Program exits without any error when I try to free a 2d matrix in a struct

Question

I create and fill my 2d array of unsigned char in my struct using parser of a file that contains numbers separated by spaces

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


struct imagem{
    unsigned char ** matrix;
    char* file_path;
    int height;
    int width;
    int max_cinza;
    int histograma[256];
    char* location;
};

struct imagem* lerImagem(char file_path[])
{

    struct imagem* imagem = malloc(sizeof(struct imagem)); 
    imagem->file_path = file_path;
    FILE* ptr;
    ptr = fopen(file_path, "r");
    
    if (NULL == ptr) {
        printf("A imagem não está no file_path selecionado \n");
        imagem->height = 0;
        return imagem;
    }
    int MAX_LEN = 10000;
    char buf[] = "";

    fgets(buf, MAX_LEN, ptr); 
    char width[10], height[10];

    fscanf(ptr, "%s %s", width, height);

    int int_largura = atoi(width);
    int int_altura = atoi(height);
    imagem->height = int_altura;
    imagem->width = int_largura;


    imagem->matrix = (unsigned char**) malloc(imagem->height * sizeof(unsigned char*)); 
    for (int i = 0; i < int_altura; i++) {
        imagem->matrix[i] = (unsigned char*) malloc(imagem->width* sizeof(unsigned char)); 
    }
    

    
    fscanf(ptr, "%u", &imagem->max_cinza);

    for(int row = 0; row < imagem->height; row++){
        for(int column = 0; column < imagem->width; column++){
            fscanf(ptr, "%u ", &(imagem->matrix[row][column]));
            }
        }   

    fclose(ptr);

    return imagem;
}

void freeImagem(struct imagem* imagem_input){
    
    printf("iniciando free de matrix");
    
    for(int i=0; i < imagem_input->height; i++){
            free(imagem_input->matrix[i]);
    }
    free(imagem_input->matrix);
    
    free(imagem_input);
}

I can compile and run that without any errors. The program seems to work randomly, as sometimes it frees properly and another times the program just stops without any explanation.

Answer 1

These lines appear problematic:

    char buf[] = "";
    fgets(buf, MAX_LEN, ptr); 

buf should point to an array that is at least MAX_LEN long, since fgets might write up to MAX_LEN bytes read from ptr.

I suggest using malloc to allocate MAX_LEN bytes to buf, before calling fgets(buf, MAX_LEN, ptr)

edit: in the code you just read 10k bytes into buf, but you don't use buf anymore after that. Are you sure this is the intended behavior?

Answer 2

At least these problems:

Enable all warnings

These 4 lines all emit warnings with a well enabled compiler:

imagem->height * sizeof(unsigned char*));
imagem->width * sizeof(unsigned char));
fscanf(ptr, "%u", &imagem->max_cinza);
fscanf(ptr, "%u ", &(imagem->matrix[row][column]));

Incorrect format

"%u" matches a unsigned *, not unsigned char *.

struct imagem{
    unsigned char ** matrix;

    ...

    fscanf(ptr, "%u ", &(imagem->matrix[row][column]));  // Bad!

Better as

fscanf(ptr, "%hhu ", &(imagem->matrix[row][column]));  // Better

Drop unneeded " " and check result

if (fscanf(ptr, "%hhu", &(imagem->matrix[row][column])) != 1) {
  ; // Handle error
}  // Even better

Buffer too small

@Leon Kacowicz

Sample alternative:

// int MAX_LEN = 10000;
// char buf[] = "";
// fgets(buf, MAX_LEN, ptr); 
#define MAX_LEN 10000
char buf[10000];
if (fgets(buf, sizeof buf, ptr) == NULL) {
  ; // Handle input error
}

Use a C compiler

Comments suggest OP is using another language (C++) to compile C code.

Improve allocation

// imagem->matrix = (unsigned char**) malloc(imagem->height * sizeof(unsigned char*)); 

if (imagem->height < 0) {
  Handle_error(); // TBD code
}
imagem->matrix = malloc(sizeof imagem->matrix[0] * imagem->height); 
if (imagem->matrix == NULL && imagem->height) {
  Handle_error(); // TBD code
}

Never use "%s" without a width when scanning

char width[10], height[10];

// fscanf(ptr, "%s %s", width, height);
fscanf(ptr, "%9s %9s", width, height);

Better, scan into the int and check result.

int int_largura, int_altura;
if (fscanf(ptr, "%s %s", &int_largura, &int_altura) != 2) {
  Handle_error();
}

Mixing fscanf() and fgets() is tricky

This often fouls up learner code. Best to first use fgets()` only.

Answer 3

In the end, the problem was in file_path and location attributes, as I didn't used malloc to use them, this leads my program to broke randomly.