"Implicit declaration" warning

Question

For this code:

int i=0; char **mainp;
for(i=0;i<2;++i)
{
    mainp[i]=malloc(sizeof(char)*200);
    if(!scanf("%[^#],#",mainp[i]))
        break;
   if(i<2)
       scanf("%[^#],#",mainp[i]);
}

GCC emits the warnings:

warning: implicit declaration of function ‘scanf’
warning: incompatible implicit declaration of built-in function ‘scanf’
warning: ‘mainp’ may be used uninitialized in this function

And I get a segmentation fault at runtime

input:(P>Q),(Q>R),-R#-P output: (P>Q),(Q>R),-R (empt slot)

i expected to give me (P>Q),(Q>R),-R -P //where should i fix in my code such that it gives me expected //output

Answer 1

Problem #1:

warning: ‘mainp’ may be used uninitialized in this function

You need to allocate memory for the array of arrays first.

char **mainp = malloc(sizeof(char*)*2);

Problem #2:

warning: implicit declaration of function ‘scanf’
warning: incompatible implicit declaration of built-in function ‘scanf’

You need to include stdio.h at the top of your file:

#include <stdio.h>

Problem #3: (Not included in your compiling warnings)

Remember to free both the allocated array members and also the array of array address.

Answer 2

gcc expects this line at the beginning of your file:

#include <stdio.h>

and a declaration of mainp like this one:

char *mainp[2];

Answer 3

You shouldn't use functions without declaring them; you used scanf, but at no point in your code is scanf declared. Since it's a standard library function it's declared in one of the standard headers, stdio.h, so you just need to include it:

#include <stdio.h>

Brian's answer is good for the other part