C - File IO

File I/O

Code Properties

• Language: C

Overview

C provides file I/O operations through the <stdio.h> library. Files are accessed via file pointers (FILE *) and can be read or written in text or binary mode.

Code

Opening and Closing Files

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    FILE *file = fopen("example.txt", "r");
    
    if (file == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }
    
    // file operations here...
    
    if (fclose(file) != 0) {
        perror("Error closing file");
        return EXIT_FAILURE;
    }
    
    return EXIT_SUCCESS;
}

Writing Text Files

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    FILE *file = fopen("output.txt", "w");
    if (file == NULL) {
        perror("Error creating file");
        return EXIT_FAILURE;
    }
    
    // write using fprintf
    fprintf(file, "Hello, World!\n");
    fprintf(file, "The answer is %d\n", 42);
    
    // write using fputs
    fputs("Another line\n", file);
    
    // write single character
    fputc('X', file);
    fputc('\n', file);
    
    fclose(file);
    printf("File written successfully\n");
    
    return EXIT_SUCCESS;
}

Reading Text Files

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    FILE *file = fopen("input.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }
    
    char buffer[256];
    
    // read line by line
    printf("Contents:\n");
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        printf("%s", buffer);
    }
    
    fclose(file);
    return EXIT_SUCCESS;
}

Reading Character by Character

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    FILE *file = fopen("input.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }
    
    int ch;
    int char_count = 0;
    int line_count = 0;
    
    while ((ch = fgetc(file)) != EOF) {
        char_count++;
        if (ch == '\n') {
            line_count++;
        }
    }
    
    printf("Characters: %d\n", char_count);
    printf("Lines: %d\n", line_count);
    
    fclose(file);
    return EXIT_SUCCESS;
}

Formatted Reading with fscanf

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    FILE *file = fopen("data.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }
    
    char name[50];
    int age;
    double score;
    
    // read formatted data
    while (fscanf(file, "%49s %d %lf", name, &age, &score) == 3) {
        printf("Name: %s, Age: %d, Score: %.2f\n", name, age, score);
    }
    
    fclose(file);
    return EXIT_SUCCESS;
}

Binary File I/O

#include <stdio.h>
#include <stdlib.h>
 
typedef struct {
    int id;
    char name[50];
    double value;
} Record;
 
int write_records(const char *filename, const Record *records, size_t count) {
    FILE *file = fopen(filename, "wb");
    if (file == NULL) return -1;
    
    size_t written = fwrite(records, sizeof(Record), count, file);
    fclose(file);
    
    return (written == count) ? 0 : -1;
}
 
int read_records(const char *filename, Record *records, size_t max_count, size_t *count) {
    FILE *file = fopen(filename, "rb");
    if (file == NULL) return -1;
    
    *count = fread(records, sizeof(Record), max_count, file);
    fclose(file);
    
    return 0;
}
 
int main(void) {
    Record data[] = {
        {1, "Alpha", 100.5},
        {2, "Beta", 200.75},
        {3, "Gamma", 300.25}
    };
    
    // write
    if (write_records("records.bin", data, 3) != 0) {
        fprintf(stderr, "Error writing records\n");
        return EXIT_FAILURE;
    }
    
    // read
    Record loaded[10];
    size_t count;
    if (read_records("records.bin", loaded, 10, &count) != 0) {
        fprintf(stderr, "Error reading records\n");
        return EXIT_FAILURE;
    }
    
    printf("Read %zu records:\n", count);
    for (size_t i = 0; i < count; i++) {
        printf("  ID: %d, Name: %s, Value: %.2f\n",
               loaded[i].id, loaded[i].name, loaded[i].value);
    }
    
    return EXIT_SUCCESS;
}

File Positioning

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }
    
    // get current position
    long pos = ftell(file);
    printf("Initial position: %ld\n", pos);
    
    // seek to end
    fseek(file, 0, SEEK_END);
    long file_size = ftell(file);
    printf("File size: %ld bytes\n", file_size);
    
    // seek back to beginning
    fseek(file, 0, SEEK_SET);
    // or: rewind(file);
    
    // seek relative to current position
    fseek(file, 10, SEEK_CUR);
    printf("Position after +10: %ld\n", ftell(file));
    
    fclose(file);
    return EXIT_SUCCESS;
}

Appending to Files

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main(void) {
    FILE *file = fopen("log.txt", "a");
    if (file == NULL) {
        perror("Error opening log file");
        return EXIT_FAILURE;
    }
    
    time_t now = time(NULL);
    fprintf(file, "[%s] Application started\n", ctime(&now));
    
    fclose(file);
    return EXIT_SUCCESS;
}

Error Handling

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
 
int main(void) {
    FILE *file = fopen("nonexistent.txt", "r");
    
    if (file == NULL) {
        // method 1: perror
        perror("fopen failed");
        
        // method 2: strerror with errno
        fprintf(stderr, "Error: %s\n", strerror(errno));
        
        // method 3: check specific errors
        if (errno == ENOENT) {
            fprintf(stderr, "File does not exist\n");
        } else if (errno == EACCES) {
            fprintf(stderr, "Permission denied\n");
        }
        
        return EXIT_FAILURE;
    }
    
    // check for read/write errors
    char buffer[100];
    if (fgets(buffer, sizeof(buffer), file) == NULL) {
        if (feof(file)) {
            printf("End of file reached\n");
        } else if (ferror(file)) {
            fprintf(stderr, "Error reading file\n");
        }
    }
    
    fclose(file);
    return EXIT_SUCCESS;
}

Details

File Mode Strings

Mode	Description
"r"	Read text (file must exist)
"w"	Write text (creates/truncates)
"a"	Append text (creates if needed)
"r+"	Read/write text (file must exist)
"w+"	Read/write text (creates/truncates)
"a+"	Read/append text
"rb"	Read binary
"wb"	Write binary
"ab"	Append binary
"rb+" / "r+b"	Read/write binary
"wb+" / "w+b"	Read/write binary (creates/truncates)

Seek Origins

Constant	Description
SEEK_SET	Beginning of file
SEEK_CUR	Current position
SEEK_END	End of file

Standard Streams

Stream	Description	Default
stdin	Standard input	Keyboard
stdout	Standard output	Terminal
stderr	Standard error	Terminal

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Appendix

Note created on 2025-12-31 and last modified on 2025-12-31.

See Also

• C - Hello World and Program Structure
• C - Error Handling
• C - Structs and Unions
• C Code Index
• MOC - Computer Science
• MOC - Development

Backlinks

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(c) No Clocks, LLC | 2025

creation_date: 2025-12-31 modification_date: 2025-12-31 author: Jimmy Briggs jimmy.briggs@noclocks.dev tags:

• Type/Code
• Status/WIP aliases:
• C - File IO publish: true permalink: description: cssclasses:
• code

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C - File IO

Code Properties

• Language:
• Packages:

Overview

Sources:

• Source URL or reference

Description of this code snippet/script/module.

Code

# code goes here

Usage

How to use this code:

# usage example

Notes

Additional notes about the code.

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Appendix

Note created on 2025-12-31 and last modified on 2025-12-31.

See Also

• Code Index

Backlinks

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(c) No Clocks, LLC | 2025