I'm trying to make a replace function in C. I know there are many out there that I could copy, but I decided to make my own function in order to practice.
However, I'm stuck at this:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void replace_content(char *rep, char *with, char **text) {
int len_rep = strlen(rep);
int len_with = strlen(with);
char *p = *text;
int new_text_size = 0;
char *new_text = malloc(new_text_size);
do {
if (!strncmp(p, rep, len_rep)) {
new_text_size += len_with;
new_text = (char *) realloc(new_text, new_text_size + 1);
strcat(new_text, with);
p += len_rep;
} else {
new_text_size++;
new_text = (char *) realloc(new_text, new_text_size);
new_text[new_text_size-1] = *p;
p++;
}
} while (*p != '\0');
*text = malloc(new_text_size);
strcpy(*text, new_text);
}
int main() {
printf("Testing a replace function:\n");
char *text =
"<serviceName>\n"
" <label1>a</label1>\n"
" <label2>b</label2>\n"
" <label3>c</label3>\n"
"</serviceName>\n";
printf("Before replace:\n%s", text);
replace_content("serviceName>", "serviceNameResponse>", &text);
printf("After replace:\n%s", text);
return 0;
}
This is the output I'm seeing so far:
Testing a replace function:
Before replace:
<serviceName>
<label1>a</label1>
<label2>b</label2>
<label3>c</label3>
</serviceName>
After replace:
<0�serviceNameRespons
<label1>a</label1>
<label2>b</label2>
<label3>c</label3>
</serviceNameResponse>
My guess is that I'm doing something wrong with dynamic memory, but the more I look into my code the more confused I am.
These two statements are problematic:
new_text = (char *) realloc(new_text, new_text_size + 1);
strcat(new_text, with);
The first problem is that you should never assign back directly to the pointer you reallocate. That is because realloc may fail and return NULL, making you lose the original pointer.
The second problem is that new_text doesn't initially point to a null-terminated string, which makes the call to strcat undefined behavior.
There is also a problem in the else branch:
new_text = (char *) realloc(new_text, new_text_size);
new_text[new_text_size-1] = *p;
Besides the same problem with reassigning back to the pointer being reallocated, you don't terminate the string in new_text.
May the reason is malloc(0) in line 10 char *new_text = malloc(new_text_size);.
The malloc() function allocates size bytes and returns a pointer to
the allocated memory. The memory is not initialized. If size is 0,
then malloc() returns either NULL, or a unique pointer value that
can later be successfully passed to free().
I suggest using char *new_text = NULL; instead.
Related
I try to code my own concatenation function in C without library, but I have issue and I don't know where it comes from.
To do my function I use pointers of char.
This is my Code :
#include <stdio.h>
#include <stdlib.h>
int longueur(char *str)
{
int i =0;
while(str[i] != '\0')
{
i++;
}
return i;
}
void concat(char* source, char* dest)
{
int longStr1 = (longueur(source));
int longStr2 = (longueur(dest));
int i=0, j=0;
char* temp = dest;
free(dest);
dest = (char*) realloc(dest, ((longStr1 + longStr2)* sizeof(char)));
/*dest[0] = temp[0]; <------ If I do this it will generate issue, so the bellow code too*/
while(temp[i] != '\0')
{
dest[i] = temp[i];
i++;
}
while(source[j] != '\0')
{
dest[i] = source[j];
i++;
j++;
}
dest[i] = '\0';
}
int main()
{
char *str1 = "World";
char *str2 = "Hello";
concat(str1, str2);
printf("-------------\n%s", str2);
return 0;
}
EDIT
I read all your answer, so I changed my concat function to :
void concat(char* source, char* dest)
{
int longStr1 = (longueur(source));
int longStr2 = (longueur(dest));
int i=0, j=0;
dest = (char*) malloc((longStr1 + longStr2)* sizeof(char) + sizeof(char));
while(dest[i] != '\0')
{
dest[i] = dest[i];
i++;
}
while(source[j] != '\0')
{
dest[i] = source[j];
i++;
j++;
}
dest[i] = '\0';
}
Now I don't have issue but my code only display "Hello"
In addition to all the good comments and solutions: realloc can give you a different pointer and you must return that pointer. So your function signature should be:
void concat(char* source, char** dest)
{
int longStr1 = (longueur(source));
int longStr2 = (longueur(dest));
int i=0, j=0;
char* temp = *dest, *temp2;
if ((temp2 = realloc(dest, ((longStr1 + longStr2)+1))==NULL) return;
*dest= temp2;
while(temp[i] != '\0')
{
*dest[i] = temp[i];
i++;
}
while(source[j] != '\0')
{
*dest[i] = source[j];
i++;
j++;
}
*dest[i] = '\0';
}
..and this assumes the function will only be called with a dest that was allocated with malloc. And sizeof(char) is always 1. (This resulting function is not optimal.)
