Deallocating 2D array in C -

for whatever reason getting following error when trying free 2d array created:

error in `./a.out': free(): invalid next size (fast): 0x0000000001759310 *** aborted (core dumped)

i printed out contents of array , correct , able access of elements. however, yet able free it. error occurs when going through freeing loop, i.e. freeing double *. appreciate help.

thanks!

here code:

/*allocation*/ double **block_mat = (double **) malloc (sizeof(double *) * num_blocks); int i; (i = 0; <num_blocks; i++){     block_mat[i] = (double *) malloc (sizeof(double) * s); }  /*freeing*/ (i = 0; < num_blocks; i++){     free(block_mat[i]); } 

free(block_mat);

edit: error found! under-allocated memory...so when printed out arrays looked fine... allocated arrays of sizes s, used s^2 instead. thank everyone!

you allocate space s doubles each block_mat[i]. later, access

block_mat[i][block_index] = 0; block_index++; 

but never check block_index goes out of bounds, does.

if write beyond s allocated doubles, might corrupt internal control data subsequent pointer, placed before pointer returned malloc , required intact free.