C File I/O

Open a File

You can usefopen( )The function used to create a new file or open an existing file. This call initializes a typeFILEan object of typeFILEthat contains all the necessary information to control the stream. Below is the prototype of this function call:

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

Here,filenameis a string used to name the file; the access modemodeThe value can be one of the following values:

If you are dealing with binary files, use the following access modes to replace the access modes above:

"rb", "wb", "ab", "rb+", "r+b", "wb+", "w+b", "ab+", "a+b"

Close a File

To close a file, use the fclose( ) function. The prototype of the function is as follows:

 int fclose( FILE *fp );

If the file is closed successfully,fclose( )The function returns zero; if an error occurs when closing the file, the function returnsEOFEOF. This function actually flushes the data in the buffer, closes the file, and releases all memory used for the file. EOF is defined in the header filestdio.hthe constant in.

The C standard library provides various functions to read and write files character by character or in the form of fixed-length strings.

Write to a File

The following is the simplest function for writing a character to a stream:

int fputc( int c, FILE *fp );

Functionfputc()Writes the character value of parameter c to the output stream pointed to by fp. If successful, it returns the written character; if an error occurs, it returnsEOFYou can use the following function to write a null-terminated string to a stream:

int fputs( const char *s, FILE *fp );

Functionfputs()the stringswrites to the output stream pointed to by fp. If successful, it returns a non-negative value; if an error occurs, it returnsEOFYou can also useint fprintf(FILE *fp,const char *format, ...)The function writes a string to the file. Try the following example:

Note:Please make sure you have a usabletmpdirectory. If the directory does not exist, you need to create it on your computer first.

/tmpThis is generally the temporary directory on Linux systems. If you are running on Windows, you need to change it to an existing directory in your local environment, for example: C:\tmp、D:\tmpetc.

When the above code is compiled and executed, it creates a new file in the /tmp directorytest.txtand use two different functions to write two lines. Next, let's read this file.

Read from a File

The following is the simplest function to read a single character from a file:

int fgetc( FILE * fp );

fgetc()The function reads a character from the input file pointed to by fp. The return value is the character read; if an error occurs, it returnsEOFThe following function allows you to read a string from a stream:

char *fgets( char *buf, int n, FILE *fp );

Functionfgets()It reads n - 1 characters from the input stream pointed to by fp. It copies the read string to the bufferbuf, and appends anullcharacter to terminate the string.

If this function encounters a newline '\n' or the end-of-file EOF before reading the last character, it returns only the characters read, including the newline. You can also useint fscanf(FILE *fp, const char *format, ...)function to read strings from a file, but it stops reading when it encounters the first space or newline.

When the above code is compiled and executed, it reads the file created in the previous section and produces the following results:

1: This
2: is testing for fprintf...

3: This is testing for fputs...

First,fscanf()The method only readThisbecause it encountered a space afterward. Next, callfgets()to read the remaining part until the end of the line. Finally, callfgets()Completely reads the second line.

Binary I/O functions

The following two functions are used for binary input and output:

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

freadandfwriteare the core functions in the C standard library for reading and writing binary file blocks. They directly operate on memory data blocks, avoiding the overhead of text format conversion, and are especially suitable for batch reading and writing of large files and structured data.

Parameter description

The four parameters have the same meaning in both functions, but the data flow direction is opposite:

The actual number of bytes operated on issize × countthe return value is the number of blocks successfully read or written, not the number of bytes. If the return value is less than count, it may be caused by end-of-file (EOF) or a read/write error, which need to be checked separately.

Binary mode vs. text mode

When using fread/fwrite, you must open the file in binary mode; otherwise, data corruption may occur.

Why binary mode must be used

Text mode on Windows automatically converts newline characters\nConverted to\r\n, which will destroy the integrity of binary data.

On Linux/macOS, text mode and binary mode behave the same, but cross-platform code should explicitly use"b"flag to ensure portability.

Comparison with text functions

Which function to choose depends on the data type and the requirement scenario:

Data read/write flowchart

The following figure shows the data flow between fread and fwrite and the file and memory:

Typical usage scenarios

Reading and writing struct data

This is the most common application scenario of fread/fwrite; you can read or write an entire structure as a single data block at once.

学号: 101, 姓名: Alice, 成绩: 95.5

Structs cannot contain pointer members (such aschar *name), because fwrite only saves the address value of the pointer, not the content of the string it points to. When read back, that address is no longer valid, resulting in a dangling pointer. The correct practice is to use a fixed-length array, such aschar name[50]。

Reading and writing array data

Arrays are stored contiguously in memory, so the entire array can be read or written at once.

已写入 5 个整数到 array.bin
读取到的数据: 1 2 3 4 5

Batch reading and writing large files

For large files, you should use a loop to read/write in chunks, avoiding loading all content into memory at once.

文件复制成功: example_input.bin -> example_output.bin

The key to this mode isfreadReturns the actual number of bytes read, then usefwriteWrite only the number of bytes actually read, avoiding writing extra data at the end.

Key considerations

Return value checking

After calling fread/fwrite, you must check the return value to distinguish normal completion from abnormal conditions.

已到达文件末尾，实际读取 47 个元素

Struct read/write pitfalls

The memory layout of structures is affected by the compiler's alignment strategy, which may cause cross-platform compatibility issues.

Memory alignment inconsistency

With different compilers or different compilation options, the same structure may have different memory layouts.

默认对齐 Mixed 大小: 8 字节
紧凑对齐 PackedMixed 大小: 5 字节

Recommended practiceFor cross-platform or long-term storage, prefer manual serialization (reading/writing member by member) over directly writing the structure to a file. This can completely avoid alignment and byte-order issues.

Pointer members are prohibited

This is one of the most common mistakes made by beginners.

演示完成，但 load 出来的 name 指针是无效的

Buffer flush

C standard library I/O is buffered; critical data needs to be explicitly flushed to ensure persistence.

使用 fflush 强制刷新到磁盘
fclose 也会自动刷新缓冲区

File pointer positioning

Combining fseek and ftell enables random access to a file, reading data at specified positions as needed.

文件大小: 6000 字节
包含 100 条学生记录
第 6 条记录: id=106, name=EXAMPLE

Summary of common errors

The following table summarizes the most common problems and correct practices when using fread/fwrite: