Unexpected segmentation fault when trying to mix some string

Question

I need to manipulate an string in this way:

If the character is '+' or '-' or '/' or '*', move them to the end of buffer, if not, move to the beginning of the buffer.

My solution is quite simple:

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

void mix_the_string(char ** buff, char ** string)
{
    printf("The string which will be printed is : %s\n",*string);

    int i = 0;
    int j = strlen(*string) - 1;

    while(i< strlen(*string))
    {
        if(*string[i] != '+' || *string[i] != '-' || *string[i] != '*' || *string[i] != '/')
        {

            printf("in the first if, i = %d, *string[i] = '%d'\n",i,(int)*string[i]);

            *buff[i] = *string[i];
        }
        else
        {
            printf("in the second if, i = %d, *string[i] = '%d'\n",i,(int)*string[i]);
            *buff[j] = *string[i];
        }

        i++;
        j--;
    }
}

int main()
{    
    char * buff  = (char *) malloc(50);
    char * string  = (char *) malloc(50);

    string = "1+2+3";

    mix_the_string(&buff,&string);
    puts(buff);
    free(buff);
    free(string);

    return 0;
}

The output of this code is:

 The string which will be printed is : 1+2+3
 in the first if, i = 0, *string[i] = '49'
 in the first if, i = 1, *string[i] = '49'
 Segmentation fault

I expected with this example that output would be like:

The string which will be printed is : 1+2+3
in the first if, i = 0, *string[i] = '49'
in the second if, i = 1, *string[i] = '43'
in the first if, i = 2, *string[i] = '50'
in the second if, i = 3, *string[i] = '43'
in the first if, i = 4, *string[i] = '51'
123++

Where am I going wrong?

why are you passing the addresses of pointers? this makes it more complex than it should be. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Mar 26, 2018 at 14:49
Which line is causing the crash? Run the code in a debugger and update the question with this vital piece of information — Chris Turner
– Chris Turner, Commented Mar 26, 2018 at 14:50
int j = strlen(*string) - 1; if string is empty you get -1. UB... — Jean-François Fabre
– Jean-François Fabre ♦, Commented Mar 26, 2018 at 14:52
you're decreasing j each time, increasing i each time. maybe you should do that only when storing in the buffer... — Jean-François Fabre
– Jean-François Fabre ♦, Commented Mar 26, 2018 at 14:53

rafix07 · Accepted Answer · 2018-03-26 15:20:45Z

5

There is difference between *string[i] and (*string)[i] notation. [] has higher precedence than * operator. You pass string by pointer to pointer, so you should call (*string)[i] in every line of your code.

(*string)[i] means - dereference pointer to array and get i-th element of string array

now you are doing

*string[i] - get i-th element of string array (!! but this is not array, this is only one element) and dereference first element

Do the same with buff.

And in main function you should copy 1+2+3 string literal into string buffer for example by strcpy function.

edited Mar 26, 2018 at 15:20

answered Mar 26, 2018 at 15:03

rafix07

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1 Comment

frslm Over a year ago

Also, in main(), string is overwritten with "1+2+3" and then free()ed. Instead, OP should copy "1+2+3" into string.

machine_1 · Accepted Answer · 2018-03-26 16:50:26Z

There are lot of bugs in your code.

first string = "1+2+3"; and then doing free(string) is not correct. First copy it using strcpy(string,"1+2+3"); and then free(string)
string is double pointer in mix_the_string(). Accessing like *string[i] is not correct because string is not array of pointer. instead of this use *(*string+i). Same applicable for buf also.
finally casting of malloc is not required. just char * buff = malloc(50); is fine.
And moreover you don't need to pass &string to mix_the_string(). just pass string thats enough.

Here is the expected code which you want

void mix_the_string(char ** buff, char ** string)
{
    printf("The string which will be printed is : %s\n", *string);

    int i = 0;
    int j = strlen(string[0]) - 1;

    while (i < strlen(string[0]))
    {
        if (*(*string + i) != '+' || *(*string + i) != '-' || *(*string + i) != '*' || *(*string + i) != '/')
        {
            printf("in the first if, i = %d, *string[i] = '%d'\n", i, *(*string + i));

            *(*buff + i) = *(*string + i);
        }
        else
        {
            printf("in the second if, i = %d, *string[i] = '%d'\n", i, *(*string + i));
            *(*buff + j) = *(*string + i);
        }
        i++;
        j--;
    }
}

int main()
{
    char * buff = (char *)malloc(50);
    char * string = (char *)malloc(50);

    strcpy(string, "1+2+3");

    mix_the_string(&buff, &string);
    puts(buff);
    free(buff);
    free(string);

    return 0;
}

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