--EDIT--
Below the correct, optimized version:
void concat(char* source, char** dest)
{
int longSrc = longueur(source);
int longDst = longueur(dest);
char *pDst, *pSrc;
if ((pDst = realloc(*dest, longSrc + longDst + 1))==NULL) return;
if (pDst != *dest) *dest= pDst;
pDst += longSrc;
pSrc= source;
while(pSrc)
*pDst++ = *pSrc++;
*pDst = '\0';
}
In your code
free(dest);
and
dest = (char*) realloc(dest, ((longStr1 + longStr2)* sizeof(char)));
invokes undefined behavior as none of them use a pointer previously allocated by malloc() or family.
Mostly aligned with your approach, you need to make use of another pointer, allocate dynamic memory and return that pointer. Do not try to alter the pointers received as parameters as you've passed string literals.
That said, you need to have some basic concepts clear first.
You need not free() a memory unless it is allocated through malloc() family.
You need to have a char extra allocated to hold the terminating null.
Please see this discussion on why not to cast the return value of malloc() and family in C..
If your concatenation function allocates memory, then, the caller needs to take care of free()-ing the memory, otherwise it will result in memory leak.
After you have freed dest here:
free(dest);
You cannot use this pointer in following call to realloc:
dest = (char*) realloc(dest, ((longStr1 + longStr2)* sizeof(char)));
/*dest[0] = temp[0]; <------ If I do this it will generate issue, so the bellow code too*/
man realloc
void *realloc(void *ptr, size_t size);
The realloc() function changes the size of the memory block
pointed to by ptr to size bytes. (...)
But this pointer is invalid now and you cannot use it anymore. When you call free(dest), the memory dest points to is being freed, but the value of dest stays untouched, making the dest a dangling pointer. Accessing the memory that has already been freed produces undefined behavior.
NOTE:
Even if free(dest) is technically valid when called on pointer to memory allocated by malloc (it is not an error in your function to call free(dest) then), it is incorrect to use this on pointer to literal string as you do in your example (because str2 points to string literal it is an error to pass this pointer to function calling free on it).
Given your original use, perhaps you would find a variant like this useful
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
size_t longueur ( const char * str ) { /* correct type for string lengths */
size_t len = 0;
while (*str++ != '\0') ++len;
return len;
}
char * concat ( const char * first, const char * second ) {
const char * s1 = first ? first : ""; /* allow NULL input(s) to be */
const char * s2 = second ? second : ""; /* treated as empty strings */
size_t ls1 = longueur(s1);
size_t ls2 = longueur(s2);
char * result = malloc( ls1 + ls2 + 1 ); /* +1 for NUL at the end */
char * dst = result;
if (dst != NULL) {
while ((*dst = *s1++) != '\0') ++dst; /* copy s1\0 */
while ((*dst = *s2++) != '\0') ++dst; /* copy s2\0 starting on s1's \0 */
}
return result;
}
int main ( void ) {
const char *str1 = "Hello";
const char *str2 = " World";
char * greeting = concat(str1, str2);
printf("-------------\n%s\n-------------\n", greeting);
free(greeting);
return 0;
}
In this variant, the two inputs are concatenated and the result of the concatenation is returned. The two inputs are left untouched.
There's a function. It is add_lexem and adds an element (char *) in the end of specified array and. If no memory left, it allocates some extra memory (100 * sizeof(char *)). That function causes segfault, which is the problem.
