Unable to print to file from shared object

Question

I have seen questions similar to this one on here before but I was unable to solve my problem from the answers to those.

I have a file libfoo.c from which I'm creating a shared object. Using gcc's __attribute__((constructor)), I'd like to print a message to a file when this shared object is loaded:

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

static char *term = NULL;

static void init (void) __attribute__((constructor));

static void
init (void)
{
  FILE *f = fopen ("lib.log", "w");

  if (f)
    {
      if (isatty (STDOUT_FILENO))
        {
          char *tmp = ttyname (STDERR_FILENO);
          if (tmp && *tmp)
            {
              term = strdup (tmp); // nevermind the memory leak
              fprintf (f, "Found terminal %s\n", term);
            }
        }
      else
        fprintf (f, "Failed to find terminal\n");

      fclose(f);
   }
}

void *
malloc (size_t size)
{
  return NULL;
}

The dummy malloc implementation (to be extended later) should replace stdlib.h's malloc in another dummy program, whose source code resides in main.c:

#include <stdlib.h>

int main (void)
{
  char *m = malloc(1024);
  (void)m;
  return 0;
}

I'm compling and linking both files as follows:

gcc -o main main.c
gcc -fpic -shared -o libfoo libfoo.c

And then when I execute the following no file is created and no output is logged:

LD_PRELOAD=$(readlink -f libfoo) ./main

What is going on here? As a sidenote: how could I try to debug a problem like this using ltrace? ltrace "LD_PRELOAD=... ./main" is not a valid command.

EDIT: What is going on here? It seems at least printf works inside the shared object, but only inside the setup function, if I call printf inside malloc the program segfaults. In trying to at least figure out what terminal stdout is connected to I did the following (inside the setup function):

// ...

char buf[100];
buf[readlink ("/proc/self/fd/1", buf, 100)] = '\0';
printf ("STDOUT (buf): %s\n", buf);

char *tmp = ttyname (1);
printf ("STDOUT (ttyname): %s, %s\n", tmp, strerror (errno));

// ...

This prints:

STDOUT (buf): /dev/pts/1
STDOUT (ttyname): (null), Success

According to the ttyname manpage, that second line of output should be impossible. Am I completely misunderstanding something here?

Answer 1

here is what I ran, created from your code, and it ran perfectly, generating a file: lib.log

The contents of the file: lib.log

Found terminal /dev/pts/1

and now, the code I ran:

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

static char *term = NULL;

static void init (void) __attribute__((constructor));

static void init (void)
{
    FILE *f = fopen ("lib.log", "w");

    if (f)
    {
        if (isatty (STDOUT_FILENO))
        {
            char *tmp = ttyname (STDERR_FILENO);
            if (tmp && *tmp)
            {
                term = strdup (tmp); // nevermind the memory leak
                fprintf (f, "Found terminal %s\n", temp);
            }
        }

        else
            fprintf (f, "Failed to find terminal\n");

      fclose(f);
   }

   else
   {
       perror( "fopen failed" );
   }
}


void *
myMalloc (size_t size)
{
    (void)size;
    return NULL;
}


int main (void)
{
    char *m = myMalloc(1024);
    (void)m;
    return 0;
}

Of course, your posted code is missing a header file. Such a header file should be referenced/included by your main.c file and your libfoo.c file