关于C#：fscanf while循环永远不会运行

fscanf while-loop never runs

我正在一个项目上，我似乎无法弄清楚为什么我查找素数的功能无法运行。本质上，我想编写代码以首先检查文本文件日志中是否有以前遇到的质数，但是无论我为包含fscanf()的while循环添加什么内容，似乎我的代码都从未输入过。

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int filePrime(int a) {
int hold = 0;
FILE *fp = fopen("primes.txt","a+");

if (fp == NULL) {
printf("Error while opening file.");
exit(2);
}

/*
the while loop below this block is the one with the issue.
on first run, it should skip this loop entirely, and proceed
to finding prime numbers the old-fashioned way, while populating the file.
instead, it is skipping this loop and proceeding right into generating a
new set of prime numbers and writing them to the file, even if the previous
numbers are already in the file
*/

while (fscanf(fp,"%d", &hold) == 1){
printf("Inside scan loop.");
if (hold >= a) {
fclose(fp);
return 1;
}
if (a % hold == 0) {
fclose(fp);
return 0;
}
}

printf("Between scan and print.
");

for (; hold <= a; hold++) {
if (isPrime(hold) == 1) {
printf("Printing %d to file
", hold);
fprintf(fp,"%d
", hold);
if (hold == a)
return 1;
}
}

fclose(fp);
return 0;
}

我已经尝试了while循环测试的各种更改。
例如！= 0，！= EOF，完全消除== 1。

我只是似乎无法使我的代码使用fscanf进入循环。

非常感谢您的帮助，非常感谢您的宝贵时间。