### Install Hook Function for Library Call

Source: https://github.com/kubo/plthook/blob/master/README.md

Use this code to intercept a function call (e.g., 'recv') within a specified shared library ('libfoo.so.1'). Ensure 'plthook.h' and the appropriate C file are included in your project. This method is suitable when the hook function is not part of the library being modified.

```c
#include "plthook.h"

/* This function is called instead of recv() called by libfoo.so.1  */
static ssize_t my_recv(int sockfd, void *buf, size_t len, int flags)
{
    ssize_t rv;
    
    ... do your task: logging, etc. ...
    rv = recv(sockfd, buf, len, flags); /* call real recv(). */
    ... do your task: logging, check received data, etc. ...
    return rv;
}
    
int install_hook_function()
{
    plthook_t *plthook;
    
    if (plthook_open(&plthook, "libfoo.so.1") != 0) {
        printf("plthook_open error: %s\n", plthook_error());
        return -1;
    }
    if (plthook_replace(plthook, "recv", (void*)my_recv, NULL) != 0) {
        printf("plthook_replace error: %s\n", plthook_error());
        plthook_close(plthook);
        return -1;
    }
    plthook_close(plthook);
    return 0;
}
```

--------------------------------

### Install Hook Function When Hook is Inside Modified File

Source: https://github.com/kubo/plthook/blob/master/README.md

This code handles the scenario where the hook function resides within the same file being modified by PLTHook. It uses `plthook_open_by_address` and passes the address of the original function pointer to `plthook_replace`. Platform-specific handling for obtaining the original function address (e.g., using `dlsym` on Unix) is included.

```c
static ssize_t (*recv_func)(int sockfd, void *buf, size_t len, int flags);

/* This function is called instead of recv() called by libfoo.so.1  */
static ssize_t my_recv(int sockfd, void *buf, size_t len, int flags)
{
    ssize_t rv;
    
    ... do your task: logging, etc. ...
    rv = (*recv_func)(sockfd, buf, len, flags); /* call real recv(). */
    ... do your task: logging, check received data, etc. ...
    return rv;
}
    
int install_hook_function()
{
    plthook_t *plthook;
    
    if (plthook_open_by_address(&plthook, &recv_func) != 0) {
        printf("plthook_open error: %s\n", plthook_error());
        return -1;
    }
    if (plthook_replace(plthook, "recv", (void*)my_recv, (void**)&recv_func) != 0) {
        printf("plthook_replace error: %s\n", plthook_error());
        plthook_close(plthook);
        return -1;
    }
#ifndef WIN32
    // The address passed to the fourth argument of plthook_replace() is
    // available on Windows. But not on Unixes. Get the real address by dlsym().
    recv_func = (ssize_t (*)(int, void *, size_t, int))dlsym(RTLD_DEFAULT, "recv");
#endif
    plthook_close(plthook);
    return 0;
}
```

--------------------------------

### Enumerate PLT/IAT Entries in C

Source: https://github.com/kubo/plthook/blob/master/README.md

Use this function to iterate through and print information about PLT/IAT entries within a specified executable file. Ensure the plthook_t pointer is initialized to NULL and the position counter is set to zero before calling.

```c
void print_plt_entries(const char *filename)
{
    plthook_t *plthook;
    unsigned int pos = 0; /* This must be initialized with zero. */
    const char *name;
    void **addr;

    if (plthook_open(&plthook, filename) != 0) {
        printf("plthook_open error: %s\n", plthook_error());
        return -1;
    }
    while (plthook_enum(plthook, &pos, &name, &addr) == 0) {
        printf("%p(%p) %s\n", addr, *addr, name);
    }
    plthook_close(plthook);
    return 0;
}
```

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