Function that concatenates two pointer strings together - c

Question

I am having issues when it comes to concatenating these two pointer strings together, below is my concatenating function, I am supposed to take string 1 and add it to string 2. Also I cannot use any functions in the string library, that's the point of this is to help us understand what code is actually in the functions by writing it ourself.
char strconcat(char *user2p, char *user1p) {
while (*user2p) {
user2p++;
}
while (*user1p) {
*user2p = *user1p;
*user2p++;
*user1p++;
}
*user2p = '\0';
printf("test: %c", *user2p);
return *user2p;
}
And here is the part of my main that is relevant to the function.
int main() {
char userString1[21], userString2[21];
char *user1p, *user2p;
user1p = userString1;
user2p = userString2;
printf("Please enter the first string: ");
gets(userString1);
printf("Please enter the second string: ");
gets(userString2);
printf("String 1 after concatenation: ");
puts(userString1);
printf("String 2 after concatenation: %c\n", strconcat(user2p, user1p));
The terminal keeps giving me this, I didn't include the code for the length and alphabetical order. It gives me a null when I try to run the test printf in the function and it gives me nothing when I return the function. I'm at a loss and any help is much appreciated!
Please enter the first string: jackhammer
Please enter the second string: jacky
The length of string 1 is: 10
The length of string 2 is: 5
String 1 comes before string 2 alphabetically.
String 1 after concatenation: jackhammer
(null)
String 2 after concatenation:

Answer 1

Your concat algorithm is fine, but you have to return a pointer to the original [leftmost] value, so your function needs to save it before looping:
char *
strconcat(char *user2p, char *user1p)
{
char *orig2p = user2p;
while (*user2p) {
user2p++;
}
while (*user1p) {
*user2p = *user1p;
user2p++;
user1p++;
}
*user2p = '\0';
printf("test: %s\n", orig2p);
return orig2p;
}
UPDATE:
To come up with a completely bulletproof test program for the concat function, we can use [overly] large input buffers and clip the input length to a maximum of 1/2 of the target buffer.
gets strips the newline but fgets does not. So, I've created an xgets function that is similar to gets but uses fgets and strchr to get [nearly] the same effect.
Although I believe it's okay to use standard string functions as part of the test code, I've created a hand coded version of strchr [hope that's not your next assignment :-)].
Anyway, here's the full program:
#include <stdio.h>
char *
strconcat(char *user2p, char *user1p)
{
char *orig2p = user2p;
while (*user2p) {
user2p++;
}
while (*user1p) {
*user2p = *user1p;
*user2p++;
*user1p++;
}
*user2p = '\0';
printf("test: %s\n", orig2p);
return orig2p;
}
char *
xstrchr(char *buf,int chrwant)
{
int chrcur;
char *res = NULL;
for (chrcur = *buf++; chrcur != 0; chrcur = *buf++) {
if (chrcur == chrwant) {
res = buf - 1;
break;
}
}
return res;
}
char *
xgets(char *buf,int maxlen)
{
char *cp;
char *res;
res = fgets(buf,maxlen,stdin);
if (res != NULL) {
cp = xstrchr(buf,'\n');
if (cp != NULL)
*cp = 0;
}
return res;
}
#define MAXLEN 800
int
main(void)
{
char userString1[MAXLEN], userString2[MAXLEN + 1];
char *user1p, *user2p;
printf("Please enter the first string: ");
user1p = xgets(userString1,MAXLEN / 2);
printf("Please enter the second string: ");
user2p = xgets(userString2,MAXLEN / 2);
if ((user2p != NULL) && (user1p != NULL))
printf("String 2 after concatenation: %s\n",strconcat(user2p, user1p));
return 0;
}

Answer 2

There's a number of issues. First is this.
while (*user1p) {
*user2p = *user1p;
*user2p++;
*user1p++;
}
This is working by accident. If you have compiler warnings on you should get a warning...
test.c:13:9: warning: expression result unused [-Wunused-value]
*user2p++;
^~~~~
test.c:14:9: warning: expression result unused [-Wunused-value]
*user1p++;
^~~~~~~
The reason it's unused is because C is interpreting it like so:
*(user1p++)
Increment the pointer, then dereference it. You just want to increment the pointers, no dereferencing required.
while (*user1p) {
*user2p = *user1p;
user2p++;
user1p++;
}
Then down here.
printf("String 2 after concatenation: %c\n", strconcat(user2p, user1p));
%c prints an individual char. You want %s which prints a char *. This reveals you have the wrong signature. strconcat should return a char * (ie. what C uses for strings) and return user2p (a char *).
char *strconcat(char *orig_to, const char *from) {
...
return user2p;
}
And since you're not changing from it should be const char * to let the compiler know and warn you if its accidentally changed.
Finally, when you return *user2p it's already been moved to the end of the string.
while (*user1p) {
*user2p = *user1p;
user2p++;
user1p++;
}
*user2p = '\0';
printf("test: %c", *user2p);
// This points to the null byte just set above
return user2p;
So printing the result of strconcat will print nothing. To get around this, store the original pointer for user2p and return that.
char *strconcat(char *orig_to, const char *from) {
char *orig_user2p = user2p;
...
return orig_user2p;
}
And some tips. It's easier to follow the code with good variable names that describe what they're doing.
char *strconcat(char *orig_to, const char *from) {
char *to = orig_to;
...
}
char foo[NN] already makes foo a pointer. There's no need to declare separate char * variables and copy the pointer.
char from[21], to[21];
Never use gets. There's no limit to how much memory it can use and it can easily overflow your buffer. Use fgets which can limit how much can be read to available memory.
printf("Please enter the string to concat from: ");
fgets(from, sizeof(from), stdin);
Though it's annoying that it keeps the newline and there's no simple function to strip it. You can use scanf which will strip whitespace, but beware its many pitfalls.
printf("Please enter the string to concat from: ");
scanf("%20s", from);
printf("Please enter the string to concat to: ");
scanf("%20s", to);
Finally, be sure the string you're concatenating to can hold its own contents and the new contents.
char from[21], to[41];
printf("Please enter the string to concat to: ");
// Be sure to leave enough room in `to` to fit `from`.
fgets(to, sizeof(to) - sizeof(from), stdin);

Answer 3

I would have created a more dynamic memory model. This code is more generic and concatenates strings creating a new string containing both strings.. Free when done :-)...
char *strconcat(char *string1, char *string2) {
int lenStr1=0,lenStr2=0;
char *tmpStr1=string1,*tmpStr2=string2,*returnStr;
while (*tmpStr1++)lenStr1++;
while (*tmpStr2++)lenStr2++;
if((returnStr=(char *)malloc(lenStr1+lenStr2+1))){
memcpy(returnStr,string1,lenStr1);
memcpy(&returnStr[lenStr1],string2,lenStr2);
returnStr[lenStr1+lenStr2]=0;
return returnStr;
} else {
return 0;
}
}
int main() {
char *string1="String 1 ",*string2="String 2 ",*result;
if((result=strconcat(string1, string2))) {
printf("-> %s \n",result);
free(result);
} else {
printf("Out of memory");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}