Breaking down a pointer str to smaller pointers str - c

Question

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAXLINES 25
int get_lines(char *studentinfo[]);
int main()
{
int onswitch=0;
char *studentinfo[100];
char *fname[100];
char *lname[100];
char *score[100];
int counter;
int x,y;
char temp,temp2,temp3;
counter=get_lines(studentinfo);
for (y=0; y<counter; y++)
{
temp=strtok(studentinfo, " ");
fname[y]=malloc(strlen(temp));
strcpy(fname[y],temp);
temp2=strtok(NULL, " ");
lname[y]=malloc(strlen(temp2));
strcpy(lname[y],temp2);
temp3=strtok(NULL," ");
score[y]=malloc(strlen(temp3));
strcpy(score[y],temp3);
int get_lines(char *studentinfo[])
{
int n=0;
char buffer[80];
puts("Enter one line at a time; enter a blank when done.");
while ((n<MAXLINES) && (gets(buffer) !=0) && (buffer[0] != '\0'))
{
if ((studentinfo[n]=(char*)malloc(strlen(buffer)+1))==NULL)
return -1;
strcpy(studentinfo[n++],buffer);
}
return n;
}
Alright guys I am trying to make program that takes in student information for sorting later. I have taking the input down with the function on the bottom. I am trying to break down to the student information into three different pointers for sorting. The problem I am having is trying to allocate enough memory to store then information. Then actually storing the memory in that pointer location.
A simple input is
John Smith 80
^fname ^lname ^score
I thought the for loop I had would work in theory but it didnt (error: Unhandled exception at 0x0F3CFA50 (msvcr110d.dll) in ConsoleApplication3.exe: 0xC0000005: Access violation reading location 0xFFFFFF8) can anybody point me in the right direction (not just give me a loop that works) ?

Answer 1

With your implementation, you get an access violation. You are trying to touch a dirty region of memory. Here is the solution with an explanation below
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAXLINES 25
int get_lines(char *studentinfo[]);
int main()
{
int onswitch=0;
char *studentinfo[100];
char *fname[100];
char *lname[100];
char *score[100];
int counter;
int x,y;
char *temp,*temp2,*temp3;
counter=get_lines(studentinfo);
for (y=0; y<counter; y++)
{
temp=strtok(studentinfo[y], " ");
fname[y]=malloc(strlen(temp));
strcpy(fname[y],temp);
temp2=strtok(NULL, " ");
lname[y]=malloc(strlen(temp2));
strcpy(lname[y],temp2);
temp3=strtok(NULL," ");
score[y]=malloc(strlen(temp3));
strcpy(score[y],temp3);
printf("%s %s %s", fname[y], lname[y], score[y]);
}
}
int get_lines(char *studentinfo[])
{
int n=0;
char buffer[80];
puts("Enter one line at a time; enter a blank when done.");
while ((n<MAXLINES) && (gets(buffer) !=0) && (buffer[0] != '\0'))
{
if ((studentinfo[n]=(char*)malloc(strlen(buffer)+1))==NULL)
return -1;
strcpy(studentinfo[n++],buffer);
}
return n;
}
First off, you are missing an ending bracket } for your for loop as well as your main function. So add those.
Your getlines function is all good.
Your for loop is screwed up. In particular, you confused the data types you are passing. Remember, you have declared an array of POINTERS.
temp=strtok(studentinfo, " ");
This is saying, hey, let's tokenize my array pointer. You don't want this. You want to tokenize the yth element in that array! So element 0 in your array is a pointer to the string "JOHN SMITH 80". This is what we want to tokenize. Otherwise what you had was trying to tokenize somthing along the lines of 0xabcdabcd or whatever the memory address of the allocated array was.
temp=strtok(studentinfo[y], " ");
This is the correct way. It says tokenize the first element, which is a pointer to our string.
Your next problem is your temp variables. You are calling strlen(temp). strlen expects a pointer to a string. You are passing the data of the char itself. Actually, you are passing the return pointer (likely null) of the strtok function that was stored in your char byte.
char temp,temp2,temp3;
You declared three bytes for the type char. What you wanted was three char * to hold pointers to your string tokens.
char *temp,*temp2,*temp3;
With this, strlen takes in these pointers, mallocs some space for your fname elements, and then you proceed to copy into this element using strcpy.
Note: strcpy also takes two pointers, one for destination, one for source, so again your temp variables needed to be pointers to your strings.
Hope this helps let me know if you are confused with my explanation at all.

Answer 2

strcpy takes characters until it reaches a \0 character. You want to check out the strncpy or memcpy functions, and then manually add the null terminator.