C programming File size [closed] - c

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Closed 10 years ago.
I am trying to acquire the size of a file after creating it and writing data to it. I get values that don't seem to correspond to the actual file size. Here is my program. Please show me how I can display the file size in Bits, Bytes, Kilobytes, and Megabytes. According to me the file size should be 288 Bits, 36 Bytes, 0.03515626 Kilobytes, and 0.000034332 Megabytes.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#define PERMS 0777
int main(int argc, char *argv[])
{
int createDescriptor;
int openDescriptor;
char fileName[15]="Filename1.txt";
umask(0000);
if ((openDescriptor = creat(fileName, PERMS )) == -1)
{
printf("Error creating %s", fileName);
exit(EXIT_FAILURE);
}
if(write(openDescriptor,"This will be output to testfile.txt\n",36 ) != 36)
{
write(2,"There was an error writing to testfile.txt\n",43);
return 1;
}
if((close(openDescriptor))==-1)
{
write(2, "Error closing file.\n", 19);
}
struct stat buf;
fstat(openDescriptor, &buf);
int size=buf.st_size;
printf("%d\n",size);
printf("%u\n",size);
return 0;
}

Answer 1

The fstat() function has a return code, check it.
int r = fstat(openDescriptor, &buf);
if (r) {
fprintf(stderr, "error: fstat: %s\n", strerror(errno));
exit(1);
}
This will print:
error: fstat: Bad file descriptor
Yep... you closed the file descriptor, it's not a file descriptor any more. You have to fstat() before calling close().
The code worries me.
This is extremely fragile, and cannot be recommended under any circumstances:
if (write(openDescriptor,"This will be output to testfile.txt\n",36 ) != 36)
You can do this:
const char *str = "This will be output to testfile.txt\n";
if (write(fd, str, strlen(str)) != strlen(str))
It will compile to the same machine code, and it's obviously correct (as opposed to the original code, where you have to count the number of characters in a string to figure out if it's correct or not).
Even better, when you are using stderr, just use the standard <stdio.h> functions:
fprintf(stderr, "There was an error writing to %s: %s\n",
fileName, strerror(errno));
The same error appears when defining fileName...
// You should never have to know how to count higher than 4 to figure
// out if code is correct...
char fileName[15]="Filename1.txt";
// Do this instead...
static const char fileName[] = "Filename1.txt";
You actually miscounted this time, [15] should have been [14], but better to leave it to the compiler. There's no benefit to making the compiler's job easier, since the compiler presumably doesn't have better things to do.
About the machine code:
$ cat teststr.c
#include <unistd.h>
void func(int openDescriptor) {
write(openDescriptor,"This will be output to testfile.txt\n",36 );
}
$ cat teststr2.c
#include <string.h>
#include <unistd.h>
void func(int openDescriptor) {
const char *str = "This will be output to testfile.txt\n";
write(openDescriptor, str, strlen(str));
}
$ cc -S -O2 teststr.c
$ cc -S -O2 teststr2.c
$ diff teststr.s teststr2.s
1c1
< .file "teststr.c"
---
> .file "teststr2.c"
Yep. As demonstrated, the call to strlen() does not actually result in different machine code.