C将指针解引用为不完整类型时出错

Question

I have some issues with a main file.I have a matrix of structures (theoretically) and i want to modify all my "p" parameters in all the structures. This is the main file:

int main(int argc, char** argv) {
int i, j;

struct PQ *queue;
queue = createQ(5);
for (i = 0; i <= 5; i++) {
    for (j = 0; j = 20; j++);
    queue->mem[i][j].p = 1;
}

for (i = 0; i <= 5; i++) {
    puts("\n");
    for (j = 0; j <= 20; j++);
    printf("%d ",queue ->mem[i][j].p);
}


return (EXIT_SUCCESS);
}

And this is another file which contains structures definiton and generate function:

typedef struct newLine{
    unsigned p;
} newLine;


struct PQ{
    struct newLine ** mem;
};

struct PQ *createQ(unsigned min){
    int i=0;
    struct PQ *newQ = malloc(sizeof(PQ));
    newQ->mem = malloc(min*sizeof(newLine *));

    for(i=0;i<=min;i++){
        newQ->mem[i]=calloc(20,sizeof(newLine));
    }

    return newQ;
}

Any ideas?

Answer 1

You are seeing the error because struct is part of the type in C, so you have to use struct newline in struct PQ. Another way is to use typedef to create type alias:

typedef struct newline {
    int p;
} newline;

The way to access struct's member is use . on struct, use -> on pointer to struct, so use queue->mem[i][j].p instead.

There are other problems.

You cannot dereference an uninitialized pointer, it yields undefined behavior:

PQ *newQ;

should be:

struct PQ *newQ = malloc(sizeof(struct PQ));

You should allocate with correct indirections:

newQ->mem = malloc(min * sizeof(newline*));
for(int I = 0;i < min; i++){
    newQ->mem[i] = calloc(20, sizeof(newLine));
}