Why does this cause a segfault?

Question

I'm just trying to malloc an array of strings and copy input from a file into this array. This combination of lines causes a segfault and I don't know why.

int count = 0;
char **output = (char**)malloc(numLines*257);
fgets(output[count], 257, input);

Don't cast malloc: http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Barmar, Oct 10 '13 at 16:56

score 5 · Accepted Answer · answered Oct 10 '13 at 16:58

You've allocated space for an array of pointers, but you haven't initialized any of those pointers.

int count = 0;
char **output = malloc(numLines*sizeof(char *));
int i;
for (i = 0; i < numLines; i++) {
  output[i] = malloc(257);
}
fgets(output[count], 257, input);

score 1 · Answer 2 · answered Oct 10 '13 at 17:00

I think what you actually wanted to do here was allocate a memory for numLines pointers (strings) and then allocate memory for every string so that each of these is capable of holding 257 chars:

int i, count = 0;
char **output = malloc(sizeof(char*) * numLines);
for (i = 0; i < numLines; ++i)
    output[i] = malloc(257);
...
fgets(output[count], 257, input);

just don't forget to clean it up once you don't need it anymore:

for (i = 0; i < numLines; ++i)
    free(output[i]);
free(output);
output = NULL;

score 1 · Answer 3 · answered Oct 10 '13 at 17:01

int count = 0;
char **output = (char**)malloc(numLines*257);
fgets(output[count], 257, input); // here You are going wrong, with out allocating memory you are trying to read.

if you want to read string

char *output = malloc(MAX_LENGTH+1); //allocate memory
    fgets(output[count], MAX_LENGTH+1, input);

if you want to read array of strings

char **output = malloc(MAX_NUM_STRINGS * sizeof(char *)); //allocate Number of pointers 
for(count=0;count<MAX_NUM_STRINGS;count++)
{   output[count]=malloc(SIZE_OF_EACH_STRING+1);  //allocate memory for each pointer,  
    //You are accessing with out allocating memory  
    fgets(output[count], SIZE_OF_EACH_STRING+1, input); 
}

Why does this cause a segfault?

3 Answers3