How to write a file not at the start or tail

Question

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I have a exist file named "Test.txt" which contents a simple string "0123456789".
Look at the code,
1.open the file in the mode "r+" //read and write
2.read 3 bytes from the file
3.at the the 3rd byte position, write 10 bytes
==>Problem:just can write 7 bytes to file, and the file not changed.

C++

#include <stdio.h>

int main(void)
{
	FILE* fp = fopen("Test.txt", "r+");
	char ReadBuff[3] = {0};
	int ReadLength = fread(ReadBuff, 1, 3, fp); //ReadLength=3
	int FpPos = ftell(fp); //FpPos=3
	char WriteBuff[10] = {0};
	char* Str = "abcdefghij"; //10 bytes
	int WriteLength = fwrite(Str, 1, 10, fp);//WriteLength=7
	
	fclose(fp);
	return 0;
}

What I have tried:

After reading from the file, If I call fseek() to reset the file descriptor, the code works well.

Posted 27-Feb-16 3:17am

LubinLewCHN

Updated 27-Feb-16 3:42am

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Patrice T 27-Feb-16 9:29am

What is the question ?

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Jochen Arndt · Accepted Answer · 2016-02-27T03:41:00

See fopen[^]:

Quote:
When a file is opened with update mode ( '+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, the application shall ensure that output is not directly followed by input without an intervening call to fflush() or to a file positioning function ( fseek(), fsetpos(), or rewind()), and input is not directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.