CODEWITHSUNDEEP
clinux
Zé Miguel Coutinho
Zé Miguel Coutinho

Reputation: 75

fgets() causing segmentation fault

In college I am learning communication between processes, and I have this exercise where I have to get a parent process link to a child through a pipe, the child runs the cmd sort fx.txt and sends to the parent the output, which the parent should print.

The content of fx.txt is

b
a

After some search, found out popen() would be a good choice, and following some example code this is what I wrote:

#include <stdio.h>
#include <unistd.h>

#define SIZE 255

int main() {
    FILE *file;
    char *cmd = "sort fx.txt";
    file = popen(cmd, "r");
    if (file == NULL) {
        printf("error opening pipe\n");
        return 1;
    }

    char *str;

    //print sorted lines
    while (fgets(str, SIZE, file) != NULL) {
        printf("%s", str); // \n already stored in str
    }
    pclose(file);
    return 0;
}

I get segmentation fault on the line of fgets(). After that, I randomly tried to fix it by creating another pointer and assigning the return of fgets() to it.

The change is as follows:

    char *str;
    char *s;
    //print sorted lines
    while ((s = fgets(str, SIZE, file)) != NULL) {
        printf("%s", str); // \n already stored in str
    }

It worked and the output is: 

a
b

I would be thankful if someone could explain me this behaviour.

Upvotes: 1

Views: 297

Answers (1)

Carl Norum
Carl Norum

Reputation: 224944

char *str;

Is an uninitialized pointer. When you call fgets with it as an argument, you cause undefined behaviour. Allocate some actual space and use that:

char str[MAX_LINE_LENGTH];

Where MAX_LINE_LENGTH is something reasonable for your program.

Upvotes: 1

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