stdin instead. See if this does what you want:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define MAX_LENGTH 100
int main (void)
{
char *str = (char *) malloc (sizeof(char) * MAX_LENGTH + 1);
bzero(str, MAX_LENGTH + 1);
printf("Enter something: ");
fgets(str, MAX_LENGTH, stdin);
// Remove the newline
if ((strlen(str) > 0) && (str[strlen (str) - 1] == '\n'))
{
str[strlen (str) - 1] = '\0';
}
char *p = strchr(str, ' ');
if (p != NULL)
{
// Pointing to the space, which we will skip
p++;
printf ("%s\n\n", p);
}
return 0;
}
Here is my full code, it looks like to work, but it's not working very well.
I would accept any code, that is working like this.
Firstly, the code works, but when I want to add the third name to the struct, it crashes.
Is there any other way to do this?
I need struct, because in the future, I want to add some other params, like age, average, gender, etc.
Please, help me out.
//The student table
typedef struct students {
char name[50];
} students;
//Global params
int scount = 0;
students *s;
//Basic functions
void addNewStudent();
int main()
{
int loop = 1;
char in;
int ch;
printf("Willkommen.\n Wahlen Sie bitte von die folgenden Optionen:\n");
while (loop)
{
printf("\t[1] Neue Student eingeben\n");
printf("\t[9] Programm beenden\n");
scanf(" %c", &in);
while ((ch = getchar()) != '\n');
switch (in)
{
case '1':
addNewStudent();
break;
case '9':
loop = 0;
break;
default: printf("------\nOption nicht gefunden.\n------\n");
break;
}
}
free(s);
return 0;
}
void addNewStudent()
{
int index = 0;
if (scount == 0)
{
s = (students*)malloc(sizeof(students));
}
else
{
realloc(s, sizeof(students) * scount);
}
printf("Geben Sie Bitte die Name:\n");
fgets(s[scount].name, sizeof(s[scount].name), stdin);
while (s[scount].name[index] != '\n')
{
index++;
}
s[scount].name[index] = '\0';
scount++;
}
I'm using Visual Studio.
Thanks for help!
students *mynew= realloc(s, sizeof(students)* (scount+1));
if( mynew != NULL )
s=mynew;
Otehrwise you are having a memory leak. You didn't use the return value of realloc.
Don't cast the return type of malloc.
As per standard §7.22.2.35
void *realloc(void *ptr, size_t size)
The realloc function deallocates the old object pointed to by ptr and
returns a pointer to a new object that has the size specified by size.
It is good not to use the same pointer variable on which you are calling malloc because in case it fails you will lose reference to the old one too (unless it is stored by other means).
Also you didn't check the return value of malloc.
s = malloc(sizeof(students));
if( s == NULL ){
frpntf(stderr,"%s","Memory allocation failed");
exit(1);
}
Also you should check the return value of fgets().
if( fgets(s[scount].name, sizeof(s[scount].name), stdin) == NULL){
fprintf(stderr,"%s","Error in input");
exit(1);
}
Also trying to compile your code it showed this
warning: ignoring return value of ‘realloc’, declared with attribute warn_unused_result [-Wunused-result]
realloc(s, sizeof(students) * scount);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When compiling try not to ignore any warning messages. It showed the problem you had.
Important point: (why scount+1 in realloc?)
When reallocating the general idea is increase the number of students. And for that you need to have extra memory allocated for an student. That's why the scount+1 in the code.(realloc).
Some other points:
while (s[scount].name[index] != '\n')
{
index++;
}
s[scount].name[index] = '\0';
You can do it like this also
size_t len = strlen(s[scount].name);
if(len){
s[scount].name[len-1]='\0';
}
To understand why from standard §7.21.7.2
char *fgets(char * restrict s, int n,FILE * restrict stream)
The fgets function reads at most one less than the number of
characters specified by n from the stream pointed to by stream into
the array pointed to by s. No additional characters are read after a
new-line character (which is retained) or after end-of-file. A null
character is written immediately after the last character read into
the array.
\0 character was there already in the inputted string. You can get the length of it but you know that the one before the \0 is the \n character 1 that you entered by pressing the Enter key. We are overwriting it with the \0.
1. This is the usual case but not the only one. There are two cases where this might not be the right way to look at the thing.
The input line has n-1 or more characters before the '\n'. The the one before \0 will not be the \n rather it will be some character inputted by the user.
The last line is a stream which may not have a '\n'. (stdin closed). In that case also the input doesn't contain the \n.
