Binary files in C

Binary files are very similar to arrays of records, except the records are in a disk file rather than in an array in memory. Because the records in a binary file are on disk, you can create very large collections of them (limited only by your available disk space). They are also permanent and always available. The only disadvantage is the slowness that comes from disk access time.

Binary files have two features that distinguish them from text files: You can jump instantly to any record in the file, which provides random access as in an array; and you can change the contents of a record anywhere in the file at any time. Binary files also usually have faster read and write times than text files, because a binary image of the record is stored directly from memory to disk (or vice versa). In a text file, everything has to be converted back and forth to text, and this takes time.

#include

/* random record description - could be anything */
struct rec
{
int x,y,z;
};

/* writes and then reads 10 arbitrary records from the file "junk". */
void main()
{
int i,j;
FILE *f;
struct rec r;

/* create the file of 10 records */
f=fopen("junk","w");
for (i=1;i<=10; i++)
{
r.x=i;
fwrite(&r,sizeof(struct rec),1,f);
}
fclose(f);

/* read the 10 records */
f=fopen("junk","r");
for (i=1;i<=10; i++)
{
fread(&r,sizeof(struct rec),1,f);
printf("%d\n",r.x);
}
fclose(f);
printf("\n");

/* use fseek to read the 10 records in reverse order */
f=fopen("junk","r");
for (i=9; i>=0; i--)
{
fseek(f,sizeof(struct rec)*i,SEEK_SET);
fread(&r,sizeof(struct rec),1,f);
printf("%d\n",r.x);
}
fclose(f);
printf("\n");

/* use fseek to read every other record */
f=fopen("junk","r");
fseek(f,0,SEEK_SET);
for (i=0;i<5;>
{
fread(&r,sizeof(struct rec),1,f);
printf("%d\n",r.x);
fseek(f,sizeof(struct rec),SEEK_CUR);
}
fclose(f);
printf("\n");

/* use fseek to read 4th record, change it, and write it back */
f=fopen("junk","r+");
fseek(f,sizeof(struct rec)*3,SEEK_SET);
fread(&r,sizeof(struct rec),1,f);
r.x=100;
fseek(f,sizeof(struct rec)*3,SEEK_SET);
fwrite(&r,sizeof(struct rec),1,f);
fclose(f);
printf("\n");

/* read the 10 records to insure 4th record was changed */
f=fopen("junk","r");
for (i=1;i<=10; i++)
{
fread(&r,sizeof(struct rec),1,f);
printf("%d\n",r.x);
}
fclose(f);
}

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