problems about insert some data to a file - c

Question

I write a function to insert some data to a file. And it works fine when i only use it once. but when i wrote a loop to test this function, the program crashed. how can i fix this?(sorry for my terrible English, i promise i will try to improve it^_^)
here is the code(written in C)
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
int finsert(char *insert,int size,long offset,FILE *fp,int memory);
int main()
{
FILE *fp;
FILE *record;
char insert[100]="2";
long start,end;
int i=0;
if((fp=fopen("test","rb+"))==NULL)
{
printf("error!\n");
exit(1);
}
if((record=fopen("record.txt","w+"))==NULL)
{
printf("error!\n");
exit(1);
}
for(i=1;i<=10000;i+=1)
{
start=clock();
finsert(insert,strlen(insert),0,fp,i);
end=clock();
fprintf(record, "%d,%ld\n",i,end-start);
printf("\ri=%d ",i);
}
fclose(fp);
fclose(record);
return 0;
}
/*
this is the function and int memory is how many data to write to the file everytime
*/
int finsert(char *insert,int size,long offset,FILE *fp,int memory)
{
long i;
long getsize=size;
long writesize=size;
char *read;
char *write;
char *fill;
char *read_keep;
char *write_keep;
char *fill_keep;
long offset_read,offset_write;
if(size>=memory)
{
read=(char *) malloc(size);
write=(char *) malloc(size);
read_keep=read;
write_keep=write;
strcpy(write,insert);
while(getsize==size)
{
fseek(fp,offset,SEEK_SET);
getsize=fread(read,1,size,fp);
fseek(fp,offset,SEEK_SET);
fwrite(write,1,writesize,fp);
offset+=getsize;
strcpy(write,read);
writesize=getsize;
}
fwrite(write,1,writesize,fp);
free(read_keep);
free(write_keep);
}
else
{
writesize=memory;
getsize=memory;
offset_write=offset;
offset_read=offset-size+memory;
read=(char *) malloc(memory+3);
write=(char *) malloc(memory+3);
fill=(char *) malloc(memory-size);
for(i=0;i<memory+3;i+=1)
{
read[i]=0;
write[i]=0;
}
if(read==NULL||write==NULL||fill==NULL)
{
printf("malloc error!\n");
exit(1);
}
strcpy(write,insert);
fseek(fp,offset,SEEK_SET);
fread(fill,1,memory-size,fp);
strcpy(write+size,fill);
while(getsize==memory)
{
fseek(fp,offset_read,SEEK_SET);
getsize=fread(read,1,memory,fp);
fseek(fp,offset_write,SEEK_SET);
fwrite(write,1,writesize,fp);
offset_read+=getsize;
offset_write+=getsize;
if(memory+1<strlen(read)) printf("copy error!\n");
strcpy(write,read);
writesize=getsize;
}
fwrite(write,1,writesize,fp);
free(read);
free(write);
free(fill);
}
}

Answer 1

maybe you could describe a bit more what your program is intended to do.
When I run it in valgrind, it reported quite a lot invalid memory reads and writes...
The fact is that it runs fine once, because after one run program is not able to destroy its memory so much, however if you run it 10000 time, damage is severe (causing SEGFAULT).
Most of memory overwrites happens here:
strcpy(write+size,fill);
and here:
strcpy(write,insert);
strcpy(write,read);
caused by:
read=(char *) malloc(size);
write=(char *) malloc(size);
I was not able to determine why it happens in first case, but in second case it looks to be obvious that parameter size will always be 1 byte and you need to least 2 bytes for copy string, as ti must be terminated by '\0'.
so might to change the code to something like:
read=(char *) malloc(size + 1);
write=(char *) malloc(size + 1);
And for following code:
for(i=0;i<memory+3;i+=1)
{
read[i]=0;
write[i]=0;
}
You might have a look on memset().
P.S
I was able to make it run removing last free(read); but this is hardly correct solution ...