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
void add_lexem(char **lexems, int *lexemsc, int *lexem_n, const char *lexem)
{
if (*lexem_n >= *lexemsc) {
lexems = realloc(lexems, sizeof(char *) * (*lexemsc + 100));
*lexemsc += 100;
}
char *for_adding = malloc(sizeof(char) * strlen(lexem));
strcpy(for_adding, lexem);
lexems[*lexem_n] = for_adding;
(*lexem_n)++;
}
int main(void)
{
char **D = malloc(sizeof(char *) * 2);
int lexemsc = 2;
int lexem_n = 0;
add_lexem(D, &lexemsc, &lexem_n, "MEOW");
printf("%s\n", D[0]);
add_lexem(D, &lexemsc, &lexem_n, "BARK");
printf("%s\n", D[1]);
// in this place lexem_n becomes equal lexemsc
add_lexem(D, &lexemsc, &lexem_n, "KWARK");
printf("%s\n", D[2]);
return 0;
}
The output must be
MEOW
BARK
KWARK
but it is
MEOW
BARK
Segmentation fault (core dumped)
You're passing your lexeme parameter by value, when it should be by address:
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
// removed unused void ccat(char *str, char c)
void add_lexem(char ***lexems, int *lexemsc, int *lexem_n, const char *lexem)
{
if (*lexem_n >= *lexemsc) {
*lexems = realloc(*lexems, sizeof(char *) * (*lexemsc + 100));
*lexemsc += 100;
}
char *for_adding = malloc(sizeof(char) * strlen(lexem)+1);
strcpy(for_adding, lexem);
(*lexems)[*lexem_n] = for_adding;
(*lexem_n)++;
}
int main(void)
{
char **D = malloc(sizeof(char *) * 2);
int lexemsc = 2;
int lexem_n = 0;
add_lexem(&D, &lexemsc, &lexem_n, "MEOW");
printf("%s\n", D[0]);
add_lexem(&D, &lexemsc, &lexem_n, "BARK");
printf("%s\n", D[1]);
// in this place lexem_n becomes equal lexemsc
add_lexem(&D, &lexemsc, &lexem_n, "KWARK");
printf("%s\n", D[2]);
return 0;
}
Output
MEOW
BARK
KWARK
Note: Triple indirection (i.e. a 3-start-programming) is not something to enter into lightly, though it actually fits what you appear to be trying to do here. Read the above code carefully and make sure you understand how it works.
Edit: added terminator space for added string. (don't know why I missed it, since it was what everyone else seemed to be catching on first-review, duh).
Note: See #wildplasser's answer to this question. Honestly it is the best way to do this, as it tightens the relationship between the string pointer array and the magnitude of said-same. If it is possible to retool your code to use that model, you should do so, and in-so-doing select that answer as the the "correct" solution.
Alternative to avoid the three-star-programming: put everything you need inside a struct:
struct wordbag {
size_t size;
size_t used;
char **bag;
};
void add_lexem(struct wordbag *wb, const char *lexem)
{
if (wb->used >= wb->size) {
wb->bag = realloc(wb->bag, (wb->size+100) * sizeof *wb->bag );
wb->size += 100;
}
wb->bag[wb->used++] = strdup(lexem);
}
The main problem is that you are passing D to the function by value: the assignment
lexems = realloc(...);
has no effect on D. In cases when realloc performs reallocation, D becomes a dangling pointer, so dereferencing it becomes undefined behavior.
You need to pass D by pointer in the same way that you pass lexemsc and &lexem_n, so that the realloc's effect would be visible inside the main function as well.
In addition, your add_lexem does not allocate enough memory for the string being copied: strlen does not count the null terminator, so these two lines
char *for_adding = malloc(sizeof(char) * strlen(lexem));
strcpy(for_adding, lexem);
write '\0' one byte past the allocated space.
The problem may come from :
char *for_adding = malloc(sizeof(char) * strlen(lexem));
strcpy(for_adding, lexem);
try char *for_adding = malloc(sizeof(char) * (strlen(lexem)+1)); to leave some space for the '\0 character.
Edit : and #WhozCraig seems to be right !
Bye,
I have writen a code to split the string with multiple char delimiter.
It is working fine for first time of calling to this function
but i calling it second time it retuns the correct word with some unwanted symbol.
I think this problem occurs because of not clearing the buffer.I have tried a lot but cant solve this. please help me to solve this problem.
