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I’m trying to use the LD_PRELOAD to inject my custom function in a program. I’ve compiled a shared library `getpid.so` having a modified getpid function that prints a message:
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
pid_t getpid(void)
{
printf("Initializing..n");
return syscall(SYS_getpid);
}I’ve also compiled a program testpid.c that calls the regular getpid function:
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main()
{
printf( "pid = %d!n", getpid() );
return 0;
}When I run the command:
LD_PRELOAD=./getpid.so ./testpidI get the following error:
ERROR: ld.so: object 'getpid.so' from LD_PRELOAD cannot be preloaded: ignoredI’ve compiled both files with `gcc -Wall -fPIC -shared -o getpid.so getpid.c` and `gcc testpid -o testpid.c`
Can anyone help me understand why LD_PRELOAD is not working and how to fix the issue?
hi friend @marveeamasi the issue you’re encountering with `LD_PRELOAD` is likely due to the naming and location of the shared library `getpid.so`. You can go through it again to ensure everything is correctly set up, if you need further guidance do well to reach out
Yh thanks @destynin I had to specify the full path to getpid.so in the LD_PRELOAD environment variable
Hi! @marveeamasi One common issue with `LD_PRELOAD` is that the shared library must be compiled with position-independent code (`PIC`). Make sure you compiled `getpid.so` with the `-fPIC flag`. Also, check that `getpid.so` is in the current directory when you run the `LD_PRELOAD` command. Try running `ldd` `getpid.so` to check if there are any missing dependencies.
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