So in these cases the idea of removing \n would fail.Discussed in comment. (chux)
A better and safe solution than overwriting this way:
s[scount].name[strcspn(s[scount].name, "\n")] = '\0';
The explanation from the link is that if a \0 is given as input then we will basically write to s[scount].name[SIZE_MAX] which is not desired.
From the standard §7.24.5.3
size_t strcspn(const char *s1, const char *s2)
The strcspn function computes the length of the maximum initial
segment of the string pointed to by s1 which consists entirely of
characters not from the string pointed to by s2.
How to correctly malloc a struct in C ?
p = malloc(sizeof *p);
if (p == NULL) Handle_OutOfMemory();
How to correctly re-allocate a struct in C ?
void *t = realloc(p, sizeof *p * number_of_elements);
if (t == NULL && number_of_elements > 0) {
Handle_OutOfMemory();
} else {
p = t;
}
p points to some struct. Notice no coding of that type in above.
OP' primary problem is not using the return value of realloc() and allocating 1-too-small
// realloc(s, sizeof(students) * scount);
s = realloc(s, sizeof *s * (scount+1)); // or use above code with check for out-of-memory.
realloc returns a new pointer that you need to keep:
students* snew = realloc(s, sizeof(students) * (scount + 1));
if (!snew) {
free(s); // If there is not enough memory, the old memory block is not freed
// handle out of memory
} else {
s = snew;
}
You are not allocating it back! Take a look at how realloc works. You need to assign the pointer back after making the re-allocation like this.
if (scount == 0)
{
s = (students*)malloc(sizeof(students));
}
else
{
students *temp = realloc(s, sizeof(students) * (scount+1));
if(temp == NULL){
free(s);
}
else{
s = temp;
}
}
By Definition, realloc returns a void pointer but you aren't collecting it.
void *realloc(void *ptr, size_t size);
realloc returns a NULL if there's not enough space. So you can re-assign it when you are sure that it is not NULL
Just make a small change above and your code works like a charm!
Cheers!
I need to get strings dynamically but as I need to get more than one string, I need to use functions. So far I wrote this
(I put //**** at places i think might be wrong)
char* getstring(char *str);
int main() {
char *str;
strcpy(str,getstring(str));//*****
printf("\nString: %s", str);
return 0;
}
char* getstring(char str[]){//*****
//this part is copy paste from my teacher lol
char c;
int i = 0, j = 1;
str = (char*) malloc (sizeof(char));
printf("Input String:\n ");
while (c != '\n') {//as long as c is not "enter" copy to str
c = getc(stdin);
str = (char*)realloc(str, j * sizeof(char));
str[i] = c;
i++;
j++;
}
str[i] = '\0';//null at the end
printf("\nString: %s", str);
return str;//******
}
printf in the function is working but not back in main function.
I tried returning void, getting rid of *s or adding, making another str2 and tring to strcpy there or not using strcpy at all. Nothing seems to working. Am I misssing something? Or maybe this is not possible at all
//Thank you so much for your answers
Getting the string part can be taken from this answer. Only put a \n as input to the getline funtion.
char * p = getline('\n');
Three things :-
don't cast malloc, check if malloc/realloc is successful and sizeof is not a function.
The problem is not with the function that you are using, but with the way you try copying its result into an uninitialized pointer.
Good news is that you don't have to copy - your function already allocates a string in dynamic memory, so you can copy the pointer directly:
char *str = getstring(str);
This should fix the crash. A few points to consider to make your function better:
main needs to free(str) when it is done in order to avoid memory leak
Store realloc result in a temporary pointer, and do a NULL check to handle out-of-memory situations properly
There are two things to take away from the lesson as it stands now:
(1) You should have one way of returning the reference to the new string, either as an argument passed by reference to the function OR as a return value; you should not be implementing both.
(2) Because the subroutine your teacher gave you allocates memory on the heap, it will be available to any part of your program and you do not have to allocate any memory yourself. You should study the difference between heap memory, global memory, and automatic (stack) memory so you understand the differences between them and know how to work with each type.
(3) Because the memory is already allocated on the heap there is no need to copy the string.
Given these facts, your code can be simplified to something like the following:
int main() {
char *str = getstring();
printf( "\nString: %s", str );
return 0;
}
char* getstring(){
.... etc
Going forward, you want to think about how you de-allocate memory in your programs. For example, in this code the string is never de-allocated. It is a good habit to think about your strategy for de-allocating any memory that you allocate.
