Store data from binary file into dynamic array of strings

Question

I have a binary file where I store numbers as strings in this fashion: 11 43 89 101 etc

I want, by using only system commands, to read the numbers stored and store them in a string dynamic array, because i don't know how long the strings will end up being or how many. Here is the relevant code:

char **positions;
int all_names=0,i,j;

   fd=open(argv[2],O_RDWR|O_CREAT,S_IRWXU);
        i=0;
        j=0;
        do{
            positions=(char**)malloc(sizeof(char*));
            (*positions)[i]=(char*)malloc((MAX_SIZE+1)*sizeof(char));
            do{
                read(fd,positions[i][j],1);
            }while(positions[i][j+1]!='\0');
            i++;
        }while(i<all_names);

        for(i=0; i<all_names; i++){
            for(j=0; positions[i][j]!='\0';j++){
                printf("%c", positions[i][j]);
            }
            printf("\n");
        }
    }

All names keeps track of the amount of entries in the binary file.

When I run it I get a segmentation fault. The part where I store the numbers works fine I have checked the file. It always stores the number and a '\0' after it.

I get this as warning but don't know how to fix it

warning: incompatible integer to pointer conversion passing 'char' to parameter of type 'void *' [-Wint-conversion] read(fd,positions[i][j],1);

About positions[i][j].

Thanks for any help

Edit: changed code to:

char **positions;
int all_names=0,i,j;

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

fd=open(argv[2],O_RDWR|O_CREAT,S_IRWXU);
        i=0;
        j=0;
        for(i=0; i<all_names; i++){
            positions=(char**)realloc(*positions,(all_names) * sizeof(char*));
            positions[i]=(char*)malloc((all_names+1)*sizeof(char));
            for(j=0; ;j++){
                read(fd,&positions[i][j],1);
                if (positions[i][j] == ' ') {
                    break;
                }
            }
        }

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

Now I get an error on runtime:

malloc: * error for object 0x20400036: pointer being realloc'd was not allocated * set a breakpoint in malloc_error_break to debug Abort trap: 6

I really think I am supposed to realloc every time cause all_names value gets updated at an earlier part of my code. What am I doing wrong?

Answer 1

You need to check for the null byte after you read, not before.

The second argument to read() needs to be a pointer to the array element, use &positions[i][j].

You have to allocate memory for all the strings, you're just allocating one element of positions.

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

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] == '\0') {
            break;
        }
    }
}

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

You also need to set all_names to the actual number of strings in the file, not 0. I'm not sure where you get that from.