fopen() in c : segmentation fault - c

Question

I'm trying to wrap the C library functions malloc and free to detect whether there is memory leakage in my code. I extend the malloc/free functions by adding to them an fprintf to write to a file the address of the malloc/free and the size.
Compiling this code with gcc or clang gives a segmentation fault in the fopen() line.
Here is the command:
gcc -o mainapp main.c -Wall -Wextra
I placed the fopen inside the malloc and free function but also get the same issue: Segmentation fault (core dumped)
I can't find an explanation for the issue.
Here is my complete code:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#define __USE_GNU
#include <dlfcn.h>
#define TEST_MEM_LEAK 1 // a value of 1 means to join the memory leak detection, and a value of 0 means not to join
#if TEST_MEM_LEAK
typedef void *(*malloc_t)(size_t size);
malloc_t malloc_f = NULL;
typedef void (*free_t)(void *p);
free_t free_f = NULL;
int malloc_flag = 1; // It is used to prevent repeated recursion and cannot exit because the printf function will call malloc for memory allocation
int free_flag = 1;
const char* logFileName = "/home/hammamiw/Documents/HeapMonitor/allocs.log";
FILE* fp = NULL;
void initCheck()
{
fp = fopen("/home/hammamiw/Documents/HeapMonitor/allocs.log", "w");
}
void *malloc(size_t size)
{
if(malloc_flag) {
initCheck();
malloc_flag = 0; // Used to prevent printf from causing an error when calling malloc recursively
void *p = malloc_f(size);
fprintf(fp, "malloc, %lx, %lu\n", (uintptr_t)p, size);
//printf("m\n");
malloc_flag = 1; // It is used to ensure that the initial value of flag flag is consistent when malloc in this file is called again
return p;
}
else {
return malloc_f(size); // Here, the malloc function in the system library obtained by dlsym is called
}
}
void free(void *p)
{
initCheck();
if(free_flag) {
//initCheck();
free_flag = 0;
fprintf(fp, "F, %lx\n", (uintptr_t)p);
//printf("f\n");
free_f(p);
free_flag = 1;
} else {
free_f(p);
}
}
#endif
int main()
{
#if TEST_MEM_LEAK // the part from if to endif can be divided into function calls
malloc_f = dlsym(RTLD_NEXT, "malloc");
if(!malloc_f) {
printf("load malloc failed: %s\n", dlerror());
return 1;
}
free_f = dlsym(RTLD_NEXT, "free");
if(!free_f) {
printf("load free failed: %s\n", dlerror());
return 1;
}
#endif
void *p1 = malloc(10); //The malloc function in this article will be called first
void *p2 = malloc(20);
//Here, p2 is not released and there is a memory leak. Judge by checking whether the number of malloc and free times printed is the same
free(p2);
free(p1);
return 0;
}
NOTE: the code works well if I use printf instead of fprintf -> prints "f" and "m" at each free and malloc call.
Environment: Ubuntu 22.04, C language, GCC compiler version 11.3.0

Answer 1

You need to set malloc_flag before calling fopen(). Then you will protect against recursion not only via fprintf() but also via fopen().