Let's simplify the code a bit:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* getstring()
{
char c = 0;
int i = 0, j = 2;
char *str = NULL;
if ((str = (char*) malloc(sizeof(char))) == NULL)
return NULL;
printf("Input String: ");
while (c = getc(stdin)) {
if (c == '\n') break;
str = (char*) realloc(str, j * sizeof(char));
str[i++] = c;
j++;
}
str[i] = '\0';
printf("getstring() String: %s\n", str);
return str;
}
int main()
{
char *str = getstring();
printf("main() String: %s\n", str);
free(str);
return 0;
}
Then execute:
$ make teststring && ./teststring
cc teststring.c -o teststring
Input String: asdfasfasdf
getstring() String: asdfasfasdf
main() String: asdfasfasdf
In C, what is the best way of prompting and storing a string without wasted space if we cannot prompt for the string length. For example, normally I would do something like the following...
char fname[30];
char lname[30];
printf("Type first name:\n");
scanf("%s", fname);
printf("Type last name:\n");
scanf("%s", lname);
printf("Your name is: %s %s\n", fname, lname);
However, I'm annoyed with the fact that I have to use more space than needed so I do not want to use char fname[30], but instead dynamically allocate the size of the string. Any thoughts?
You can create a function that dynamically allocates memory for the input as the user types, using getchar() to read one character at a time.
#include <stdio.h>
#include <stdlib.h>
void* safeRealloc(void* ptr, size_t size) {
void *newPtr = realloc(ptr, size);
if (newPtr == NULL) { // if out of memory
free(ptr); // the memory block at ptr is not deallocated by realloc
}
return newPtr;
}
char* allocFromStdin(void) {
int size = 32; // initial str size to store input
char* str = malloc(size*sizeof(char));
if (str == NULL) {
return NULL; // out of memory
}
char c = '\0';
int i = 0;
do {
c = getchar();
if (c == '\r' || c == '\n') {
c = '\0'; // end str if user hits <enter>
}
if (i == size) {
size *= 2; // duplicate str size
str = safeRealloc(str, size*sizeof(char)); // and reallocate it
if (str == NULL) {
return NULL; // out of memory
}
}
str[i++] = c;
} while (c != '\0');
str = safeRealloc(str, i); // trim memory to the str content size
return str;
}
int main(void) {
puts("Type first name:\n");
char* fname = allocFromStdin();
puts("Type last name:\n");
char* lname = allocFromStdin();
printf("Your name is: %s %s\n", fname, lname);
free(fname); // free memory afterwards
free(lname); // for both pointers
return 0;
}
From man scanf:
• An optional 'm' character. This is used with string conversions (%s,
%c, %[), and relieves the caller of the need to allocate a
corresponding buffer to hold the input: instead, scanf() allocates a
buffer of sufficient size, and assigns the address of this buffer to
the corresponding pointer argument, which should be a pointer to a
char * variable (this variable does not need to be initialized before
the call). The caller should subsequently free(3) this buffer when it
is no longer required.
this however is a POSIX extension (as noted by fiddling_bits).
To be portable I think that in your usage case I would prepare a function like the following:
char *alloc_answer() {
char buf[1000];
fgets(buf,sizeof(buf),stdin);
size_t l = strlen(buf);
if (buf[l-1]=='\n') buf[l]=0; // remove possible trailing '\n'
return strdup(buf);
}
even if this solution will break lines longer than 1000 characters (but it prevents buffer overflow, at least).
A fully featured solution would need to read input in chunks and realloc the buffer on every chunk...
I have lineget function that returns char *(it detects '\n') and NULL on EOF.
In main() I'm trying to recognize particular words from that line.
I used strtok:
int main(int argc, char **argv)
{
char *line, *ptr;
FILE *infile;
FILE *outfile;
char **helper = NULL;
int strtoks = 0;
void *temp;
infile=fopen(argv[1],"r");
outfile=fopen(argv[2],"w");
while(((line=readline(infile))!=NULL))
{
ptr = strtok(line, " ");
temp = realloc(helper, (strtoks)*sizeof(char *));
if(temp == NULL) {
printf("Bad alloc error\n");
free(helper);
return 0;
} else {
helper=temp;
}
while (ptr != NULL) {
strtoks++;
fputs(ptr, outfile);
fputc(' ', outfile);
ptr = strtok(NULL, " ");
helper[strtoks-1] = ptr;
}
/*fputs(line, outfile);*/
free(line);
}
fclose(infile);
fclose(outfile);
return 0;
}
Now I have no idea how to put every of tokenized words into an array (I created char ** helper for that purpose), so that it can be used in qsort like qsort(helper, strtoks, sizeof(char*), compare_string);.
