使用指向字符串的指针数组将字符串中的子字符串替换为c中的另一个子字符串

Question

I am trying to replace a substring inside a string with another substring. However, I am getting a segmentation fault at line strcpy(p, str2); I do not understand why. Here is my code

#include<stdio.h>
#include<string.h>
#include<stdlib.h>


int main()
{
    char *str[] = {"We will teach you how to",
        "Move a mountain",
        "Level a building",
        "Erase the past",
        "Make a million",
        "...all through C"
    };
    char str1[20], str2[20];
    printf("Enter first string\n");
    scanf("%s", str1);
    printf("Enter second string\n");
    scanf("%s", str2);
    char *news, *t, *p;
    int i;
    if(strlen(str2) > strlen(str1))
    {
        printf("Second string should be smaller than first string\n");
        exit(1);
    }
    for(i = 0; i < 6; i++)
    {
        p = strstr(str[i], str1);
        if(p)
        {
            news = p + strlen(str1);
            strcpy(t,news);
            strcpy(p, s);
            strcat(p,t);
            break;
        }
    }
    printf("the new string is\n");
    for( i = 0; i < 6; i++)
    {
        printf("%s\n", str[i]);
    }
    return 0;
}

Answer 1

You're copying to the string t , but you've never initialized it to point anywhere.

strcpy(t, news);

This is bad news — oh, no, it's bad t and good news.

You have a second problem, too. You're trying to modify the strings in the str array of pointers, but those are string literals. You can't modify string literals reliably. You'll have to modify things so that you don't do that.

p = strstr(str[i], str1);  // Generates a pointer into str[i]
...
strcpy(p, s);    // Modifies str[i] -- unsuccessfully

Probably the easiest (but not necessarily best) fix for this is to use:

char str[][64] = {"We will teach you how to",
    "Move a mountain",
    "Level a building",
    "Erase the past",
    "Make a million",
    "...all through C"
};

I've not spent a long time cogitating on the correct size for the second dimension of the array, but 64 looks like it probably leaves enough space for you to grow the strings a bit. Maybe that doesn't matter since there's a comment about the second string must be shorter than the first, so you're only dealing with shrinking strings. Lucky you to be able to do that. You'd have to be careful to avoid troubles if your strings can grow.

As first noted by t0mm13b in a comment , there's no variable s in the code. Please make sure you post compilable code.

Looking further at that inner loop, we can see all sorts of problems:

1. The news = p + strlen(str1); assignment means that news is pointing to tail of str[i] after the str1 was found in str[i] .
2. You copy an indeterminate amount of information from news into the space pointed at by an uninitialized pointer, t . This is a first source of core dump.
3. You copy the undefined variable s over p . Is that meant to be str2 , since str2 is not used after the length check? This is a second source of core dump.
4. You copy the material from t after the material copied from s . If you've not already dumped core, this could be a third source of core dump (though maybe if you've not dumped core yet, this won't trigger one either).

You probably need:

char *p = strstr(str[i], str1);
if (p != 0)
{
    char t[64];
    strcpy(t, p + strlen(str1));
    strcpy(p, str2);
    strcat(p, t);
}

This is far from being proof from overflows, but is more nearly sound.