2-Dimensional array with malloc, function call to allocate - c

Question

I'm working on a project and I'm kinda stuck at a problem. I'm trying to read a file that contains a number in the first line that gives the number of rows, and then following the matrix of integers, separated by spaces.
I want to make a pointer in the main, then call the function with the pointer as parameter, the function should read the first number in the txt file, then create a 2d array with malloc, then read the matrix in the textfile and return.
but either i get it so the function can allocate and read the matrix, but then i have something wrong with the pointer when calling the function so i cant use the allocated and read stuff in the main, or im getting errors when trying to call by reference and allocate stuff in the function then.
void readjobs(FILE* fp, int ***array, int linesToRead, int facilityCount) {
int ch = 0;
int rows = 0;
while ((ch = fgetc(fp)) != '\n')
{
rows = ch - 48;
//rows = rows * 10 + (ch - 48);
}
if (rows > linesToRead) rows = linesToRead;
*array = (int**)malloc(rows * sizeof(int*));
for (int i = 0; i < rows; i++) {
/* size_y is the height */
*array[i] = (int*)malloc(facilityCount * sizeof(int));
}
int i = 0, j = 0;
while ((ch = fgetc(fp)) != EOF)
{
if (ch == '\n')
{
i++;
printf("\n");
}
else if (ch == ' ')
{
j++;
printf("%i ", *array[i][j]);
}
else //wenn es ne nummer ist
{
*array[i][j] = (*array[i][j] * 10) + (ch - 48);
}
}
}
int main(int argc, char** argv) {
int facilities_count = -1;
int jobs_count = -1;
int **jobs = NULL;
FILE *fp; //Zeiger für Datei
fp = fopen("jobs.txt", "r"); //Dateizugriff, Datei als read
if (fp == NULL) { //falls die Datei nicht geoeffnet werden kann
printf("Datei konnte nicht geoeffnet werden!!\n");
}
else { //Datei konnte geoeffnet werden
printf("Datei ist lesbar\n");
readjobs(fp, &jobs, 6, 6);
if (jobs == NULL)printf("nullpionter");
else {
for (int i = 0; i < 6; i++) {
printf("\n");
for (int j = 0; j < 6; j++) {
printf("%x ", jobs[i][j]);
}
}
}
fclose(fp); //Dateizugriff wieder freigeben
}
MPI_Finalize();
getchar();
return 0;
}
Textfile example:
6
3 2 2 1 5 4
1 1 3 4 2 0
1 2 3 4 5 1
3 4 2 0 1 5
1 0 5 2 3 4
4 0 1 3 5 2
the first number "6" in this case is how many lines, and the rest is the matrix to be read

Answer 1

Your main problem
I compiled your code with debugging on:
$ cc -g mat.c -Wall -Wextra
Ran it in debugger:
$ gdb a.out
(gdb) run
Starting program: /tmp/a.out
Datei ist lesbar
Program received signal SIGSEGV, Segmentation fault.
0x00005555555548ff in readjobs (fp=0x555555756010, array=0x7fffffffe740, linesToRead=6, facilityCount=6)
at mat.c:18
18 *array[i] = (int*)malloc(facilityCount * sizeof(int));
Ok, so it crashes at this line:
*array[i] = (int*)malloc(facilityCount * sizeof(int));
Doing so is always a good idea to find out what the problem is. What could be the cause? Unfortunately it is not trivial in this case. Unless you really understand pointers. *array[i] is not what you want. You want (*array)[i].
Remember that x[i] is just a shortcut for *(x+i). Furthermore, [] has higher priority than *. So *x[i] = *(x[i]) = *(*(x+i)), but what you want is (*x)[i] = *((*x) + i) which clearly is not the same thing.
Other stuff
I would definitely extract the creation of the matrix, like this:
int ** createMatrix(int rows, int columns) {
printf("%d %d\n", rows, columns);
int **ret;
ret = malloc(rows * sizeof *ret);
if(!ret) { perror("Error: "); exit(EXIT_FAILURE); }
for(int i=0; i<columns; i++) {
ret[i] = malloc(columns * sizeof (*ret)[0]);
if(!ret[i]) { perror("Error: "); exit(EXIT_FAILURE); }
}
return ret;
}
and then in your code:
*array = createMatrix(rows, facilityCount);
// Don't trust my code. Check the pointer.
if(!array) { perror("Error: "); exit(EXIT_FAILURE); }
Remember to ALWAYS check if malloc succeeded.
And your way of reading the numbers is very weird. If you are a beginner who came up with this method on your own, that's actually pretty impressive, but it's not a good method. Read about fscanf and getline.