Segmentation fault when appending char to string in string array - c

Question

So I want to get all the lines from a file and turn them into a char* array. Problem is that whenever I try to append the character onto the end of the element it gives a segmentation fault.
char** loadOutputs(char *fileName, int *lineCount)
{
FILE *file = fopen(fileName, "r");
if (file) {
char c;
int lines = 0;
while ((c = fgetc(file)) != EOF)
if (c = '\n')
lines++;
rewind(file);
char **output = malloc(lines * sizeof(char*));
for (int i = 0; i < lines; i++)
output[i] = "";
int index = 0;
while ((c = fgetc(file)) != EOF)
if (c == '\n')
index++;
else
strcat(output[i], &c);
return output;
}
return NULL;
}
I always get a segmentation fault at strcat(output[i], &c);. I'd rather not create a fixed array size for the output because this could get fairly large and I don't want to use too much memory.

Answer 1

The following code:
for (int i = 0; i < lines; i++)
output[i] = "";
Is setting the pointer to an empty read only string.
You need to allocate some memory for the string:
I.e.
for (int i = 0; i < lines; i++) {
output[i] = malloc(MAX_LINE_LENGTH + 1);
}
Where MAX_LINE_LENGTH is some defined constant - perhaps #define MAX_LINE_LENGTH 100.
You will need to check that when reading the lines you do not exceed this length.
The following code will do this. This will resolve the other problem in that the address of c will not point to a null terminated string.
int index = 0;
int position = 0;
while ((c = fgetc(file)) != EOF) {
if (c == '\n') {
output[index][position] = 0; // Null terminate the line
position = 0; // Restart next line
index++;
} else {
if (position < MAX_LINE_LENGTH) { // Check if we have space!
output[index][position] = c; // Add character and move forward
position++;
}
}
}
output[index][position] = 0; // Add the null to the final line
Also you need to declare c as and int - i.e. change char c to int c. This is because EOF is outside the range for a char