how to enter a function to a file in c

printf() prints to stdout. You need to fopen() that file and then use fprintf() with the returned from fopen() FILE* pointer as the first argument.

/* Open the file for writing */
FILE* fp = fopen("filename.txt", "w");
/* Check for errors */
if (fp == NULL)
    /* Notify the user of the respective error and exit */
    fprintf(stderr, "%s\n", strerror(errno));
/* Write to the file */
fprintf(fp, "Hello!\n");
/* Close the file */

Note: Your question was quite unclear and this answer is based on what I could understand out of it.

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