使用指针字符串会产生分段错误

Question

The task was mainly to use pointers to input a string and slice it at places where there is a '\\' character and output them in separate lines, using pointers. The program runs fine when I use arrays instead of pointers. However using pointers to store strings give the message "Segmentation fault". The code is as follows :

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

int main() {
char *name;
char *sep[100];
int i = 0, j = 0, k = 0;
scanf("%[^\n]s", name);

for(i = 0; (*(name+i)) != '\0'; i++) {
    if((*(name+i)) == '\\') {
        *((*(sep+k))+j) = '\0';
        j = 0;
        k++;
    } else {
        *((*(sep+k))+j) = *(name+i);
        j++;
    }
}

for(i = 0; i <= k; i++) {
    printf("%s\n", *(sep+i));
}

return 0;
}

It would be awesome if you could point out what and where the problem is, instead of giving me an alternative solution. TIA.

Answer 1

your pointers are null pointers.you are invoking undefined behavior by using them without assigning them to allocated memory.Allocate memory to them so that you can use them correctly and store words separated by \\.Also,you can use [] instead of * .

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

int main()
{
    char name[256];
    char *sep[100];
    for( int n = 0 ; n < 100 ; n++ )
    {
        sep[n] = malloc(30*sizeof(char));
    }
    int i = 0, j = 0, k = 0;
    scanf(" %255[^\n]s", name);

    for(i = 0; name[i] != '\0'; i++)
    {
        if( name[i] == '\\')
        {
            sep[k][j] = '\0';
            j = 0;
            k++;
        }
        else
        {
            sep[k][j] = name[i];
            j++;
        }
    }

    sep[k][j] = '\0';
    for(i = 0; i <= k ; i++)
    {
        printf("%s\n",sep[i]);
    }
    for( int n = 0 ; n < 100 ; n++ )
    {
        free(sep[n]);
    }
    return 0;
}

Answer 2

In your code,

 scanf("%[^\n]s", name);

name is an unintialized pointer. It does not point to any valid memory location. You need to allocate memory before you can use it.

The same goes out for sep array, too.

You can consider using an array for this purpose or see the man page of malloc() if you want to stick to a pointer.

FWIW, using an unitialized pointer can lead to undefined behavior .

Answer 3

You must allocate space for your pointers to avoid undefined behaviour: you cannot use a pointer without initializing it.

int main() {
   char *name = malloc(MAX_DIM_OF_NAME+1);
   char *sep[100];

   for (int i=0; i<100; i++)
      sep[i] = malloc(MAX_DIM_OF_NAME+1);

   ....

Answer 4

您使用未初始化的name调用scanf 。