My code is to allocate and initialize the array as follows (floatalloc2 is a function used to allocate the 2D array):
#include <stdio.h>
#include <stdlib.h>
void alloc_arr(int adjwfd, int nx, int nz, float **G, float **L1, float **L2)
{
G = floatalloc2(nx, nz);
switch (adjwfd){
case 1:
L1 = floatalloc2(nx, nz);
break;
case 2:
L2 = floatalloc2(nx, nz);
break;
}
}
void init_arr(int adjwfd, int nx, int nz, float **G, float **L1, float **L2)
{
memset(G[0],0,nx*nz*sizeof(float));
switch (adjwfd){
case 1:
memset(L1[0],0,nx*nz*sizeof(float));
break;
case 2:
memset(L2[0],0,nx*nz*sizeof(float));
break;
}
}
int main(int argc, char *argv[])
{
int adjwfd_P=1, nx=10, nz=10;
float **glob=NULL, **local1=NULL, **local2=NULL;
alloc_arr(adjwfd_P, nx, nz, glob, local1, local2);
init_arr(adjwfd_P, nx, nz, glob, local1, local2);
exit(0);
}
It passes the compilation. But when I run this code, it goes wrong says:
YOUR APPLICATION TERMINATED WITH THE EXIT STRING: Segmentation fault (signal 11)
However, I found that if I change the alloc_arr as follows, it runs successfully:
#include <stdio.h>
#include <stdlib.h>
void alloc_arr(int adjwfd, int nx, int nz, float ***G, float ***L1, float ***L2)
{
*G = floatalloc2(nx, nz);
switch (adjwfd){
case 1:
*L1 = floatalloc2(nx, nz);
break;
case 2:
*L2 = floatalloc2(nx, nz);
break;
}
}
void init_arr(int adjwfd, int nx, int nz, float **G, float **L1, float **L2)
{
memset(G[0],0,nx*nz*sizeof(float));
switch (adjwfd){
case 1:
memset(L1[0],0,nx*nz*sizeof(float));
break;
case 2:
memset(L2[0],0,nx*nz*sizeof(float));
break;
}
}
int main(int argc, char *argv[])
{
int adjwfd_P=1, nx=10, nz=10;
float **glob=NULL, **local1=NULL, **local2=NULL;
alloc_arr(adjwfd_P, nx, nz, &glob, &local1, &local2);
init_arr(adjwfd_P, nx, nz, glob, local1, local2);
exit(0);
}
My question is why I have to take the address of the 2D array, and in the alloc_arr define 3D array only for the allocation part, whereas in other functions such as init_arr, I can just pass the original 2D array into the function?
why I have to take the address of the 2D array, and in the alloc_arr define 3D array only for the allocation part,......
In this case:
alloc_arr(adjwfd_P, nx, nz, glob, local1, local2);
You are actually passing the last three argument as NULL to alloc_arr() function parameter G, L1 and L2 as the glob, local1 and local2 are initialized with NULL and in alloc_arr()
void alloc_arr(int adjwfd, int nx, int nz, float **G, float **L1, float **L2)
{
G = floatalloc2(nx, nz);
.....
L1 = floatalloc2(nx, nz);
.....
L2 = floatalloc2(nx, nz);
.....
the memory is getting allocated to G, L1 and L2 which are local variables of alloc_arr() function. In the main(), the glob, local1 and local2 are still null pointers after alloc_arr() function returned. These null pointers passed to init_arr() and when trying to access them in init_arr() your program is giving segmentation fault.
and in this case
alloc_arr(adjwfd_P, nx, nz, &glob, &local1, &local2);
you are passing the address of pointers glob, local1 and local2 as argument to alloc_arr() function parameters G, L1 and L2, which means G, L1 and L2 will hold address of glob, local1 and local2 pointers respectively.
void alloc_arr(int adjwfd, int nx, int nz, float ***G, float ***L1, float ***L2)
{
*G = floatalloc2(nx, nz);
.....
*L1 = floatalloc2(nx, nz);
.....
*L2 = floatalloc2(nx, nz);
.....
Dereferencing the pointer G will give glob pointer i.e. *G is glob. Similarly, dereferencing *L1 and *L2 will give pointer local1 and local2 respectively. So, when allocating memory to *G, *L1 and *L2 will actually allocate memory to glob, local1 and local2 pointers.
whereas in other functions such as init_arr, I can just pass the original 2D array into the function?
Look at this:
alloc_arr(adjwfd_P, nx, nz, &glob, &local1, &local2);
init_arr(adjwfd_P, nx, nz, glob, local1, local2);
the alloc_arr() will allocate memory to glob, local1 and local2 which means after returning from alloc_arr() (assuming everything works as expected) the pointers glob, local1 and local2 are pointing to valid memory locations. In the init_arr() you are just passing those memory locations as argument to the init_arr() function parameters G, L1 and L2. In the init_arr(), accessing G, L1 and L2 means it is accessing those memory locations.
Additional:
Follow good programming practice, make sure to free the allocated memory before program exits.