[SOLVED] read() doesn't read properly

gursewaks056 · 11-30-2017, 10:08 PM

When I use read() function for reading from file then it read the text but also print 2 or 3 unwanted characters.

for example :
file1.txt : My name is John. This is file contents.

coding : char buff[10]; int f; f=open("file1.txt",O_RDONLY);
fread(f,buff,5); printf("%s\n",buff);

output : My na$%#

giis · 11-30-2017, 10:20 PM

That's because your buf size 10 and you read only 5 chars.

And also you are not reading entire file content. Just reading first 5 bytes of it. I don't think thats your requirement.

gursewaks056 · 11-30-2017, 11:09 PM

If I used my total buffer size to read then also it does same thing. Thanks for reply.

giis · 11-30-2017, 11:32 PM

Quote:

Originally Posted by gursewaks056

If I used my total buffer size to read then also it does same thing.

Yes, Just cleaning your buffer won't solve the problem.

Can you explain whats going on here:

Quote:

fread(f,buff,5);

in plain english?

michaelk · 12-01-2017, 06:49 AM

printf("%s\n",buff) will output the contents buff up to but not including a NULL character. Any printable character that is in buff between the 5th element and a NULL character will be displayed.

You can add a NULL (0) character to buff or only print 5 characters. Here are a few examples.

Code:

   buff[5]=0;
   printf("%s\n", buff); // will print to the first NULL character
   printf("%.5s\n",buff);  // Will print first 5 characters
   length=5;
   printf("%.*s\n", length, buff);  // will print number of characters defined by length

gursewaks056 · 12-01-2017, 09:04 AM

Quote:

Originally Posted by giis

Yes, Just cleaning your buffer won't solve the problem.

Can you explain whats going on here:

in plain english?

Oh! Spelling mistake is here. I wanted to use read() function as that place of fread(). read(f,buff,5); statement reads first 5 characters from the file opened by f descriptor.

rhubarbdog · 12-01-2017, 10:23 PM

The %s format takes a nul terminated string you need to do the following

Code:

char buf[10];
  f=open("file.txt",O_RDONLY);
  ok=fead(f,buf,5);
  if (ok<0) buf[0]='\0';
  else buf[ok]='\0';
  printf("%s\n",buf);

Note you have to nul terminate all strings explicitly. And have to allow space for it, that is buf[10] gives 10 character positions one of which has to be the nul byte leaving ipto 9 for data

giis · 12-04-2017, 04:48 AM

Here's simple char-char version with file operation using sys.call directly:

Quote:

#include <stdio.h>

int main(){
char c;
int fp,ret=0;

fp=open("/tmp/f1.txt",0);
if (fp < 0){
printf("file open error");
return -1;
}

ret=read(fp,&c,1);
while(ret > 0){
printf("%c",c);
ret = read(fp,&c,1);
}
close(fp);
}

HTH