Understanding C Language File Operations – An Array in Essence

There are four main components of C language files: Opening (Creating) and Closing Files、Read and Write Operations、Positioning Operations、Error Checking，this article will explain the first two topics. 🌟

1. Opening and Closing Files

• •<span>fopen</span>
• •Function：Opens a file and returns a pointer to that file.
• •Prototype：<span>FILE *fopen(const char *filename, const char *mode);</span>
• •Parameters：<span>filename</span> is the name of the file to be opened;<span>mode</span> is the mode in which to open the file, described as follows:

Mode	Description
r	Opens an<span>existing text file</span>, allowing<span>reading</span>from the file.
w	Opens a text file, allowing<span>writes</span>to the file.<span>If the file does not exist, a new file will be created</span>. Here, the program will<span>write content from the beginning of the file</span>. If the file exists, the content of the file<span>will be cleared</span> (i.e., the file length is truncated to 0).
a	Opens a text file for<span>appending</span>to the file. If the file does not exist,<span>a new file will be created</span>. Here, the program will append content to the existing file content.
r+	Opens a text file, allowing<span>reading and writing</span>to the file.
w+	Opens a text file, allowing<span>reading and writing</span>to the file. If the file already exists, it will be truncated to zero length; if the file does not exist, a new file will be created.
a+	Opens a text file, allowing<span>reading and writing</span>to the file. If the file does not exist, a new file will be created. Reading starts from the beginning of the file, while writing can only be in append mode.

The main difference between modes with + and without + is whether<span>read and write permissions are available simultaneously</span>

• •<span>fclose</span>
• •Function：Closes an already opened file.
• •Prototype：<span>int fclose(FILE *stream);</span>
• •Parameters：<span>stream</span> is a pointer to the file to be closed.
• •Return Value：Returns <span>0</span> if successful; returns <span>EOF</span> if an error occurs.

2. File Read and Write Operations

Character Read and Write

• •<span>fgetc</span>
• •Function：Reads a character from the file.
• •Prototype：<span>int fgetc(FILE *stream);</span>
• •Return Value：Returns the character read; returns <span>EOF</span> if the end of the file is reached or an error occurs.
• •<span>fputc</span>
• •Function：Writes a character to the file.
• •Prototype：<span>int fputc(int c, FILE *stream);</span>
• •Parameters：<span>c</span> is the character to be written;<span>stream</span> is a pointer to the file.
• •Return Value：Returns the character written if successful; returns <span>EOF</span> if an error occurs.

String Read and Write

• •<span>fgets</span>
• •Function：Reads a line of string from the file.
• •Prototype：<span>char *fgets(char *s, int size, FILE *stream);</span>
• •Parameters：<span>s</span> is the buffer to store the read string;<span>size</span> is the size of the buffer;<span>stream</span> is a pointer to the file.
• •Return Value：Returns a pointer to <span>s</span> if successful; returns <span>NULL</span> if the end of the file is reached or an error occurs.
• •<span>fputs</span>
• •Function：Writes a string to the file.
• •Prototype：<span>int fputs(const char *s, FILE *stream);</span>
• •Parameters：<span>s</span> is the string to be written;<span>stream</span> is a pointer to the file.
• •Return Value：Returns a non-negative value if successful; returns <span>EOF</span> if an error occurs.

Formatted Read and Write

• •<span>fscanf</span>
• •Function：Reads data from the file according to the specified format.
• •Prototype：<span>int fscanf(FILE *stream, const char *format, ...);</span>
• •Parameters：<span>stream</span> is a pointer to the file;<span>format</span> is the format control string; the following are variable arguments used to store the read data.
• •Return Value：Returns the number of input items successfully matched and assigned; returns <span>EOF</span> if the end of the file is reached or an error occurs.
• •<span>fprintf</span>
• •Function：Writes data to the file according to the specified format.
• •Prototype：<span>int fprintf(FILE *stream, const char *format, ...);</span>
• •Parameters：<span>stream</span> is a pointer to the file;<span>format</span> is the format control string; the following are variable arguments containing the data to be written.
• •Return Value：Returns the number of characters actually written; returns a negative value if an error occurs.

