fopen 在 gdb 中返回 NULL

我试图从picoCTF解决二进制开发问题，但我在gdb上遇到了问题。 这是问题的源代码(我已经评论了一些东西来帮助我(。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <wchar.h>
#include <locale.h>
#define BUF_SIZE 32
#define FLAG_LEN 64
#define KEY_LEN 4
void display_flag() {
char buf[FLAG_LEN];
FILE *f = fopen("flag.txt","r");
if (f == NULL) {
printf("'flag.txt' missing in the current directory!n");
exit(0);
}
fgets(buf,FLAG_LEN,f);
puts(buf);
fflush(stdout);
}
// loads value into key, global variables ie not on stack
char key[KEY_LEN];
void read_canary() { 
FILE *f = fopen("/problems/canary_3_257a2a2061c96a7fb8326dbbc04d0328/canary.txt","r");
if (f == NULL) {
printf("[ERROR]: Trying to Read Canaryn");
exit(0);
}
fread(key,sizeof(char),KEY_LEN,f);
fclose(f);
}
void vuln(){
char canary[KEY_LEN];
char buf[BUF_SIZE];
char user_len[BUF_SIZE];
int count;
int x = 0;
memcpy(canary,key,KEY_LEN); // copies "key" to canary, an array on the stack
printf("Please enter the length of the entry:n> ");
while (x<BUF_SIZE) {
read(0,user_len+x,1);
if (user_len[x]=='n') break;
x++;
}
sscanf(user_len,"%d",&count); // gives count the value of the len of user_len
printf("Input> ");
read(0,buf,count); // reads count bytes to buf from stdin
// compares canary (variable on stack) to key
// if overwriting need to get the value of key and maintain it, i assume its constant
if (memcmp(canary,key,KEY_LEN)) { 
printf("*** Stack Smashing Detected *** : Canary Value Corrupt!n");
exit(-1);
}
printf("Ok... Now Where's the Flag?n");
fflush(stdout);
}
int main(int argc, char **argv){
setvbuf(stdout, NULL, _IONBF, 0);

int i;
gid_t gid = getegid();
setresgid(gid, gid, gid);
read_canary();
vuln();
return 0;
}

当我正常运行时，使用./vuln，我得到正常的执行。但是当我用gdb ./vuln在gdb中打开它，然后用run运行它时，我收到[ERROR]: Trying to Read Canary消息。这是为了使问题具有挑战性吗？我不想要解决方案，我只是不知道这是预期行为还是错误。谢谢