File processing examples of fopen, FREAD, fwrite, fseek

Like any operating system , File processing is Linux Core concepts in . Any system programmer will regard it as him / One of her initial programming tasks is learning . This aspect of programming involves system files .

Through file processing , You can create system files 、 modify 、 Delete and other operations . In this paper , Try to introduce very basic file processing .

File processing function

In this paper , We will introduce the following functions commonly used in file processing ：

fopen()

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

fopen() Function to open a file and put I/O Flow and its association . This function has two arguments . The first parameter is a pointer to the string containing the file name to open , The second parameter is the mode to open the file . The pattern can be ：

• 'r' : Open a text file to read . The stream is at the beginning of the file .
• 'r+' : Turn on read write . The stream is at the beginning of the file .
• 'w' ： Truncate the file to zero length or create a text file for writing . The stream is at the beginning of the file .
• 'w+' : Turn on read write . If the file doesn't exist , Then create the file , Otherwise, cut it off . The stream is at the beginning of the file .
• 'a' ： Open for attachment （ Write... At the end of the file ）. If the file doesn't exist , Then create the file . The stream is at the end of the file .
• 'a+' ： Open for reading and attaching （ Write... At the end of the file ）. If the file doesn't exist , Then create the file . The initial file position read is at the beginning of the file , But the output is always appended to the end of the file .

fopen() The function returns... On success FILE Flow pointer , And when it fails, it returns NULL.

fread() and fwrite()

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

fread/fwrite The function is used from fopen Function to open the file / Write data . These functions take three parameters . The first parameter is for reading / Pointer to the buffer where data is written . Read / The written data adopts “nmemb” Element form , Every “size” Byte length .

If it works ,fread/fwrite Return from fopen Function to actually read from the stream opened / Bytes written . If you fail , Returns a smaller number of bytes （ Then request to read / Write ）.

fseek()

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

fseek() The function is used to set the file location indicator of the stream to the new location . This function takes three arguments . The first parameter is fopen() Function return FILE Flow pointer . The second parameter 'offset' Tell us the number of bytes to find . The third parameter “whence” Tell me where to look “offset” Number of bytes .wherece The available values of are SEEK_SET、SEEK_CUR or SEEK_END. These three values （ According to the order ） Describes the beginning of the file 、 Current location and end of file .

success , This function returns 0, Otherwise return to -1.

fclose()

int fclose(FILE *fp);

fclose() Function first refreshes fopen() Open stream , Then close the underlying descriptor . After successful completion , This function returns 0, Otherwise, it returns to the end of the file (eof). In the event of failure , If you further access the stream , Then the behavior remains undefined .

Code

#include<stdio.h>#include<string.h>#define SIZE 1#define NUMELEM 5int main(void){    FILE* fd = NULL;    char buff[100];    memset(buff,0,sizeof(buff));    fd = fopen("test.txt","rw+");    if(NULL == fd)    {        printf("\n fopen() Error!!!\n");        return 1;    }    printf("\n File opened successfully through fopen()\n");    if(SIZE*NUMELEM != fread(buff,SIZE,NUMELEM,fd))    {        printf("\n fread() failed\n");        return 1;    }    printf("\n Some bytes successfully read through fread()\n");    printf("\n The bytes read are [%s]\n",buff);    if(0 != fseek(fd,11,SEEK_CUR))    {        printf("\n fseek() failed\n");        return 1;    }    printf("\n fseek() successful\n");    if(SIZE*NUMELEM != fwrite(buff,SIZE,strlen(buff),fd))    {        printf("\n fwrite() failed\n");        return 1;    }    printf("\n fwrite() successful, data written to text file\n");    fclose(fd);    printf("\n File stream closed through fclose()\n");    return 0;}

The above code assumes that you have a test file “test.txt” Put it in the same location where the executable is running .

The content of the original document is ：

$ cat test.txthello everybody

Now? , Run code ：

$ ./fileHandling  File opened successfully through fopen() Some bytes successfully read through fread() The bytes read are [hello] fseek() successful fwrite() successful, data written to text file File stream closed through fclose()

Check the file again test.txt The content of . As shown below , The contents of the file have been modified .

$ cat test.txthello everybodyhello