not able to read data from file using fread - c

Question

unsigned long int nextOffset, currOffset, len;
nextOffset = read offset from file (eg. 15)
currOffset = read prev offset from file (eg. 0 )
len = nextOffset-currOffset;
str = malloc((int)len);
fread(str,(int)(len)-1,1,dataFile);
str[(int)len]='\0';
rowAddr = ftell(tempDataFile);
fwrite(&rowAddr,sizeof(unsigned long int),1,tempOffsetFile);
fwrite(str,(int)(len)-1,1,tempDataFile);
free(str);
for some reason i'm not able to read from datafile using fread.. i debugged it and what i found was that the striing str is showing random data.. when i did this strlen(str) it shows 1709936.....
what is possibly wrong with this code.. all these files are opeend in binary mode...

Answer 1

What Ptival said.
But the most eggregious problem is that if you allocate n bytes, they are numbered from 0 to n-1. You're setting byte n to zero, and that byte is beyond the end of what you've malloc()ed. So you're probably unintentionally stomping on some other data. C won't keep you from shooting yourself in the foot this way.
Otherwise, based on what you said you needed, there doesn't seem to be much wrong with your code. I fleshed it out a bit, and wrapped it all in a little shell script for easy running. The script:
#!/bin/sh
# pgm 1 generates a binary file.
# pgm 2 dumps a file.
# pgm 3 demos a solution to your problem.
rm -f 1 2 3; \
cat > 1.c <<EOD; cat > 2.c <<EOD; cat > 3.c <<EOD; \
gcc -Wall -Werror 1.c -o 1; \
gcc -Wall -Werror 2.c -o 2; \
gcc -Wall -Werror 3.c -o 3; \
./1; ./2 dataFile.dat; ./3; \
./2 tempDataFile.dat; ./2 tempOffsetFile.dat
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char**argv)
{
unsigned int jndex;
unsigned char buffer[4];
FILE *phyle;
phyle=fopen("dataFile.dat","w+");
if(!phyle)
{
fprintf(stderr,"%s: fopen() fail\n",argv[0]);
exit(1);
}
for(jndex='A';
jndex<='Z';
jndex++
)
{
buffer[0]=jndex;
if(!fwrite(buffer,sizeof(char),1,phyle))
{
fprintf(stderr,"%s: fwrite() fail\n",argv[0]);
}
}
fclose(phyle);
printf("%s complete\n",argv[0]);
return 0;
}
EOD
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char**argv)
{
int jndex;
unsigned char buffer[4];
FILE *phyle;
if(argc!=2)
{
fprintf(stderr,"%s: arg error\n",argv[0]);
exit(1);
}
phyle=fopen(argv[1],"r");
if(!phyle)
{
fprintf(stderr,"%s: fopen fail\n",argv[0]);
exit(1);
}
for(jndex=0;
;
jndex++
)
{
if(!fread(buffer,sizeof(char),1,phyle))
{
break;
}
printf("%02X",buffer[0]);
if(jndex%16==15)
{
printf("\n");
}
else
if(jndex%2==1)
{
printf(" ");
}
}
if(jndex%16)
{
printf("\n");
}
fclose(phyle);
printf("%s %s complete\n",argv[0],argv[1]);
return 0;
}
EOD
#include <stdio.h>
#include <stdlib.h>
FILE *dataPhyle;
FILE *tempDataPhyle;
FILE *tempOffsetPhyle;
void do_one_guy(char *pgmName,
unsigned long int nextOffset,
unsigned long int curOffset
)
{
unsigned long int len;
long rowAddr;
char *str;
len=nextOffset-curOffset;
str=malloc(len);
if(str==NULL)
{
fprintf(stderr,"%s: malloc() fail\n",pgmName);
exit(1);
}
if(fread(str,sizeof(char),len-1,dataPhyle)!=len-1)
{
fprintf(stderr,"%s: fread() fail\n",pgmName);
}
str[len-1]='\0';
printf("record content is %s\n",str);
rowAddr=ftell(tempDataPhyle);
if(fwrite(&rowAddr,1,sizeof(rowAddr),tempOffsetPhyle)!=sizeof(rowAddr))
{
fprintf(stderr,"%s: fwrite(first) fail\n",pgmName);
}
if(fwrite(str,sizeof(char),len-1,tempDataPhyle)!=len-1)
{
fprintf(stderr,"%s: fwrite(second) fail\n",pgmName);
}
free(str);
}
int
main(int argc, char**argv)
{
dataPhyle=fopen("dataFile.dat","r");
if(!dataPhyle)
{
fprintf(stderr,"%s: fopen(\"dataFile.dat\") fail\n",argv[0]);
exit(1);
}
tempOffsetPhyle=fopen("tempOffsetFile.dat","w+");
if(!tempOffsetPhyle)
{
fprintf(stderr,"%s: fopen(\"tempOffsetFile.dat\") fail\n",argv[0]);
exit(1);
}
tempDataPhyle=fopen("tempDataFile.dat","w+");
if(!tempDataPhyle)
{
fprintf(stderr,"%s: fopen(\"tempDataFile.dat\") fail\n",argv[0]);
exit(1);
}
do_one_guy(argv[0],32,16);
do_one_guy(argv[0],12,8);
printf("%s complete\n",argv[0]);
return 0;
}
EOD
The output:
./1 complete
4142 4344 4546 4748 494A 4B4C 4D4E 4F50
5152 5354 5556 5758 595A
./2 dataFile.dat complete
record content is ABCDEFGHIJKLMNO
record content is PQR
./3 complete
4142 4344 4546 4748 494A 4B4C 4D4E 4F50
5152
./2 tempDataFile.dat complete
0000 0000 0F00 0000
./2 tempOffsetFile.dat complete
Hope this helps.