使用read将文本从c的文件形式转换成矩阵形式

Question

I currently try to fix a problem I encountered during a project. I have to read the content of a file (a map) into a matrix, but the first line contains 4 separate characters:

1. The number of rows
2. The empty character's representative
3. The obstacle character's representative
4. The full character's representative. These I need to enter in an array.

Code:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include "head.h"

#define BUF_S 4096

char    **ft_build_martrix(char *str, int *pr, int *pc, char *blocks)
{
  int     fd;
  int     ret;
  int     ret2;
  int     j;
  char    *res[BUF_S + 1];

  j = 0;
  fd = open(str, O_RDONLY);
  if (fd == -1)
  {
      ft_putstr("map error");
  }
  ret2 = read(fd, blocks, 4); //take the first row out and put it into the array
  ret = read(fd, res, BUFF_S); //read the file's content into the matrix
  res[ret][0] = '\0';
  *pr = blocks[0] - '0'; // give the number of rows to the r variable that is declared in main and passed as argument, I think that's the equivalent of the $r in c++ ?
  blocks[0] = blocks[1]; // delete the
  blocks[1] = blocks[2]; // first character
  blocks[2] = blocks[3]; // that is already in *pr
  while (res[1][j] != '\n')
      j++;
  *pc = j; // get the number of column
  return (res);
}

Answer 1

You are returning pointer to local variable. Local variables are removed from scope (their lifetime has ended) after returning from function, so accessing this address (which doesn't belong to you anymore) has undefined behavior.

Also this is declaration of pointer-to-array

char * res[BUF_S + 1];

+---+          +---+
| * |--------->|   | res[0][0] (*ret)[0]
+---+          +---+
 res           |   | res[0][1]
               +---+
               |   | ...
               +---+
               |   | res[0][BUF_S + 1 - 1]
               +---+

If you apply this operation

res[ret][0] = '\0';

with ret != 0 you are invoking undefined behavior.

---

You have to declare it like this ( pointer-to-pointer-to-char )

char ** res = malloc(sizeof(char*) * X);  // Where X is number of rows

And every row separately

for (int i = 0; i < X; i++)
{
    res[i] = malloc(BUF_S + 1);
}

Then you can access different 'rows' (max X-1 , X and more would again lead to access outside of bounds and undefined behavior).

Then you will free memory.