Seg Fault using strcmp with * char

Question

I have this struct

typedef struct no
{
    char command[MAX_COMMAND_LINE_SIZE];
    struct no * prox;
} lista;

lista *listaCommand = NULL;

and I'm filling listaCommand with a simple function that seems to work ok, as I can read the values without any problem, but if I try to compare, like

strcmp(listaCommand->prox>command, ">")

I just get a segmentation fault, even though the value > is there, why this is happening?

Your are missing Correct it to -> strcmp(listaCommand->prox->command, ">") — Navnath Godse, May 24 '13 at 07:24
@Navnath uhhh.. clever! I though it was a typo, but you are right, if this is the actual code it is a comparison and it returns either 0 or 1.. Nice! :) — Lorenzo Dematté, May 24 '13 at 07:25
The fact that there's a problem in *this* code doesn't mean that's the only problem. @Dante003 Should Navnaths answer not solve your problem, please devise a minimal, **compilable** testcase using `strcmp` to produce your segfault. — autistic, May 24 '13 at 07:29
Can anyone tell me how come nobody sees anything wrong with trying to dereference a **NULL** pointer?Either my basics are rusty or you are not seeing the elephant in the room!! — Rüppell's Vulture, May 24 '13 at 07:37
@Rüppell'sVulture From what we can see, "I'm filling listaCommand with a simple function that seems to work ok". The OP *could* be inadvertently blinding us to other errors, which is why I asked for a minimal, compilable testcase... — autistic, May 24 '13 at 07:44
Are you asking about the difference between the `>` operator and the `->` operator? — autistic, May 24 '13 at 07:52

score 10 · Answer 1 · edited May 24 '13 at 07:32

10

strcmp(listaCommand->prox>command, ">")

Should be

strcmp(listaCommand->prox->command, ">")

In your code listaCommand->prox>command will be seen as a comparison operation, using the > operator. A comparison in C returns an integer, 0 if false, non-zero otherwise. There are good chances it will return 0, which is not a valid memory address. Hence, the segmentation fault.

edited May 24 '13 at 07:32

Lorenzo Dematté

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answered May 24 '13 at 07:26

Navnath Godse

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score 0 · Answer 2 · edited May 25 '13 at 16:09

0

Change

strcmp(listaCommand->prox>command, ">")

to

strcmp(listaCommand->prox->command, ">")

edited May 25 '13 at 16:09

Ry-

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answered May 24 '13 at 07:26

Raghu Srikanth Reddy

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kundrata · Answer 3 · 2013-05-24T11:29:04.207

Allocate memory !!!

typedef struct no
{
    char str[20];
    struct no * prox;
} lista;

lista *listaCommand = NULL;

int main(int argc, char** argv)
{
    listaCommand = malloc(sizeof(lista));
    listaCommand->prox = malloc(sizeof(lista));
    strcpy(listaCommand->prox->str, "aaa");
    printf("%d\n", strcmp(listaCommand->prox->str, ">>"));

    free(listaCommand->prox);
    free(listaCommand);

    return 0;
}

Seg Fault using strcmp with * char

3 Answers3