fgets segmentation fault when reading input from user

0
votes

here's the offending code using ubuntu

char *name;

int main(void)
{
  fgets(name, sizeof(name), stdin);
}
void HUD()
{
  printf("%s ", name); 
}

Here's my problem. I started with scanf("%s", &name) and was getting junk at the end of the string. Through the last 2 hours have been reading docs on scanf, and fgets, because apparently scanf shouldn't be used when you don't know the size of the array you want, (and since user input can vary in size) I decided to try using fgets. I've also tried setting a fixed value both by char name[100]; and by fgets(name, 100, stdin)

Now I'm getting a segmentation fault, and through reading every result I found on the first 2 pages of google, my syntax appears correct, and I've found nothing on cboard or here to fix my problem.

Any ideas?

2
cfgets
i've done that already. No success. Still getting segmentation fault. – nanthil
You can use scanf() just fine when you don't know the length of the input, as long as you specify a field width that prevents your array from being overrun. fgets() has exactly the same limitation; it's just more explicit about making you tell it the limit on how many characters it should read. – John Bollinger
Do note, however, that scanf() and fgets() differ on how to determine when to stop reading. – John Bollinger

2 Answers

3
votes

sizeof(name) Will be the size of the pointer on your system, on mine it's 8 bytes. Not the size of the buffer, as you might have been expecting

Also char* name is uninitialised. You will try to write to an uninitialised buffer and it will end in undefined behaviour.

To resolve either make it a fixed size buffer or allocate some space on the heap.

Allocate

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

#define NAME_SIZE 100
char *name;

void HUD()
{
  printf("%s ", name); 
}

int main(void)
{
    name=calloc(NAME_SIZE, sizeof(char));
    fgets(name, NAME_SIZE, stdin);

    HUD();

    free(name);
}

Static Array

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

#define NAME_SIZE 100
char name[NAME_SIZE];

void HUD()
{
  printf("%s ", name); 
}

int main(void)
{
    fgets(name, NAME_SIZE, stdin);

    HUD();
}

You must pass the size of the buffer to fgets so it know how much space it has to write in to.

-2
votes

char *fgets(char *restrict s, int n, FILE *restrict stream);

The fgets() function shall read bytes from stream into the array pointed to by s, until n-1 bytes are read, or a is read and transferred to s, or an end-of-file condition is encountered. The string is then terminated with a null byte. [0]

You need to allocate it to a specific size and call fgets with that size. This code can help you accomplish the same thing, but it has a fixed size buffer.

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

char name;
char* buffer;
int buffer_size = 16;
int i = 0;


void HUD()
{
        printf("%s ", buffer);
}

int main(void)
{
        buffer = malloc(buffer_size);
        if(!buffer) return;

        for(;;) {
                name = getchar();
                if(name < 0) {
                        buffer[i] = '\0';
                        goto finish;
                } else if(i < (buffer_size -1)) {
                        buffer[i++] = name;
                } else if(name == '\n') {
                        break;
                }
        }
        buffer[i] = '\0';
        finish:
            HUD();
            free(buffer);
            return 0;
}

[0] http://pubs.opengroup.org/onlinepubs/009695399/functions/fgets.html