Comprehensive Guide to C Language File Operations

A complete explanation of the basic knowledge of C language files — “Create, Flush, Seek, Read, Write”, recommended for collection.

Overview of this section
1. Introduction to the essence of C files
2. Overview of core APIs
3. File creation API
4. Disk flush API
5. File pointer API
6. File read API
7. File write API

0. C File Knowledge Points

If you thoroughly understand the above 5 knowledge points, you will have completely mastered C files!

1. Essence of C Files

A file is essentially a binary storage area on the disk. When we operate on a file, we first read the file from the disk into the high-speed buffer of physical memory, and then the CPU reads and writes data in that area according to the actual commands, followed by triggering the write disk command to flush the data to the disk.

2. Overview of Core APIs

Function
1. Create File	fopen
2. Flush Disk	fflush
3. Close File	fclose
4. File Pointer	ftell	fseek	rewind
5. Write File	fwrite	fputc	fputs	fprintf
6. Read File	fread	fgetc	fgets	fscanf

3. File Creation

1. fopen

API Name	fopen
Signature	FILE *fopen(const char *filename, const char *mode);
Signature Explanation	Open the file at the specified path filename in the specified mode (mode) and return a pointer to this file.
Notes	The value of mode determines how the file is opened, detailed as follows:Detailed explanation of modes for C language file operations.

4. Flush Disk

1. Explicit Flush — fflush

API Name	fflush
Function Prototype	int fflush(FILE *stream);
Function Description	Force the data in the buffer to be written to the corresponding file or stream. For output streams, it immediately writes the data in the buffer to the file; for input streams, its behavior depends on the specific implementation (standard C does not specify, but it is usually used to clear the input buffer).
Return Value	– Returns <span>0</span> on success. – Returns <span>EOF</span> on error, and sets <span>errno</span>.
Parameter Explanation	<span>stream</span>: A pointer to the <span>FILE</span> object representing the stream to be flushed. If it is <span>NULL</span>, it flushes all open output streams.
Note 1	1. File operations in C rely on buffers; without a flush operation, data cannot be persisted to the disk.
Note 2	2. Process termination, modification of file pointers, etc., often implicitly trigger fflush, which is why data appears on the disk even when fflush seems not to have been called. For details, see implicit flush:

2. Implicit Flush

For specific implicit triggers, please click here:Summary of behaviors that trigger flush to disk in C language.

5. File Pointer

Since files are mediated by high-speed buffers, the file pointer points to this memory area. Its core APIs are as follows:

Overview of Subsections

Operation	Core API
1.Return Internal Offset of File	ftell
2.Specify File Offset	fseek
3.Reset File Pointer to Start	rewind

1. ftell

Function: Returns the internal offset of the file (file internal pointer)

Signature	long ftell(FILE *_File)
Return Value	Returns the offset of the internal pointer from the beginning of the file.
Explanation	Note that the value of FILE* fp remains unchanged, only the internal offset of the file pointer changes, similar to the relationship between an array address and its offset.

2. fseek

Function: <span>Move the internal file pointer</span>, moving to the position reached by moving a specified offset from a specified starting position.

Difference between Internal Pointer and File Pointer:

fseek() only modifies the internal position pointer (offset) of the file, and does not change the value of FILE fp itself (i.e., the memory address pointing to the FILE structure). Therefore, the fp address output by two printf statements remains the same.

Signature	int fseek(FILE *stream, long offset, int whence);
Signature Explanation	offset is the offset; whence is the starting position, which can be SEEK_SET (beginning of file), SEEK_CUR (current position), or SEEK_END (end of file).
Return Value	Returns 0 on success; returns a non-zero value on error.
Notes	Automatic Movement: Note that any read or write operation will move the file pointer to the next unread character. This facilitates subsequent reading.

3. rewind

Function: Resets the internal pointer to the beginning of the file.

Signature	void rewind(FILE *stream);
Signature Explanation	Resets the file pointer to the beginning of the file and clears the end-of-file and error flags.
Return Value	No return value (void type).
Notes	1. Equivalent to fseek(stream, 0, SEEK_SET); but also clears <span>feof</span> and <span>ferror</span> states.2. After execution, the file pointer points to the start of the file, and subsequent read and write operations start from the beginning.3. Commonly used in scenarios where the entire file content needs to be re-read, such as when looping through file data.

#include <stdio.h>

int main(void)
{
    FILE *fp = fopen("C:\\Users\\29001\\Desktop\\file.txt", "w+");
    printf("%p,%d\n", fp/*Address of the file structure itself*/, ftell(fp)/*Get the offset of the internal file pointer*/);
    fseek(fp, 1000, SEEK_SET);//Move the internal file pointer
    printf("%p,%d\n", fp, ftell(fp));
    rewind(fp);//Reset
    printf("%p,%d\n", fp, ftell(fp));
    return 0;
}

Output

00007FFA0CC78A90,0
00007FFA0CC78A90,1000
00007FFA0CC78A90,0

6. File Write API

Classification of C language file read and write