redirecting shell command output to C program

Question

I have the following program :

/* a.c */ 
#include <stdio.h>

int   
main(int argc, char* argv[]){     

size_t size=0;    
char* lineptr;      

    while(getline(&lineptr, &size, stdin)){           
        fprintf(stderr, "line = %s\n", lineptr);      
        if(lineptr){      
            free(lineptr);     
            lineptr = NULL;     
        }      
    }      

return 0;      
}

I redirected the output of shell command "ls" to this program using the
following line :

ls | ./a.out

Expected output :
program should print the name of all files in the current directory
and terminate.

Actual output :
The program prints the name of all the files but does not terminate,
instead it loops infinitely and prints the last entry infinitely.

Thanks

I was ignoring the return value of getline. at the end it returns -1 which happens to be a true value, thus the infinite loop. — Abhishek Choubey
– Abhishek Choubey, Commented Feb 16, 2015 at 18:36
Since you aren't using the arguments, you'd do better to be explicit about that and write int main(void). — Jonathan Leffler
– Jonathan Leffler, Commented Feb 16, 2015 at 18:43

Wintermute · Accepted Answer · 2015-02-16 18:42:03Z

4

GNU's getline function returns -1 upon end-of-file (or error). Use

while(-1 != getline(&lineptr, &size, stdin))

...and set lineptr to NULL before the first call to getline.

Also, you don't have to free the pointer in every iteration of the loop; you can reuse the previous pointer and free once at the end:

size_t size = 0;
char* lineptr = NULL;

while(-1 != getline(&lineptr, &size, stdin)){
  fprintf(stderr, "line = %s", lineptr);
}

free(lineptr);

getline will use realloc internally as needed. Note that you have to make sure that lineptr and size are not changed between calls to getline for this to work (although you may change the string to which lineptr points).

edited Feb 16, 2015 at 18:42

answered Feb 16, 2015 at 18:36

Wintermute

44.3k5 gold badges84 silver badges85 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Abhishek Choubey Over a year ago

I read the manual, it says if lineptr is NULL, then the value of size is ignored, so i made lineptr to be NULL after every iteration

Wintermute Over a year ago

That works, but it is inefficient. There is no need to do a heap allocation every iteration, only when the allocated space is too small and has to be enlarged. getline handles this internally, so there's no need not to take advantage of it.

Collectives™ on Stack Overflow

redirecting shell command output to C program

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related