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File Operations in C

File operations follow four steps every time:
  1. Declare a file pointer
  2. Open the file with the appropriate mode
  3. Perform operations (read/write)
  4. Close the file

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1. Writing to a File

#include <stdio.h>

int main() {
    FILE *fp;

    fp = fopen("write.txt", "w");   // open in write mode
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    fprintf(fp, "This is a text file.\n");
    fprintf(fp, "File handling in C.\n");

    fclose(fp);   // always close after use

    printf("Data written successfully!");
    return 0;
}
After running this, a file named write.txt is created in the same directory containing the two lines.

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2. Reading from a File

#include <stdio.h>

int main() {
    FILE *fp;
    char line[100];

    fp = fopen("write.txt", "r");   // open the file we just wrote
    if (fp == NULL) {
        printf("File not found!\n");
        return 1;
    }

    printf("Contents of File:\n");

    while (fgets(line, sizeof(line), fp)) {
        printf("%s", line);
    }

    fclose(fp);
    return 0;
}
Output: 
Contents of File:
This is a text file.
File handling in C.

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3. Appending to a File

#include <stdio.h>

int main() {
    FILE *fp = fopen("write.txt", "a");
    if (fp == NULL) {
        printf("Error!\n");
        return 1;
    }

    fprintf(fp, "Appended line.\n");
    fclose(fp);

    printf("Data appended successfully!");
    return 0;
}

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4. Character-by-Character Read

#include <stdio.h>

int main() {
    FILE *fp = fopen("write.txt", "r");
    if (fp == NULL) return 1;

    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);   // print each character
    }

    fclose(fp);
    return 0;
}
EOF (End Of File) is a constant that signals no more data is available.

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Summary of Modes

NeedMode
Only read an existing file"r"
Create/overwrite a file"w"
Add data to end of file"a"
Read and write existing file"r+"
Read and write (create/clear)"w+"