segmentation fault occurs when using fclose()

I am creating a function that when run reads a file and brings another file to the position described in the file "PREVIOUSSTOP" (the file just contains a number) , the number is then used to move the file known as "info" up to the position described in "PREVIOUSSTOP" (if the number in "PREVIOUSSTOP" is x, the info file is moved up x many sentences). When I use fclose() in a certain function I get a Segmentation Fault.
Can someone help fix this, I have no idea on why this occurs as the file is not equal to NULL.
I have tried to use gdb to debug this but that just results in a backtrace with over 600 question marks

#include <stdio.h>
#include <stdlib.h>
int stopnum = 0;

void GetInfoFromFile(FILE* info, char* Sentence1) {
    int i = 0;
    while (1) {
        char c;
        c = fgetc(info);
        Sentence1[i] = c;
        if (Sentence1[i] == '.' || Sentence1[i] == '!' || Sentence1[i] == '?') {
            break;
        }
        i++;
    }
    Sentence1[i + 1] = '
#include <stdio.h>
#include <stdlib.h>
int stopnum = 0;
void GetInfoFromFile(FILE* info, char* Sentence1) {
int i = 0;
while (1) {
char c;
c = fgetc(info);
Sentence1[i] = c;
if (Sentence1[i] == '.' || Sentence1[i] == '!' || Sentence1[i] == '?') {
break;
}
i++;
}
Sentence1[i + 1] = '\0';
}
void RestartPreviousTrainingSession(FILE* info) {
FILE* stop = fopen("PREVIOUSSTOP", "r");
if (stop != NULL) {
char* stopstr = malloc(32);
fgets(stopstr, 32, stop);
int stopint = strtol(stopstr, NULL, 10);
stopnum = stopint;
char dummysentence[400];
for (int i = 0; i != stopint; i++) {
GetInfoFromFile(info, dummysentence);
}
/* segmentation fault occurs here */
fclose(stop);
} else {
printf("OOPS\n");
}
}
int main() {
FILE* f = fopen("wiki.train.tokens", "r");
RestartPreviousTrainingSession(f);
fclose(f);
}
';
}
void RestartPreviousTrainingSession(FILE* info) {
    FILE* stop = fopen("PREVIOUSSTOP", "r");
    if (stop != NULL) {
        char* stopstr = malloc(32);
        fgets(stopstr, 32, stop);
        int stopint = strtol(stopstr, NULL, 10);
        stopnum = stopint;
        char dummysentence[400];
        for (int i = 0; i != stopint; i++) {
            GetInfoFromFile(info, dummysentence);
        }
        /* segmentation fault occurs here */
        fclose(stop);
    } else {
        printf("OOPS\n");
    }
}
int main() {
    FILE* f = fopen("wiki.train.tokens", "r");
    RestartPreviousTrainingSession(f);
    fclose(f);
}

the file known as "wiki.train.tokens" can be found at https://huggingface.co/datasets/wikitext/blob/main/wikitext.py

You did not supply the input files your program relies on, and I didn't feel like search for an example.

1. I concur with @MarkAdler that you are most likely writing more than 400 bytes to dummysentence in GetInfoFromFile(). The way to fix that is add a length parameter.

2. (Not fixed) In the current implementation RestartPreviousTrainingSession() would need to know if partial token was read as not to increment i. That said, I suggest you write a function to skip stopint tokens instead and make it an internal affair. You can then optimize it by reading a block of data (say, 4k), look for the stopint'th token, and use ftell() / fseek() to set the file position indicator.

3. (Not fixed) Avoid global variables whenever possible. Shared state is hard to test.

4. fgetc() returns an int; needed to check if EOF was returned. Reworked logic so the terminating `\0' ends up at the right place for all 3 ways the loop terminates.

5. malloc() may return NULL.

6. strtol() may fail to parse the number in PREVIOUSSTOP. It may also return a value that is outside the range of int you expect.

7. Use symbolic constants instead of magic values.

8. Prefer * next to variable instead of type. FILE* a, b is an error when you expect it to mean FILE *a, *b.

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define TOKEN_LEN 32
int stopnum = 0;
#define MAX_SENTENCE_LEN 400

void GetInfoFromFile(FILE *info, size_t n, char Sentence1[n]) {
	int i = 0;
	for(; (!i || !strchr(".!?", Sentence1[i-1])) && i < n - 1; i++) {
		int c = fgetc(info);
		if(c == EOF)
			break;
		Sentence1[i] = c;
	}
	Sentence1[i] = '
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define TOKEN_LEN 32
int stopnum = 0;
#define MAX_SENTENCE_LEN 400
void GetInfoFromFile(FILE *info, size_t n, char Sentence1[n]) {
int i = 0;
for(; (!i || !strchr(".!?", Sentence1[i-1])) && i < n - 1; i++) {
int c = fgetc(info);
if(c == EOF)
break;
Sentence1[i] = c;
}
Sentence1[i] = '\0';
}
void RestartPreviousTrainingSession(FILE* info) {
FILE *stop = fopen("PREVIOUSSTOP", "r");
if (!stop) {
printf("OOPS\n");
return;
}
char *stopstr = malloc(TOKEN_LEN);
if(!stopstr) {
printf("malloc failed\n");
goto out;
}
fgets(stopstr, TOKEN_LEN, stop);
char *endptr;
long stopint = strtol(stopstr, &endptr, 10);
if(stopint < 0 || stopint > INT_MAX || stopstr == endptr) {
printf("PREVIOUSSTOP data was invalid at %s", stopstr);
goto out;
}
stopnum = stopint;
char dummysentence[MAX_SENTENCE_LEN];
for (int i = 0; i != stopint; i++) {
GetInfoFromFile(info, MAX_SENTENCE_LEN, dummysentence);
}
out:
fclose(stop);
}
int main(void) {
FILE *f = fopen("wiki.train.tokens", "r");
RestartPreviousTrainingSession(f);
fclose(f);
}
';
}

void RestartPreviousTrainingSession(FILE* info) {
	FILE *stop = fopen("PREVIOUSSTOP", "r");
	if (!stop) {
		printf("OOPS\n");
		return;
	}
	char *stopstr = malloc(TOKEN_LEN);
	if(!stopstr) {
		printf("malloc failed\n");
		goto out;
	}
	fgets(stopstr, TOKEN_LEN, stop);
	char *endptr;
	long stopint = strtol(stopstr, &endptr, 10);
	if(stopint < 0 || stopint > INT_MAX || stopstr == endptr) {
		printf("PREVIOUSSTOP data was invalid at %s", stopstr);
		goto out;
	}
	stopnum = stopint;

	char dummysentence[MAX_SENTENCE_LEN];
	for (int i = 0; i != stopint; i++) {
		GetInfoFromFile(info, MAX_SENTENCE_LEN, dummysentence);
	}
out:
	fclose(stop);
}

int main(void) {
	FILE *f = fopen("wiki.train.tokens", "r");
	RestartPreviousTrainingSession(f);
	fclose(f);
}