struct inner element's value assignment failed in c-CodePudding

context: https://stackoverflow.com/a/72238514/15603477

#include<stdio.h>
#include<stdlib.h>
#define MAT_SIZE    2
#define MAT_COUNT   3

typedef double mat[MAT_SIZE][MAT_SIZE];
typedef struct matList{
    const char *name;
    mat matrix;
}matList;

void init_mat(mat m)
{
    for(int i = 0; i < MAT_SIZE; i  ){
        for(int j = 0; j < MAT_SIZE; j  ){
            m[i][j] = i * 2.1   (double) j   1.1;
            printf("(i,j)=%f\n",m[i][j]);
        }
    }
}

//allocate matList.
matList *create_mat(const char *name)
{
    matList *tempMat = malloc(sizeof *tempMat);
    
    if(tempMat != NULL)
    {
        tempMat->name = name;
        init_mat(tempMat->matrix);
    }
    return tempMat;
}

// freematList
void free_matList(matList **mats)
{
    if(mats){
        for(int i = 0; i < MAT_COUNT; i  ){
            free(mats[i]);
        }
    }
}

// return non zero if successful
int allocate_matList(matList **mats)
{
    if(mats){
        mats[0] = create_mat("MAT_A");
        mats[1] = create_mat("MAT_B");
        mats[2] = create_mat("MAT_C");
        if(mats[0] && mats[1] && mats[2])
            return 1;
    }
}

int main(void)
{
    matList *mats[MAT_COUNT];
    if(allocate_matList(mats)){
        printf("mat[2] name: %s\n",mats[2]->name);
        size_t row = sizeof(mats[2]->matrix) / sizeof (mats[2]->matrix[0]);
        size_t col = sizeof(mats[2]->matrix[0]) / sizeof(mats[2]->matrix[0][0]);
        
        printf("row: %ld col: %ld\n",row,col);    

        for(size_t i = 0; i < row; i  ){
            for(size_t j = 0; j < col; j  ){
                printf("%f\t",mats[2]->matrix[row][col]);
            }
            printf("\n");
        }
    }
    free_matList(mats);
    exit(EXIT_SUCCESS);
}

There would be MAT_COUNT of struct matList. each matList's matrix is identical. expected last print result would be:

1.100000 2.100000
3.200000 4.200000

now the last printf return

0.000000        0.000000
0.000000        0.000000

which means that the values assignment of matrix failed?

(gdb) break 23
Breakpoint 1 at 0x11d9: file array_struct170.c, line 23.
(gdb) s
The program is not being run.
(gdb) run
Starting program: /home/jian/helloc/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, init_mat (m=0x5555555592a8) at array_struct170.c:23
23          for(int i = 0; i < MAT_SIZE; i  ){
(gdb) s
24              for(int j = 0; j < MAT_SIZE; j  ){
(gdb) s
25                  m[i][j] = i * 2.1   (double) j   1.1;
(gdb) print m[i][j]
$1 = 0

now I guess the line have problem is m[i][j] = i * 2.1 (double) j 1.1;

CodePudding user response：

    for(size_t i = 0; i < row; i  ){
        for(size_t j = 0; j < col; j  ){
            printf("%f\t",mats[2]->matrix[row][col]); // <- look closely!
        }
        printf("\n");

Since row is 2 and col is 2, you are trying to print out matrix[2][2], which is out of bounds. You probably wanted matrix[i][j] instead of matrix[row][col].

By the way, valgrind caught this immediately.