打印字符串数组行时出现段故障，逐字符打印时工作正常

Question

I have this code:

char **data;
int start = 0;

data = malloc(all_names * sizeof(char*));

        fd=open(argv[1],O_RDWR|O_CREAT,S_IRWXU);
        for(i=0; i<all_names; i++){
            data[i] = malloc((MAX_SIZE+1)*sizeof(char));

            int end = atoi(positions[i]);
            lseek(fd,0,start);
            read(fd,data[i],(end-start));
            data[i][end - start] = 0; //line edited in after answer
            start = end;

        }

        qsort(data, all_names, sizeof(char*), strcmp);

        for(int i=0; i<all_names; ++i)
        {
            printf("%s\n", data[i]);
        }

        /*//print data array
        start = 0;
        for(i=0; i<all_names; i++){
            int end = atoi(positions[i]);
            for(j=0;j<(end-start) ;j++){
                printf("%c",data[i][j]);
            }
            printf("\n");
        }*/

What I get when running it is a seg fault when trying to print.

If I comment out qsort and the printing for , and comment in the print data array part, I get, as expected, all my entries in the order I inserted them in.

If I leave qsort out, but keep the for loop as my printing method, I still get a seg fault.

1.The strings in the data array come from a file so they re probably not null terminated. However, I am hesitant to add a null byte cause it must not be there when I write the sorted array back in the file.

2. **positions array contains the starting point of every entry, apart from the first one. Eg. if I insert "alpha one" and then "beta two", then positions[0] = 9.

Please tell me to further explain anything I haven't done a good job explaining. Thanks.

Edit: Entire code cause it turns out I can't locate the problem

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

#define MAX_SIZE 50

int updateCounter(int pcounter, char *str){
int m,charcount = 0;
for(m=0; str[m]; m++) {
        charcount ++;
}
//charcount--;
printf("chars: %d \n", charcount);

pcounter = pcounter + charcount;
printf("pcounter = %d \n", pcounter);

return pcounter;
}

int main(int argc, char *argv[]){

int option,i,j;
FILE *fptr;
char *name;
int dcounter,pcounter = 0;
int fd;
char **positions,**data;
int all_names=0; //keeps track of how many names are currently stored
int start = 0; //first byte of word to read in data.bin
char *filename = argv[2];

name=(char*)malloc((MAX_SIZE+1)*sizeof(char));

do{
    printf("MENU: \n 1.Insert \n 2.Delete \n 3.Search \n 4.Display \n");


    printf("Please choose 1-4\n");
    scanf("%d", &option);
    while(getchar() != '\n');

    //Insert
    if(option==1){

        printf("Insert name: ");
        fgets(name,MAX_SIZE,stdin);
        name[strcspn(name,"\n")]=0;

        fd=open(argv[1],O_RDWR|O_CREAT|O_APPEND,S_IRWXU);
        write(fd,name,strlen(name));

        pcounter = updateCounter(pcounter, name);

        char passpos[5];
        sprintf(passpos,"%d",pcounter); //int to string
        fd=open(argv[2],O_RDWR|O_CREAT|O_APPEND,S_IRWXU);
        write(fd,passpos,3);
        write(fd," ",1);

        all_names++;
        printf("all names: %d\n",all_names);

        positions = malloc(all_names * sizeof(char*));

        //create pos array
        fd=open(argv[2],O_RDWR|O_CREAT,S_IRWXU);
        for(i=0; i<all_names; i++){
            positions[i] = malloc((MAX_SIZE+1)*sizeof(char));
            for(j=0; ;j++){
                read(fd,&positions[i][j],1);
                if (positions[i][j] == ' ') {
                    break;
                }
            }
        }
        //print pos array
        for(i=0; i<all_names; i++){
            printf("%s\n", positions[i]);
        }

        //create data array
        data = malloc(all_names * sizeof(char*));

        fd=open(argv[1],O_RDWR|O_CREAT,S_IRWXU);
        for(i=0; i<all_names; i++){
            data[i] = malloc((MAX_SIZE+1)*sizeof(char));

            int end = atoi(positions[i]);
            lseek(fd,0,start);
            read(fd,data[i],(end-start));
            data[i][end - start] = 0;
            start = end;

        }

        qsort(data, all_names, sizeof(char*), strcmp);

        for(int i=0; i<all_names; ++i)
        {
            printf("%s\n", data[i]);
        }

        /*//print data array
        start = 0;
        for(i=0; i<all_names; i++){
            int end = atoi(positions[i]);
            for(j=0;j<(end-start) ;j++){
                printf("%c",data[i][j]);
            }
            printf("\n");
        }*/

    }

}while(1);

}

Answer 1

I'm going to guess that MAX_SIZE is the maximum size of a string.

But it should be sizeof(char*) .

The function qsort expects a parameter that indicates the size of each element. The size of each element in a data type char** is the size of char* .

Here's an example, similar to your program, that's useful for quick testing.

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

int main(int argc, char** argv) {

    int all_names = argc;

    char** data = argv;

    qsort(data, all_names, sizeof(char*), strcmp);

    for(int i=0; i<all_names; ++i)
    {
        printf("%s\n", data[i]);
    }

    return 0;
}

Regarding your updated code, after this line:

read(fd,data[i],(end-start));

You should null-terminate your string.

data[i][end - start] = 0;

I say this with the assumption that end - start does not account for a null-terminating character and that you're not storing null-terminated characters in the file you're reading from.

Regarding your additional update, if you don't want to write the null-terminator to the file, then just find the length of the string using strlen before writing it to the file. Your other option would be to write a wrapper for qsort that looks something like this.

int cmp_string_block(const void* a, const void* b) {
    return memcmp(a, b, MAX_SIZE);
}

And then pass that function to qsort instead of strcmp .