Spilting a large text file by searching a key word.

I have working code that will split the text file in half also code that I can split at a certain amount of lines. However, this is not what I need. Is there a way to search for a keyword every time the program sees this word it writes a file? I want to split the file every time the text file says CHAPTER.
Here is the code I have for splitting the code in half.

Code:

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    #include <stdio.h>
      #include <string.h>
      #define MAX 256
   
      int main() {
            int i = 0, line = 0;
            FILE *fp1, *fp2, *fp3;
            char file1[MAX], file2[MAX], file3[MAX], str[MAX];
   
            /* get the name without file format(.txt) from user */
            printf("Enter your input file name:");
            scanf("%s", file1);
   
            /* form the output file names */
            sprintf(file2, "%s_part_%d.txt", file1, 1);
            sprintf(file3, "%s_part_%d.txt", file1, 2);
            strcat(file1, ".txt");
   
            /* open the input file in read mode */
            fp1 = fopen(file1, "r");
   
            /* error handling */
            if (!fp1) {
                    printf("Unable to open input file in read mode!!\n");
                    return 0;
            }
   
            /* open the output file in write mode */
            fp2 = fopen(file2, "w");
   
            /* error handling */
            if (!fp2) {
                    printf("Unable to open output file in write mode\n");
                    fclose(fp1);
                    return 0;
            }
   
            /* open the output file in write mode */
            fp3 = fopen(file3, "w");
   
            /* error handling */
            if (!fp3) {
                    printf("Unable to open output file2 in write mode\n");
                    fclose(fp1);
                    fclose(fp2);
                    return 0;
            }
   
            /* calculate the number of lines in input file */
            while (!feof(fp1)) {
                    fgets(str, MAX, fp1);
                    if (!feof(fp1))
                            line++;
            }
   
            /* moving the file pointer to the start of file */
            rewind(fp1);
   
            /* writing first half of data to first output file */
            while (i < line/2) {
                    fgets(str, MAX, fp1);
                    if (!feof(fp1)) {
                            fputs(str, fp2);
                    }
                    i++;
            }
   
            /* writing second half to second output file */
            while (!feof(fp1)) {
                    fgets(str, MAX, fp1);
                    if (!feof(fp1)) {
                            fputs(str, fp3);
                    }
            }
   
            /* close all opened files */
            fclose(fp1);
            fclose(fp2);
            fclose(fp3);
   
            /* remove the input file */
            remove(file1);
            return 0;
      }

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I just found out about the sscanf(). I will edit this post once I play with it more.

Break the problem down.

1. You need to find the word Chapter.
2. You need to find the file ending position for Chapter (either another chapter or eof).
3. You need to copy the contents between these two markers.

Code:

---

        /* calculate the number of lines in input file */
        while (!feof(fp1)) {
                fgets(str, MAX, fp1);
                if (!feof(fp1))
                        line++;
        }

---

This can be simplified (and actually made safer) by doing

Code:

---

while ( fgets(str, MAX, fp1) != NULL ) {
  line++;
}
// when the loop exits, one of feof() or ferror() is true

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