fopen() / fclose() / freopen()

// Opens a file and returns a FILE pointer.
// Return value: a FILE pointer on success, NULL on failure.
FILE *fopen(const char *filename, const char *mode);

// Closes a file.
// Return value: 0 on success, EOF on error.
int fclose(FILE *stream);

// Associates an existing FILE pointer with a different file (useful for redirection).
// Return value: stream on success, NULL on failure.
FILE *freopen(const char *filename, const char *mode, FILE *stream);

#include <stdio.h>

int main(void) {
    // Create a file in "w" mode and write some text to it.
    FILE *fp = fopen("sample.txt", "w");
    if (fp == NULL) {
        // fopen failed (e.g., invalid path or insufficient permissions).
        perror("fopen");
        return 1;
    }
    fprintf(fp, "Line 1\n");
    fprintf(fp, "Line 2\n");
    fprintf(fp, "Line 3\n");
    fclose(fp); // Always close the file after writing.

    // Open the file just created in "r" mode and read its contents.
    fp = fopen("sample.txt", "r");
    if (fp == NULL) {
        perror("fopen");
        return 1;
    }
    char buf[256];
    while (fgets(buf, sizeof(buf), fp) != NULL) {
        printf("%s", buf); // Print each line to standard output.
    }
    fclose(fp);

    // Append a new line in "a" mode (existing content is preserved).
    fp = fopen("sample.txt", "a");
    if (fp == NULL) {
        perror("fopen");
        return 1;
    }
    fprintf(fp, "Line 4 (appended)\n");
    fclose(fp);
    printf("Append complete.\n");

    return 0;
}

The FILE pointer returned by fopen() is a pointer to a structure that tracks the current read/write position and state of the file. You pass this pointer to file operation functions such as fgets(), fread(), and fseek().

Always check the return value of fopen() for NULL. NULL is returned when the file does not exist or when you do not have the required permissions. Failing to check can cause a null pointer dereference and crash your program.

Always close every opened file with fclose(). Without closing, the write buffer may not be flushed and data may not be saved correctly. In addition, the number of files a process can have open is limited by the system, so leaving files open causes resource leaks.

Use fgets() to read a file line by line, and fread() / fwrite() for reading and writing binary data.