Concatenating 2D array into string causing segmentation fault

Question

I'm trying to concatenate a matrix into one long string using strcat, but keep getting seg faults whenever I attempt to access the matrix or use strcat. The segmentation fault occurs as soon as I enter the function. The first printf never executes.

void concatMatrix(int **matrix, char *output){ 
  printf("%s", "SDFSDFDSFDSFDSF");

  char *str = "";
  char *temp = "sdds";
  for(int i = 0; i < 5; i++) {
    for (int j = 0; j < 5; j++) {
       // temp = (char)matrix[i][j];
       // strcat(str, temp);
       // strcat(str, ' ');
       // printf("%d\n", matrix[i][j]);
    }
    // strcat(str, "\n");
    strcat(output, str);
    // printf("%s", output);
  }
}

This is how matrix and output were declared, and matrix was filled with values before calling the function.

int matrix[5][5];
char output[25];

Whenever I try to use matrix or output or strcpy() I get a segmentation fault. I can use printf simply on str or temp, but that is all. All the lines commented out will cause a seg fault. Any help would be greatly appreciated!

When you pass the variable declared like this: int matrix[5][5]; to a function, it decays to a single pointer (int*) not an int**. It will point to the start of the block of data that is 'perceived' by the program as a 1D array of 25 integers. — Adrian Mole
– Adrian Mole, Commented Apr 18, 2020 at 18:45
Ref your commented-out // strcat(str, temp); you can't do that with a string literal. Apart from *str pointing to a 1-byte space, it is read-only. — Weather Vane
– Weather Vane, Commented Apr 18, 2020 at 19:02
Please post the Minimal Reproducible Example, the shortest complete code that shows the problem. — Weather Vane
– Weather Vane, Commented Apr 18, 2020 at 19:06
Please add fflush(stdout) after the printf debugging cue, to be sure it is printed. When a program crashes, buffred output is usually discarded. Back to my earlier comment, you can try char str[1024] = ""; instead, or longer, and guarded from overflow by using strncat. — Weather Vane
– Weather Vane, Commented Apr 18, 2020 at 19:09
Thanks this helped a lot! changing my string to not read-only fixed a lot of problems. — George
– George, Commented Apr 18, 2020 at 20:44

anastaciu · Accepted Answer · 2020-04-18 22:31:42Z

The argument is of type int (*)[5] and the parameter is of type int**, these are not compatible, use:

void concatMatrix(int matrix[][5], char *output);

Furthermore, strcat's second parameter is expecting a char array and you are passing single char arguments to it, aside from the fact that str points to a string literal which is constant and cannot be altered.

You wouldn't need to use strcat to do this, you can assign these directly to output with a proper conversion:

Running sample

#include <stdio.h>

void concatMatrix(int matrix[][5], char *output)
{  
    int index = 0;
    for (int i = 0; i < 5; i++)
    {
        for (int j = 0; j < 5; j++, index++)
        {        
        output[index] =  matrix[i][j] + '0'; //convert from int to char and assign to output
        }       
    }
    output[index] = '\0'; //null terminate the string
}

int main()
{
    int matrix[5][5] = {{1, 4, 3, 5, 2},
                        {7, 9, 5, 9, 0},
                        {1, 4, 3, 5, 2},
                        {1, 4, 3, 5, 2},
                        {7, 9, 5, 9, 0}};
    char output[26]; //must have space for null terminator
    concatMatrix(matrix, output);
    printf("%s", output);
}

This will work only for single digits, wich, I gather, is the intended purpose given the size of the output string and the rest of the code.

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