Binary Read and Write

• •<span>fread</span>
• •Function：Reads binary data from the file.
• •Prototype：<span>size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);</span>
• •Parameters：<span>ptr</span> is the buffer to store the read data;<span>size</span> is the size of each data item;<span>nmemb</span> is the number of data items to read;<span>stream</span> is a pointer to the file.
• •Return Value：Returns the actual number of data items successfully read.
• •<span>fwrite</span>
• •Function：Writes binary data to the file.
• •Prototype：<span>size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);</span>
• •Parameters：<span>ptr</span> is a pointer to the data to be written;<span>size</span> is the size of each data item;<span>nmemb</span> is the number of data items to write;<span>stream</span> is a pointer to the file.
• •Return Value：Returns the actual number of data items successfully written.

Test Function

#include <stdio.h>#include <stdlib.h>#include <string.h>typedef const char *string;int main(int argc, char *argv[]){    FILE *fp = fopen("/home/eyk/桌面/MCU/1.C基础/3.Unix系统其他特点/5.文件处理/test/12.读写测试.txt", "w+");    if (fp == NULL)    {        perror("fopen");    }    else    {        printf("File opened successfully\n");        printf("File descriptor: %d\n\n", fileno(fp));        printf("Testing character read and write:\n\n");        {            printf("Writing character:\n");            fputc('a', fp);            printf("Wrote 'a', current position: %ld \n", ftell(fp));            fputc('b', fp);            printf("Wrote 'b', current position: %ld \n", ftell(fp));            printf("\nReading character:\n");            printf("Current position: %ld \n", ftell(fp));            rewind(fp);            printf("Reset file pointer to start, current position: %ld\n", ftell(fp));            char c = fgetc(fp);            printf("Read character: %c\n", c);            c = fgetc(fp);            printf("Read character again: %c\n", c);        }        printf("\n\nTesting string read and write:\n\n");        {            printf("Writing string:\n");            fputs("hello world", fp);            printf("Wrote 'hello world', current position: %ld \n", ftell(fp));            printf("\nReading string:\n");            printf("Current position: %ld \n", ftell(fp));            rewind(fp);            printf("Reset file pointer to start, current position: %ld\n", ftell(fp));            char str[128];            fgets(str, 128, fp);            printf("Read string: %s\n", str);        }        printf("\n\nTesting formatted read and write:\n\n");        {            printf("Writing formatted: %d %s %c\n", 1, "hello", 'a');            fprintf(fp, "%d %s %c", 1, "hello", 'a');            printf("Wrote formatted, current position: %ld \n", ftell(fp));            printf("\nReading formatted:\n");            printf("Current position: %ld \n", ftell(fp));            rewind(fp);            printf("Reset file pointer to start, current position: %ld\n", ftell(fp));            char str[128];            while (!feof(fp))            {                /**                 * func descp: You can see that the read and write here are similar to scanf series, using spaces, tabs, and newlines as delimiters.                 */                fscanf(fp, "%s", str);                printf("Read string: %s\n", str);            }        }        printf("\n\nTesting binary read and write:\n\n");        {            string str = "hello world";            printf("Writing binary: %s\n", str);            /**             * func descp:             * fwrite reads sizeof(str) characters from str into the file             * fread reads sizeof(str) characters from the file into str             */            fwrite(str, sizeof(char), strlen(str), fp);            printf("Wrote binary, current position: %ld \n", ftell(fp));            printf("\nReading binary:\n");            printf("Current position: %ld \n", ftell(fp));            int len = ftell(fp);            rewind(fp);            printf("Reset file pointer to start, current position: %ld\n", ftell(fp));            char str2[128];            fread(str2, sizeof(char), len, fp);            str2[len] = '\0';            printf("Read binary: %s\n", str2);        }        fclose(fp);    }}

Test Results

File opened successfullyFile descriptor: 3Testing character read and write:Writing character:Wrote 'a', current position: 1Wrote 'b', current position: 2Reading character:Current position: 2Reset file pointer to start, current position: 0Read character: aRead character again: bTesting string read and write:Writing string:Wrote 'hello world', current position: 13Reading string:Current position: 13Reset file pointer to start, current position: 0Read string: abhello worldTesting formatted read and write:Writing formatted:1 hello aWrote formatted, current position: 22Reading formatted:Current position: 22Reset file pointer to start, current position: 0Read string: abhelloRead string: world1Read string: helloRead string: aTesting binary read and write:Writing binary: hello worldWrote binary, current position: 33Reading binary:Current position: 33Reset file pointer to start, current position: 0Read binary: abhello world1 hello ahello world

File After Testing

abhello world1 hello ahello world