C File Handling

File I/O in C

C provides file I/O through the <stdio.h> library. Files are accessed through a FILE pointer. Always close files after use with fclose().

File Modes

Mode	Description	File Exists	File Missing
`"r"`	Read text	Opens for reading	Returns NULL
`"w"`	Write text	Truncates to empty	Creates new file
`"a"`	Append text	Appends to end	Creates new file
`"r+"`	Read + Write	Opens for both	Returns NULL
`"w+"`	Write + Read	Truncates to empty	Creates new file
`"a+"`	Append + Read	Appends, reads from start	Creates new file
`"rb"`	Read binary	Opens for binary read	Returns NULL
`"wb"`	Write binary	Truncates	Creates new file

Key File Functions

Function	Description
`fopen(path, mode)`	Opens a file; returns FILE* or NULL on failure
`fclose(fp)`	Closes the file and flushes buffer
`fprintf(fp, fmt, ...)`	Writes formatted text to file
`fscanf(fp, fmt, ...)`	Reads formatted data from file
`fputc(c, fp)`	Writes a single character
`fgetc(fp)`	Reads a single character
`fputs(str, fp)`	Writes a string (no newline added)
`fgets(buf, n, fp)`	Reads a line (up to n-1 chars)
`fwrite(ptr, size, n, fp)`	Writes n binary blocks of given size
`fread(ptr, size, n, fp)`	Reads n binary blocks of given size
`fseek(fp, offset, origin)`	Moves file position pointer
`ftell(fp)`	Returns current file position
`rewind(fp)`	Resets position to beginning
`feof(fp)`	Returns non-zero if end of file reached
`ferror(fp)`	Returns non-zero if an error occurred

Write to File with fprintf, Read with fscanf

#include <stdio.h>

int main() {
    FILE *fp;

    // --- Write to file ---
    fp = fopen("students.txt", "w");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    fprintf(fp, "Alice 20 3.85\n");
    fprintf(fp, "Bob   22 3.20\n");
    fprintf(fp, "Carol 21 3.65\n");
    fclose(fp);
    printf("Data written to students.txt\n");

    // --- Read from file ---
    fp = fopen("students.txt", "r");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char name[50];
    int  age;
    float gpa;

    printf("\nReading from file:\n");
    while (fscanf(fp, "%s %d %f", name, &age, &gpa) == 3) {
        printf("Name: %-10s Age: %d  GPA: %.2f\n", name, age, gpa);
    }
    fclose(fp);

    return 0;
}

Copy File Using fgetc / fputc

#include <stdio.h>

int main() {
    FILE *src, *dest;
    int ch;

    src = fopen("students.txt", "r");
    if (src == NULL) { printf("Cannot open source file\n"); return 1; }

    dest = fopen("students_copy.txt", "w");
    if (dest == NULL) { fclose(src); printf("Cannot create dest file\n"); return 1; }

    // Copy character by character
    while ((ch = fgetc(src)) != EOF) {
        fputc(ch, dest);
    }

    fclose(src);
    fclose(dest);
    printf("File copied successfully!\n");

    // Read back using fgets
    dest = fopen("students_copy.txt", "r");
    char line[100];
    printf("\nContents of copy:\n");
    while (fgets(line, sizeof(line), dest) != NULL) {
        printf("%s", line);
    }
    fclose(dest);

    return 0;
}

Binary File I/O with fwrite / fread

#include <stdio.h>
#include <string.h>

typedef struct {
    char  name[30];
    int   age;
    float salary;
} Employee;

int main() {
    Employee emp[3] = {
        {"Alice", 30, 75000.0f},
        {"Bob",   25, 60000.0f},
        {"Carol", 35, 90000.0f}
    };

    // Write binary data
    FILE *fp = fopen("employees.bin", "wb");
    if (!fp) { printf("Cannot open file\n"); return 1; }
    fwrite(emp, sizeof(Employee), 3, fp);
    fclose(fp);
    printf("Binary data written.\n");

    // Read binary data
    Employee read_emp[3];
    fp = fopen("employees.bin", "rb");
    if (!fp) { printf("Cannot open file\n"); return 1; }
    int count = fread(read_emp, sizeof(Employee), 3, fp);
    fclose(fp);

    printf("\nRead %d records:\n", count);
    for (int i = 0; i < count; i++) {
        printf("%-10s Age: %d  Salary: %.2f\n",
               read_emp[i].name, read_emp[i].age, read_emp[i].salary);
    }

    // fseek and ftell
    fp = fopen("employees.bin", "rb");
    fseek(fp, sizeof(Employee), SEEK_SET);  // skip first record
    Employee second;
    fread(&second, sizeof(Employee), 1, fp);
    printf("\nSecond record: %s\n", second.name);
    printf("File position: %ld bytes\n", ftell(fp));
    fclose(fp);

    return 0;
}

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C File Handling

File I/O in C

File Modes

Key File Functions

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