segmentation fault while writing user program in linux

[garimella]

I am trying to write a simple device driver and application program to access the driver.

Driver write system call looks like this

---

Code C - [expand]
1
2
3
4
5
6
7
8
9
10
11
12
static ssize_t dev_write(struct file *filp, const char *buff, size_t len, loff_t *off)
{
short c=0;
short ind =0;
short count=0;
memset(msg,0,100);
readPos=0;
while(len>0)
{
 msg[count++]=buff[ind++];
 len--;
}

---

and application program which works looks as follows

---

Code C - [expand]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
int main()
{
char buf[100];
char i = 0;
char tag=0;
 
int fp;
char *val[10];
char *rval[10];
unsigned int a =18440;
unsigned int d =1;
memset(buf, 0, 10);
tag=1;
fp = open("/dev/dd",O_RDWR);
if (fp<0)
{ 
  printf("file failed to open\n");
}  
sprintf(val[1],"%d%x%x",tag,a,d);
write(fp,val[1],strlen(val[1]));
sprintf(rval[1],"%d%x",tag,a);// statement that causes segmentation fault
return 0;
}

---

Now if I add one more sprintf function as shown above , it leads to segmentation fault.I do not know why?. If I remove this line, the code works fine. What could be the problem? The same problem happens when I call sprintf and write as a subfunction call?

 

[betwixt]

I'm not at a compiler right now but at a glance, should there be a 'close' to match the 'open' statement?
Also (again this is from my memory only so treat as suspect!) should 'fp' be declared as a pointer?

Brian.

 

[c_mitra]

I am not going through your code line by line but have a couple of suggestions:

1. Check the plocks; just see that the { and } are nicely matched.

2. Please try with printf first (whether it works) and then replace that with sprintf. Then check the syntax. What is there in val[] and rval[]??

 

[FvM]

Looks like a trivial C programming issue.

You are declaring arrays of pointers char *val[10] and char *rval[10] but never assigning an actual storage location to it, e.g. val[1] = buf.

 

[garimella]

val[1] gets assigned to the concatenated values of tag,a and d. This works fine for me. Now by adding only rval[1] statement leads to fault. Do not understand correlation. rval[1] holds tag and a in string format. Also checked the syntax and everything works without the second sprintf statement as shown in my code.

 

[FvM]

Presuming you didn't omit part of the code, please show me which string storage is addressed by pointers val[1] and rval[1].
Using uninitialized pointers isn't a syntax rather than a sematic error. Some compilers might flag a warning.

 

[c_mitra]

which string storage is addressed by pointers val[1] and rval[1]...

Correct; segmentation faults are caused by reference to memory outside the allowed range...

I do not see any proper reference for rval (which is a pointer).

I request the original poster to study pointers in a bit more detail; they can be messy and I myself get confused so often...