How to pass an array of strings to a function?

Question

Could anybody please tell me why is the output of both the prints are not same? How to pass an array of strings to a function then?

main()
{
    char b[10][10];
    printf("%p, %p, %p\n", b, b[0], b[9]);
    getStrListFromString(b);
}

void getStrListFromString(char **strList)
{
    printf("%p, %p, %p\n", strList, strList[0], strList[9]);
}

Expected OUTPUT :

0x7fffbe4ecf00, 0x7fffbe4ecf00, 0x7fffbe4ecf5a
0x7fffbe4ecf00, 0x7fffbe4ecf00, 0x7fffbe4ecf5a

Actual Output :

0x7fffbe4ecf00, 0x7fffbe4ecf00, 0x7fffbe4ecf5a
0x7fffbe4ecf00, 0x7fffbe4ecf80, (nil)

try void getStrListFromString(char strList[10][10]) for the prototype. 2D arrays are not pointers on 1D arrays. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Mar 22, 2017 at 12:35
Stop wasting your time and enable compiler warnings/errors. This code has 3-4 of them that shouldn't be ignored. — Lundin
– Lundin, Commented Mar 22, 2017 at 12:42

dbush · Accepted Answer · 2017-03-22 12:36:06Z

3

Your function expects a char ** but you're passing it a char [10][10]. These are not the same.

An array when passed to a function decayed to a pointer to the first element. So when you pass a char [10][10] (an array of size 10 of char [10]) to a function it decays into a char (*)[10] (a pointer to a char [10].

Change your function to accept either char [10][10] or char (*)[10].

answered Mar 22, 2017 at 12:36

dbush

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4 Comments

Karthik Raj Palanichamy Over a year ago

I understood. But couldn't understand why I am getting b[0] as 0x7fffbe4ecf00 in main but strList[0] as 0x7fffbe4ecf80 in the function.

dbush Over a year ago

@Karthik Undefined behavior. When you pass an incompatible pointer to a function, you can't accurately predict what will happen.

Karthik Raj Palanichamy Over a year ago

but there is certainly a predictable pattern. I always get (address + 128 bytes)?

dbush Over a year ago

@Karthik You can't depend on that behavior. For example, when I run this code I get address + 70.

Lundin · Accepted Answer · 2017-03-22 12:51:34Z

In addition to the core issue about pointer-to-pointer having absolutely nothing to do with multi-dimensional arrays, your also has various other bugs:

main() // obsolete form of main(), won't compile unless C90
{
    char b[10][10]; // uninitialized
    printf("%p, %p, %p\n", b, b[0], b[9]); // should be void pointers
    getStrListFromString(b); // wrong type of parameter
    // return statement necessary here in C90
}

void getStrListFromString(char **strList) // should not be pointer-to-pointer
{
    printf("%p, %p, %p\n", strList, strList[0], strList[9]); // same issue as in main
}

The fact that this code compiled means that your compiler is either complete crap or incorrectly configured. You need to fix that asap.

Corrected code:

#include <stdio.h>

void getStrListFromString(char strList[10][10])
{
    printf("%p, %p, %p\n", (void*)strList, (void*)strList[0], (void*)strList[9]);
}

int main (void)
{
    char b[10][10];
    printf("%p, %p, %p\n", (void*)b, (void*)b[0], (void*)b[9]);
    getStrListFromString(b);
}

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