CODEWITHSUNDEEP
cpointersstructsegmentation-faultmalloc
kanap008
kanap008

Reputation: 2820

Seg Fault during pointer manipulation

I have the following struct in C.

struct a {
    long state;
    long uid;
    long w, x, y, z, xx, yy, zz, xxx, yyy, zzz;
    char comm[64];
};

Then I do a malloc as follows. 

buf = malloc (100 * sizeof(struct a));

But when I try to access the individual structs as follows, I get a seg fault.

for (i = 0; i < 100; ++i) {
    tmp = buf + (i * sizeof(struct a));
    printf ("\t>%d>%ld,%ld\n", i, tmp->state, tmp->uid);
}

I am getting a seg fault after the first 10 entries. I have no idea why this happens. Please help.

Upvotes: 0

Views: 74

Answers (3)

chrisaycock
chrisaycock

Reputation: 37930

The compiler already handles sizeof() advancement during pointer arithmetic. Thus,

ptr + i;

is the same as

&ptr[i];

It is wrong to use

ptr + (i * sizeof(some_type));

Upvotes: 3

AusCBloke
AusCBloke

Reputation: 18492

This line is wrong:

tmp = buf + (i * sizeof(struct a));

You don't need to multiply i by the size of each element in the array, this is done implicitly for you based on the type of buf.


What you're actually doing is

tmp = &buf[i * sizeof(struct a)];

when what you're really trying to do is

tmp = &buf[i];

Upvotes: 4

Josh Petitt
Josh Petitt

Reputation: 9579

if buf is a pointer to a struct a, the pointer math should be:

tmp = buf + i;

Upvotes: 5

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