char **split(char *phrase, char *delimiter) {
int i = 0;
char **arraylist= malloc(10 *sizeof(char *));
char *loc1=NULL;
char *loc=NULL;
loc1 = phrase;
while (loc1 != NULL) {
loc = strstr(loc1, delimiter);
if (loc == NULL) {
arraylist[i]=malloc(sizeof(loc1));
arraylist[i]=loc1;
break;
}
char *buf = malloc(sizeof(char) * 256); // memory for 256 char
int length = strlen(delimiter);
strncpy(buf, loc1, loc-loc1);
arraylist[i]=malloc(sizeof(buf));
arraylist[i]=buf;
i++;
loc = loc+length;
loc1 = loc;
}
return arraylist;
}
called this function first time
char **splitdetails = split("100000000<delimit>0<delimit>hellooo" , "<delimit>");
It gives
splitdetails[0]=100000000
splitdetails[1]=0
splitdetails[2]=hellooo
but i called this second time
char **splitdetails = split("20000000<delimit>10<delimit>testing" , "<delimit>");
splitdetails[0]=20000000��������������������������
splitdetails[1]=10����
splitdetails[2]=testing
Update:-
thanks to #fatelerror. i have change my code as
char** split(char *phrase, char *delimiter) {
int i = 0;
char **arraylist = malloc(10 *sizeof(char *));
char *loc1=NULL;
char *loc=NULL;
loc1 = phrase;
while (loc1 != NULL) {
loc = strstr(loc1, delimiter);
if (loc == NULL) {
arraylist[i]=malloc(strlen(loc1) + 1);
strcpy(arraylist[i], loc1);
break;
}
char *buf = malloc(sizeof(char) * 256); // memory for 256 char
int length = strlen(delimiter);
strncpy(buf, loc1, loc-loc1);
buf[loc - loc1] = '\0';
arraylist[i]=malloc(strlen(buf));
strcpy(arraylist[i], buf);
i++;
loc = loc+length;
loc1 = loc;
}
}
In the caller function, i used it as
char *id
char **splitdetails = split("20000000<delimit>10<delimit>testing" , "<delimit>");
id = splitdetails[0];
//some works done with id
//free the split details with this code.
for(int i=0;i<3;i++) {
free(domaindetails[i]);
}free(domaindetails);
domaindetails=NULL;
then i called the same for the second as,
char **splitdetails1= split("10000000<delimit>1000<delimit>testing1" , "<delimit>");
it makes error and i can't free the function.
thanks in advance.
Your problem boils down to three basic things:
sizeof is not strlen()
Assignment doesn't copy strings in C.
strncpy() doesn't always nul-terminate strings.
So, when you say something like:
arraylist[i]=malloc(sizeof(loc1));
arraylist[i]=loc1;
thisdoes not copy the string. The first one allocates the size of loc1, which is a char *. In other words, you allocated the size of a pointer. You want to allocate storage to store the string, i.e. using strlen():
arraylist[i]=malloc(strlen(loc1) + 1);
Note the + 1 as well, because you also need room for the nul-terminator. Then, to copy the string you want to use strcpy():
strcpy(arraylist[i], loc1);
The way you had it was just assigning a pointer to your old string (and in the process leaing the memory you had just allocated). It's also common to use strdup() which combines both of these steps, i.e.
arraylist[i] = strdup(loc1);
This is convenient but strdup() is not part of the official C library. You need to assess the portability needs of your code before you consider using it.
Additionally, with strncpy(), you should be aware that it does not always nul-terminate:
strncpy(buf, loc1, loc-loc1);
This copies less bytes than were in the original string and doesn't terminate buf. Thus, it's necessary to include a nul terminator yourself:
buf[loc - loc1] = '\0';
This is the root cause of what you are seeing with the garbage. Since you didn't nul terminate, C doesn't know where your string ends and so it keeps on reading whatever happens to be in memory.
Here is my code
char* a[10];
a[0]="'example'";
char* p;
p=strstr(a[0],"'");
I know if strstr can find the ' it returns a pointer which points to first character which is '. I want to take the value between two ' and save it in a[1]. how should I do that?
As a result
a[1] is "example".
strchr() seems a more appropriate choice that strstr().
Use the result of first strchr(), + 1, as the argument to a subsequent strchr() and then malloc() and strcpy() or sprintf() into a[1]:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main()
{
char* a[10];
char* p;
a[0] = "'example'";
p = strchr(a[0], '\'');
if (p)
{
char* q = strchr(++p, '\'');
if (q)
{
a[1] = malloc((q - p) + 1);
if (a[1])
{
sprintf(a[1], "%.*s", q - p, p);
printf("[%s]\n", a[1]);
}
}
}
return 0;
}
Storing pointers to string literals and malloc() data into the same array seems a dangerous thing to do. You must free() dynamically allocated memory, but only dynamically allocated memory. The code will need to know what elements in a are pointing at dynamically allocated memory, and must be free()d, and those that are not.
Initialise a to all NULLs, so it is known what elements are populated:
char* a[10] = { NULL };
and calling free() on a NULL pointer is safe (does nothing).
