Programming

Hello,

I am trying to free memory allocation after EOF from keyboard is detected (ctrl+D) in a C program. I've written a small program to replicate my problem:

Code:

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

   char *line;
   line = (char*)malloc(sizeof(char)*(512));

   line = fgets(line, 512, stdin);

   if (feof(stdin)) {     /* Detects ctrl+D */
      free(line);         /* Attempts to free line before return */
      return(EXIT_SUCCESS);
   }

   return(EXIT_SUCCESS);
}

When I run this under valgrind with --leak-check=full --show-reachable=yes it is saying the line is not freed when I terminate with ctrl+D. What am I doing wrong here?

Oddly enough there is a thread from 2 days ago on this very topic. Consider using the forum search facility next time. That is why we have it.

Catch ctrl+d in C

There are some links to POSIX behavior and an explanation about changing the behavior of the terminal.

Also - when your program exits, the OS "frees" all memory anyway. There is no memory leak, therefore.

I don't think this is true, at least using SUS2 like I am. Any piece of malloc'd memory has to be freed before the program exits or it is considered leaked memory. If you run the sample code I posted above with valgrind and just hit enter, it will report the leaked memory (since it does not free the line if the program exits normally). My question is, why does the program leak memory when an EOF is sent from the keyboard even though it tries to free the memory before returning in that case?

You should, though. Leaked memory does not survive its allocating process death by design. Allocated memory is virtual memory which means nothing when the context of a process disappear.

I'd make double-sure the program is really doing what you think it does. print a line and free the memory when ctrl-d is found.

This is exactly what I did to make sure the code reaches the free(line). If I change the if statement to something like:

Code:

if (feof(stdin)) {
   printf("CTRL+D pressed by user\n");
   free(line);
   return(EXIT_SUCCESS);
}

The printf is printed, but the line isn't freed...

You didn't show what headers you included, are they at least stdio.h and stdlib.h ?

[edit] AHAH! On an EOF condition, fgets returns null, overwriting line with that and preventing the real value from being freed. Overwriting line with fgets' return value serves no purpose in any case, since if it finds any input it won't change.