Ad. 2 Even if it would work - I don't know how to clear that line, and proceed to sorting next one. How to do that?
I even crashed valgrind (with the code presented above) -> "valgrind: the 'impossible' happened:
Killed by fatal signal"
Where is the mistake ?
The most obvious problem (there may be others) is that you're reallocating helper to the value of strtoks at the beginning of the line, but then incrementing strtoks and adding to the array at higher values of strtoks. For instance, on the first line, strtoks is 0, so temp = realloc(helper, (strtoks)*sizeof(char *)); leaves helper as NULL, but then you try to add every word on that line to the helper array.
I'd suggest an entirely different approach which is conceptually simpler:
char buf[1000]; // or big enough to be bigger than any word you'll encounter
char ** helper;
int i, numwords;
while(!feof(infile)) { // most general way of testing if EOF is reached, since EOF
// is just a macro and may not be machine-independent.
for(i = 0; (ch = fgetc(infile)) != ' ' && ch != '\n'; i++) {
// get chars one at a time until we hit a space or a newline
buf[i] = ch; // add char to buffer
}
buf[i + 1] = '\0' // terminate with null byte
helper = realloc(++numwords * sizeof(char *)); // expand helper to fit one more word
helper[numwords - 1] = strdup(buffer) // copy current contents of buffer to the just-created element of helper
}
I haven't tested this so let me know if it's not correct or there's anything you don't understand. I've left out the opening and closing of files and the freeing at the end (remember you have to free every element of helper before you free helper itself).
As you can see in strtok's prototype:
char * strtok ( char * str, const char * delimiters );
...str is not const. What strtok actually does is replace found delimiters by null bytes (\0) into your str and return a pointer to the beginning of the token.
Per example:
char in[] = "foo bar baz";
char *toks[3];
toks[0] = strtok(in, " ");
toks[1] = strtok(NULL, " ");
toks[2] = strtok(NULL, " ");
printf("%p %s\n%p %s\n%p %s\n", toks[0], toks[0], toks[1], toks[1],
toks[2], toks[2]);
printf("%p %s\n%p %s\n%p %s\n", &in[0], &in[0], &in[4], &in[4],
&in[8], &in[8]);
Now look at the results:
0x7fffd537e870 foo
0x7fffd537e874 bar
0x7fffd537e878 baz
0x7fffd537e870 foo
0x7fffd537e874 bar
0x7fffd537e878 baz
As you can see, toks[1] and &in[4] point to the same location: the original str has been modified, and in reality all tokens in toks point to somewhere in str.
In your case your problem is that you free line:
free(line);
...invalidating all your pointers in helper. If you (or qsort) try to access helper[0] after freeing line, you end up accessing freed memory.
You should copy the tokens instead, e.g.:
ptr = strtok(NULL, " ");
helper[strtoks-1] = malloc(strlen(ptr) + 1);
strcpy(helper[strtoks-1], ptr);
Obviously, you will need to free each element of helper afterwards (in addition to helper itself).
You should be getting a 'Bad alloc' error because:
char **helper = NULL;
int strtoks = 0;
...
while ((line = readline(infile)) != NULL) /* Fewer, but sufficient, parentheses */
{
ptr = strtok(line, " ");
temp = realloc(helper, (strtoks)*sizeof(char *));
if (temp == NULL) {
printf("Bad alloc error\n");
free(helper);
return 0;
}
This is because the value of strtoks is zero, so you are asking realloc() to free the memory pointed at by helper (which was itself a null pointer). One outside chance is that your library crashes on realloc(0, 0), which it shouldn't but it is a curious edge case that might have been overlooked. The other possibility is that realloc(0, 0) returns a non-null pointer to 0 bytes of data which you are not allowed to dereference. When your code dereferences it, it crashes. Both returning NULL and returning non-NULL are allowed by the C standard; don't write code that crashes regardless of which behaviour realloc() shows. (If your implementation of realloc() does not return a non-NULL pointer for realloc(0, 0), then I'm suspicious that you aren't showing us exactly the code that managed to crash valgrind (which is a fair achievement — congratulations) because you aren't seeing the program terminate under control as it should if realloc(0, 0) returns NULL.)
You should be able to avoid that problem if you use:
temp = realloc(helper, (strtoks+1) * sizeof(char *));
Don't forget to increment strtoks itself at some point.