Just find the next occurrence of ' and copy the substring:
char* a[10];
a[0]="'example'";
char* p, *q;
p=strstr(a[0],"'");
if (p) {
q = strstr(p+1, "'");
if (q) {
size_t len = (size_t)(q - p);
char *sub = malloc(len + 2);
if (!sub) {
/* Oops */
exit(EXIT_FAILURE); /* Something more sensible rather */
}
memcpy(sub, p, len+1);
sub[len+1] = 0;
a[1] = sub;
}
else {
a[1] = NULL;
}
}
else {
a[1] = NULL;
}
Note that in this case, it would be better to use strchr to find the single character.
Bear with me. I have not coded in c in 8 years and am totally baffled why my string manipulation is not working. I am writing a program that loops forever. In the loop I initialize two char pointers each is passed to a function that add text to the char pointer (array). When the functions are done I print the char pointer and free the two char pointers. However the program dies after 7 iterations with the following error message
* glibc detected * ./test: double free or corruption (fasttop): 0x0804a168 ***
#include sys/types.h
#include sys/stat.h
#include fcntl.h
#include string.h
#include stdio.h
#include stdlib.h
#include errno.h
#include time.h
char *SEPERATOR = "|";
void getEvent (char* results);
void getTimeStamp(char* timeStamp, int timeStampSize);
void stringAppend(char* str1, char* str2);
int main (int argc, char *argv[])
{
int i = 0;
while(1)
{
i++;
printf("%i", i);
char* events= realloc(NULL, 1);
events[0] = '\0';
getEvent(events);
char* timestamp= realloc(NULL, 20);
timestamp[0] = '\0';
getTimeStamp(timestamp, 20);
printf("%s", events);
printf("timestamp: %s\n", timestamp);
free(events);
free(timestamp);
}
}
void getEvent (char* results)
{
stringAppend(results, "a111111111111");
stringAppend(results, "b2222222222222");
}
void getTimeStamp(char* timeStamp, int timeStampSize)
{
struct tm *ptr;
time_t lt;
lt = time(NULL);
ptr = localtime(<);
int r = strftime(timeStamp, timeStampSize, "%Y-%m-%d %H:%M:%S", ptr);
}
void stringAppend(char* str1, char* str2)
{
int arrayLength = strlen(str1) + strlen(str2) + strlen(SEPERATOR) + 1;
printf("--%i--",arrayLength);
str1 = realloc(str1, arrayLength);
if (str1 != NULL)
{
strcat(str1, SEPERATOR);
strcat(str1, str2);
}
else
{
printf("UNABLE TO ALLOCATE MEMORY\n");
}
}
You are reallocating str1 but not passing the value out of your function, so the potentially changed pointer is leaked, and the old value, which has been freed by realloc, is freed again by you. This causes the "double free" warning.
The problem is that while stringAppend reallocates the pointers, only stringAppend is aware of this fact. You need to modify stringAppend to take pointer-to-pointers (char **) so that the original pointers are updated.
This line in stringAppend:
str1 = realloc(str1, arrayLength);
changes the value of a local variable in stringAppend. This local variable named str1 now points to either the reallocated memory or NULL.
Meanwhile local variables in getEvent keep the values they had before, which now usually point to freed memory.
All the comments where very helpfull. Of course it makes total sense why the error was happening. I ended up solving it by making the following changes.
For both the getEvent and stringAppend I return the char pointer.
e.g.
char* stringAppend(char* str1, char* str2)
{
int arrayLength = strlen(str1) + strlen(str2) + strlen(SEPERATOR) + 1;
printf("--%i--",arrayLength);
str1 = realloc(str1, arrayLength);
if (str1 != NULL)
{
strcat(str1, SEPERATOR);
strcat(str1, str2);
}
else
{
printf("UNABLE TO ALLOCATE MEMORY\n");
}
return str1;
}
This isn't an answer to your question (and you don't need one, since the error has been pointed out), but I do have some other comments about your code:
char* events= realloc(NULL, 1);
events[0] = '\0';
You don't test that realloc successfully allocated memory.
char* timestamp= realloc(NULL, 20);
timestamp[0] = '\0';
Same problem here. In this case, you don't need realloc at all. Since this is a fixed-size buffer, you could use just:
char timestamp[20] = "";
And don't do this:
str1 = realloc(str1, arrayLength);
because if realloc fails, you'll orphan the memory that str1 was pointing to before. Instead:
char* temp = realloc(str1, arrayLength);
if (temp != NULL)
{
str1 = temp;
...
}
Note that since you're modifying stringAppend to return the new string, you should do similar checks in the calling functions.
Also, "separator" is spelled with two As, not with two Es.