How to copy char* to char* array in C

Question

I am trying to generate uint_32 random numbers which should be stored in the buffer array, but somehow, my codes only stores last value every time.

For instance, when I generate random numbers like

12365645
97897875
45458788

then the value of

buffer[0]=12365645
buffer[1]=97897875
buffer[2]=45458788

However currently, I am getting like

buffer[0]=45458788
buffer[1]=45458788
buffer[2]=45458788

Here is my corresponding code but ain't sure where I have made a mistake.

/*Required header files are added*/
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>         
#include <fcntl.h>
#include <stdint.h>

struct thread_arguments
{
    char *buffer;
    char *queue[10];
    uint32_t offset;
    uint32_t r;
    size_t bufferlen;


    size_t minlevel;


}ta;

void randomgenerate();
void constructor(int size, int filllevel);
void put_buffer(int ele);

void printbuf();
int main(void)
{
ta.offset=0;
ta.buffer=NULL;

    constructor(1,3);
    randomgenerate();
    printbuf();

    return EXIT_SUCCESS;

}
void constructor( int filllevel,int size)
{   
    //ta.buffer[size];
     ta.bufferlen=size;  
     ta.minlevel=filllevel;
}

void randomgenerate()
{
int i;
    for(i=0;i<ta.bufferlen;i++)
    {
                int myFile = open("/dev/random", O_RDONLY);             
                read(myFile, &ta.r, sizeof(ta.r)) ;

                put_buffer(i);
                close(myFile);

    }
}
void put_buffer(int ele)
{


ta.buffer = realloc(ta.buffer, sizeof(uint32_t));
sprintf(ta.buffer, "%zu", ta.r);
ta.offset += sizeof(uint32_t);
ta.queue[ele]=ta.buffer;
printf("%d\t%s\n",ele,ta.queue[ele]);

}
void printbuf()
{
int k;

    for(k=0;k<ta.bufferlen;k++)
    {
    printf("%s\n",ta.queue[k]);
    }
}

Without reading your code, I'm going to bet that all elements of buffer point to the same storage location. — Greg Hewgill
– Greg Hewgill, Commented May 11, 2015 at 2:55
Did the program compile? I get an error: 64:33: error: invalid conversion from 'void*' to 'char*' — programmer
– programmer, Commented May 11, 2015 at 3:01
You may wish to review your use of realloc() and consider whether a different function might be better suited for what you want. — Greg Hewgill
– Greg Hewgill, Commented May 11, 2015 at 3:07
You need to allocate space for each ta.queue[ele] using simple malloc(). — Ja͢ck
– Ja͢ck, Commented May 11, 2015 at 3:10
Why not malloc it during your "constructor" and then just memset to zero? You're currently not "reallocating" anything, you're just resizing the buffer to the size it is currently. — josaphatv
– josaphatv, Commented May 11, 2015 at 3:10

josaphatv · Accepted Answer · 2015-05-11 03:28:54Z

1

You're misusing realloc(). Your ta.bufferis always pointing to the same ta.buffer because it's already equal to sizeof (uint32_t). So when you do the assignment ta.queue[ele] = ta.buffer, every ta.queue pointer is the same value.

What you need is malloc().

EDIT

You should be using malloc() for each call to put_buffer(), like this:

void put_buffer (int ele)
{
    /* There are 10 decimal digits (characters) in a 32-bit unsigned integer, + 1 for the null terminator */
    ta.buffer = malloc ((sizeof *ta.buffer) * 11); 
    /* Print the string representation to the newly-allocated buffer */
    sprintf(ta.buffer, "%u", ta.r);
    /* I'm not sure what this is for so I'll leave it alone */
    ta.offset += sizeof (uint32_t);
    ta.queue[ele] = ta.buffer;
}

edited May 11, 2015 at 3:28

answered May 11, 2015 at 3:16

josaphatv

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1 Comment

josaphatv Over a year ago

I've expanded my answer to include example usage. Your size was not correct. You're making space for the string representation of the 32-bit unsigned integer in ta.buffer so you need to take into account how many characters you need. A size of 3 is not enough characters to fit the string "4294967295".

R Sahu · Accepted Answer · 2015-05-11 03:26:55Z

1

Not only are you using realloc with the same size every time, the amount of memory you are allocating is not right.

 ta.buffer = realloc(ta.buffer, sizeof(uint32_t));
 sprintf(ta.buffer, "%zu", ta.r);

sizeof(uint32_t) will be 4. That is not enough memory to store an object of type uint32_t in string form.

Your program is subject to undefined behavior.

answered May 11, 2015 at 3:26

R Sahu

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