I am writing a function to find substring. But I am not sure where i am going wrong. On running GDB i get a segmentation fault. If someone can guide me in the right direction.
here is the code
char *mystrstr(char * s1, const char * s2)
int main(){
char *s1 = "The quick brown fox jumps over the hell lazy dog";
char *s2 = "hello";
char *s4;
s4 = mystrstr(s1,s2);
printf("%s\n",s4); <--- this is where i am Seg. Faulting
return 0;
}
When s2 is not a substring of s1 you are returning null, and then you are trying to print it, that gives a segmentation fault.
Try something like this:
s4 = mystrstr(s1,s2);
if(s4 != NULL)
printf("%s\n",s4);
The problem is that in the inner loop you add the indices i+j to access s1. If you imagine i to point to the "o" in "dog" in your example, j goes from 0 to 5 (length of "hello") in the inner loop. This causes your access to s1[i+j] to look at the characters o, g, \0, garbage, garbage.
The benefit of C strings is that they are null terminated. So you can iterate over strings like
for (char* i = s1; *i != 0; i++) {
...
}
I.e. you iterate from the start of s1 until you find its terminating 0 byte. In your inner loop, this allows you to write the following:
const char *j, *k;
for (j = s2, k = i; *j == *k && *j != 0; j++, k++);
if (*j == 0)
return i;
I.e. j starts at the beginning of s2, k starts where i is currently pointing at inside s1. You iterate as long as both strings are equal, and they have not reached their terminating 0 byte. If you have indeed reached the 0 byte of s2 (*j == 0), you have found the substring.
Note that you probably want to return i instead of s1, since this gives you a pointer into s1 where the requested substring starts.
NULL to printf. However, my point above still holds and will give you nasty crashes in case you read past your string into a memory region that is protected by your CPU's memory management unit.printf("%s\n",s4? s4 : "(NULL)");
char *s1 = "The quick brown fox jumps over the hell lazy dog";
char *s2 = "hello";
According to mystrstr *s2 is not substring of *s1 because you cannot find hello in the *s1. Therefore, the method will return NULL. And printing NULL as a string is not possible and result in an error.
To verify this, try to replace *s1 by:
char *s1 = "The quick brown fox jumps over the hello lazy dog"; // replace hell by hello
The output will be:
The quick brown fox jumps over the hello lazy dog
'\0'on each iteration ... bad idea"hello"in that sentence?? I don't see it in there at all!