Segmentation Fault in C function

Question

I am trying to develop a basic shell. For that shell I need a C function to parse a string. As i am new to C I tried to develop a basic function and it gives me a segmentation fault error. Please tell me what I am missing.

#include<string.h>
#include<stdio.h>


void parse(char *msg);
int main()
{
    char *msg =  "This is a message";
    parse(msg);
}

void parse(char *msg){
    char *mm;
    mm = msg;

    char *tok;
    tok = strtok(mm," ");
    while(tok == NULL){
        tok = strtok(NULL," ");
            printf("%s \n",tok);
    }
}

Error Message (Runtime)

Segmentation fault (core dumped)

Thanks in advance

Answer 1

msg points to a string literal, and you are attempting to modify it. In C, modifying string literals is undefined behaviour (in practice, compilers often place them in read-only memory).

To fix, turn msg into an array:

int main()
{
    char msg[] =  "This is a message";
    parse(msg);
}

Also, there are a couple of issues with your while loop:

1) the condition is the wrong way round;
2) the second strtok() call should appear after the printf().

void parse(char *msg){
    char *mm = msg;
    char *tok = strtok(mm, " ");
    while (tok) {
        printf("%s \n",tok);
        tok = strtok(NULL," ");
    }
}

Answer 2

You can't reliably modify a string literal; they are often readonly (and in your case, clearly are readonly). An attempt to modify a string literal invokes undefined behaviour, which is always a Bad Thing™!

Use:

int main(void)
{
    char msg[] = "This is a message";
    parse(msg);
}

Answer 3

Your definition:

char *msg =  "This is a message";

makes msg as constant char string which cannot be modified. But strtok modifies it.

You may want to change it to

char *msg =  strdup("This is a message");

Don't forget to free the pointer after you are done.

Answer 4

Perhaps instead of

while(tok == NULL)

you meant

while(tok != NULL)

or just

while (tok) // <- because in C, conditions are always compared to 0

. However, what gives you the segmentation fault is that strtok modifies the passed string, which is why it takes a non-const pointer to char (see http://linux.die.net/man/3/strtok). Therefore, because you are passing a pointer to a string literal, which are not modifiable, you receive a segmentation fault (which is your luck; this bug might also slip through QA and go production if you are unlucky).

Answer 5

Go with it.........

#include<string.h>
#include<stdio.h>


void parse(char *msg);
int main()
{
    char msg[] =  "This is a message";
    parse(msg);
}

void parse(char *msg){
    char *mm;
    mm = msg;

    char *tok;
    tok = strtok(mm," ");
    while(tok != NULL){
        printf("%s \n",tok);
    tok = strtok(NULL," ");
    }
}

Answer 6

how about change tok==NULL to tok!=NULL