Core aborted (due to invalid next size)

Question

I am writing a program to benchmark the memory using read+write functions in c.

I have written a program below which accepts the block size of memory to be copied to another memory location using either sequential OR random access.

Each time I give a value greater than 12 bytes , the programmes give below message :

Elapsed time for reading and writing seconds in random manner : 0.000006

Error in `./a.out': free(): invalid next size (fast): 0x00000000022cc030 Aborted (core dumped)

#include <stdio.h>
#include <time.h>
#include <string.h>
#include <stdlib.h>
#include <pthread.h>
int rand_lim(int blocksize) {
    /* return a random number between 0 and limit inclusive.
    */

    int divisor = RAND_MAX/(blocksize+1);
    int random_byte;

    do { 
        random_byte = rand() / divisor;
    } while (random_byte > blocksize);

    return random_byte;
}
int main(int argc , char *argv[])
{
    char temp;
    int num_threads=0;
    char *buffer, *write_buffer, access;
    int i=0, block_size;


    printf("Enter the block size of data in number of bytes:");
    scanf("%d",&block_size);
    buffer = (char*)malloc(block_size);
    write_buffer = (char*)malloc(block_size);
    for (i=0; i<block_size; i++)
    {
        memset(&buffer[i],'a',block_size);
        //printf("%s",&buffer[i]);   
    }
    printf("\nEnter the acess method --> random(R) OR seq(S) : ");
    scanf("%s", &access);

    printf("\nEnter the number of threads 1 , 2 , 4 , 8 \n");
    scanf("%d", &num_threads);

    if (access == 'S')
    {
        clock_t tic1 = clock();
        for (i=0; i<block_size; i++)
        {
            memcpy(&write_buffer[i],&buffer[i],block_size);
            //printf("%s",&write_buffer[i]); 
        }
        clock_t toc1 = clock();
        printf("Elapsed time for reading and writing in sequential manner : %f seconds  \n", (double)(toc1 - tic1)/CLOCKS_PER_SEC);
        free(buffer);
        free(write_buffer);
    }
    else if(access == 'R')
    {
        clock_t tic1 = clock();
        for (i=0; i<block_size; i++)
        {
            int j = rand_lim(block_size); 
            memcpy(&write_buffer[j],&buffer[j],block_size);
            int dummy = i;
        }

        clock_t toc1 = clock();
        printf("Elapsed time for reading and writing seconds in random manner : %f \n", (double)(toc1 - tic1)/CLOCKS_PER_SEC);
        free(buffer);
        free(write_buffer);
    }
    else
    { 
        printf ("\nPlease enter the correct method for bench marking. Exiting...");
        exit(0);
    }
    return 0;
}

I am trying to benchmark with large data like 1 GB but the programme doesn't allow this. Any help is appreciated.

Answer 1

In addition to John Zwinck's answer:

Probably what you wanted was

int block_count = ...;
write_buffer = malloc(block_count * block_size);

...
for (i=0; i<block_count; i++)
    memcpy(write_buffer + i * block_size, buffer, block_size);

...

for (i=0; i<block_count; i++)
{
    int j = rand_lim(block_count - 1); // because rand_lim includes upper limit
    memcpy(write_buffer + j * block_size, buffer, block_size);
}

That is, write_buffer consists of N chunks with size block_size each, and you write them either sequentially or randomly.

BTW rand_lim() can be written much simpler:

int rand_lim(int limit) {
    return rand() % (limit + 1); // if you want to include the upper limit for some reason
}

Answer 2

for (i=0; i<block_size; i++)
{
   memset(&buffer[i],'a',block_size); // !!
}

You not need to run loop and every time memset the buffer with specific value. You can achieve it with single line

memset(buffer, 'a', block_size);

Also same is true for memcpy. one of the is

for (i=0; i<block_size; i++)
{
    memcpy(&write_buffer[i],&buffer[i],block_size);
    //printf("%s",&write_buffer[i]); 
}

here also

memcpy(write_buffer,buffer,block_size);

is enough

Finally

Do not cast the result of malloc

Answer 3

This is undefined behavior:

buffer = (char*)malloc(block_size);
for (i=0; i<block_size; i++)
{
   memset(&buffer[i],'a',block_size); // !!
}

You should replace the loop with:

memset(buffer, 'a', block_size);