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I need to create an array of pointers that will each point to an array of strings. The base, is a size 2 array of strings (the length of the strings is unknown at start). For example an array of 2 strings (first-name and last-name):

char *name[2];

Now I need to create an array of an unknown size (entered by the user) that will point to the type that I just created. My idea was to create it this way:

char **people=name;

And then ask the user how many names he would like to enter and allocate enough space to hold all the names.

people=(char**)malloc(sizeof(char*)*num); //num is the number received by the user.

This is where things got too complicated to me and I can't figure out how to I call each individual name to put a string in it. I built a loop that will receive all the names but I have no idea how to store them properly.

for(i=0;i<num;i++){
    printf("Please enter the #%d first and last name:\n",i+1);
    //Receives the first name.
    scanf("%s",&bufferFirstName);
    getchar();
    //Receives the last name (can also include spaces).
    gets(bufferLastName);
    people[i][0]=(char*)malloc(strlen(bufferFirstName)+1);
    people[i][1]=(char*)malloc(strlen(bufferLastName)+1);
    //^^Needless to say that it won't even compile :(
}

Can anyone please tell me how to properly use this kind of an array of points? Thanks.

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  • 1
    people[i][0]=(char*)malloc(strlen(bufferFirstName)+1); is wrong. if people is a char** then people[0] is a char* and people[i][0] is a char. Not a pointer. Also: don't cast malloc()s return value. It is not needed and possibly dangerous, since it can hide errors. Commented Dec 11, 2013 at 20:51
  • So people should be a *** type instead of ** (char ***people)? Also, is there a way of defining people in one line without the use of *name[2]? Commented Dec 11, 2013 at 20:54
  • From the question it is not clear what is your intention. I just point out the obvious error(s). And in most cases: when you start thinking of *** you should consider using structs (like in @msider(s) answer) Commented Dec 11, 2013 at 21:01

2 Answers 2

Reset to default
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from cdecl:

declare foo as array of pointer to array 2 of pointer to char

char *(*foo[])[2];

So, foo[0] is a pointer to array 2 of char *

That is the array, but for your use, you want:

declare foo as pointer to array 2 of pointer to char;

char *(*foo)[2];

Now you can do:

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

int main() {
    char *(*foo)[2];

    printf("How many people?\n");
    int n; scanf("%d", &n);

    foo = malloc(sizeof *foo * n);

    for (int i = 0; i < n; i++) {
        char bufFirstName[1024];
        char bufLastName[1024];

        printf("Please insert the #%d first and last name:\n", i+1);

        scanf("%s %s", bufFirstName, bufLastName); 

        char *firstName = malloc(strlen(bufFirstName) + 1);
        char *lastName = malloc(strlen(bufLastName) + 1);

        strcpy(firstName, bufFirstName);
        strcpy(lastName, bufLastName);

        foo[i][0] = firstName;
        foo[i][1] = lastName;
    }

    for (int i = 0; i < n; i++) {
        printf("Name: %s LastName: %s\n", foo[i][0], foo[i][1]);
    }

    return 0;
}

Compile with -std=c99

Note that using scanf, strcpy, strlen like that is unsafe because there can be a buffer overflow.

Also, remember to free your malloc's!

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4 Comments

When I'm trying to use it, it shows an error "Error: Incomplete type is not allowed".
It was when I tried using char *(*foo[])[2]; (before you edited the post).
Sorry, the first thing is the actual answer to your question (that's what you asked :P). The rest of the post is what I think that you wanted.
Thanks a lot. I have finally managed to get something going after hours of frustration XD.
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Not that your approach is wrong, but have you considered instead using a struct that includes first and last name, and then malloc'ing based on the number of names the user will enter:

typedef struct {
    char* first;
    char* last;
} person;

person* people = malloc(num * sizeof(*person));

This just simplifies pointer interaction. While the way you are doing it is a good exercise in understanding pointers better, it may not be the easiest way to understand.

If you are unable to use structs, you should instead be doing:

char** people;  
people = malloc(2*num*sizeof(char*));  

for (int i = 0; i < 2*num; i++) 
    people[i] = malloc(MAX_NAME_SIZE*sizeof(char));

Now you would need to reference the i th person via:

first name: people[i*2 + 0] 
last name: people[i*2 + 1]
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5 Comments

I am not allowed to use ***.
BTW: the downvote was mine, unintentionally, must have been a clicko. The silly CMS does not allow me to correct it. yet.
No worries, I thought it must have been a mistake by someone as it was downvoted almost instantly as I posted it.
The line people[i][0] = malloc(MAX_NAME_SIZE*sizeof(char)); shows an error "Error: a value type "char*" cannot be assigned to an entity of type "char"".
Woops, yeah I had a mistake there, but it's fixed now. @Marco has a good solution.

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