Detailed Explanation of File Operations in C Language

In C language, file operations are very important functions, mainly implemented through the functions provided by the standard library’sstdio.h header file. Below is a detailed introduction to file read and write operations in C language:

1. File Pointer

All file operations need to be performed through thefile pointer, defined as follows:

FILE *fp;  // Declare a file pointer

The file pointer is used to point to the relevant information of the file (such as file name, status, current position, etc.).

2. Opening a File (fopen function)

Use thefopen() function to open a file, with the prototype:

FILE *fopen(const char *filename, const char *mode);

filenameFile name (can include path)

mode: Open mode, commonly used modes:

"r": Read-only, the file must exist

 "w": Write-only, creates the file if it does not exist, clears it if it exists "a": Append, adds content at the end of the file "r+": Read and write, the file must exist "w+": Read and write, creates the file if it does not exist, clears it if it exists "a+": Read and write, adds content at the end of the fileReturn value: Returns the file pointer on success, returnsNULL on failure.Demo:

FILE *fp = fopen("test.txt", "r");if (fp == NULL) {    printf("Cannot open file\n");    return 1;}

3. Closing a File (fclose function)

After the operation is completed, the file must be closed, with the prototype:

int fclose(FILE *stream);

Return value: Returns 0 on success, non-zero on failure.Demo:

fclose(fp);

4. File Read and Write Operations

1. Character Read and Write

fgetc(): Reads a character from the file

int fgetc(FILE *stream);  // Returns character ASCII code on success, EOF on failure or end of file

fputc(): Writes a character to the file

int fputc(int char, FILE *stream);  // Returns the written character on success, EOF on failure

2. String Read and Write

fgets(): Reads a string from the file

char *fgets(char *str, int n, FILE *stream);  // Reads up to n-1 characters, automatically adds '\0'

fputs(): Writes a string to the file

int fputs(const char *str, FILE *stream);  // Returns a non-negative integer on success, EOF on failure

3. Formatted Read and Write

fscanf(): Reads formatted data from the file

int fscanf(FILE *stream, const char *format, ...);

fprintf(): Writes formatted data to the file

int fprintf(FILE *stream, const char *format, ...);

4. Block Read and Write (commonly used for binary files)

fread(): Reads a block of data from the file

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

fwrite(): Writes a block of data to the file

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

5. File Positioning

fseek(): Moves the file pointer

int fseek(FILE *stream, long int offset, int whence);

whenceReference position

SEEK_SETBeginning of the file
SEEK_CURCurrent position
SEEK_ENDEnd of the file

ftell(): Gets the current file pointer position

long int ftell(FILE *stream);

rewind(): Moves the file pointer to the beginning

void rewind(FILE *stream);

6. Example Code

1. Example of Writing to a File

#include <stdio.h>
int main() {    //FILE* fp = fopen("output.txt", "w");    FILE* fp;    fopen_s(&fp,"output.txt", "w");    if (fp == NULL) {        printf("Cannot open file\n");        return 1;    }
    // Write character    fputc('A', fp);
    // Write string    fputs("Hello World\n", fp);
    // Formatted write    int num = 100;    fprintf(fp, "Number: %d\n", num);
    fclose(fp);    return 0;}

2. Example of Reading from a File

#include <stdlib.h>#include <stdio.h>
int main() {    //FILE* fp = fopen("output.txt", "r");    FILE* fp;    fopen_s(&fp,"output.txt", "r");    if (fp == NULL) {        printf("Cannot open file\n");        return 1;    }
    char buffer[100];
    // Read character    char c = fgetc(fp);    printf("Read character: %c\n", c);
    // Read string    fgets(buffer, 100, fp);    printf("Read string: %s", buffer);
    // Formatted read    int num;    //fscanf(fp, "Number: %d", &num);    fscanf_s(fp, "Number: %d", &num);    printf("Read number: %d\n", num);
    fclose(fp);    return 0;}

7. Precautions

Check if the file is successfully opened before operating on it
Must close the file after the operation is completed, otherwise it may lead to data loss
The operation methods for text files and binary files are different, binary files usefread() andfwrite() for more efficiency
Usingfeof() function can determine if the end of the file has been reached

8. Unsafe Usage Tips

Comparison of fopen_s and fopen usage

    FILE* fp = fopen("output.txt", "r");
    FILE* fp;    fopen_s(&fp,"output.txt", "r");

Comparison of fscanf_s and fscanf usage

    int num;    fscanf(fp, "Number: %d", &num);    fscanf_s(fp, "Number: %d", &num);