truncate

Name

truncate, ftruncate
- truncate or extend a file to a specified length

Library

libc.lib

Synopsis

#include <unistd.h>

int truncate (const char *path, off_t length);

int ftruncate (int fd, off_t length);

Return values

Upon successful completion, both truncate() and ftruncate() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.

Detailed description

The truncate system call causes the file named by path or referenced by fd to be truncated to length bytes in size. If the file was larger than this size, the extra data is lost. If the file was smaller than this size, it will be extended as if by writing bytes with the value zero. With ftruncate, the file must be open for writing.

Examples

//example for truncate
#include<unistd.h>
#include<stdio.h>
#include <sys/stat.h>
int test_truncate()
{
        int retVal, retVal2, retSize, retSize2;
        struct stat buf;
        ssize_t size;
        char *buffer = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwx";
        int fp = open("c:\test.txt", O_RDWR|O_CREAT);
        size = write(fp,buffer,50);
        close(fp);
        retVal2 = stat("c:\test.txt", &buf );
        if ( !retVal2 )
        {
        retSize = buf.st_size;
        printf("Size before: %d", retSize);
        retVal = truncate("c:\test.txt", retSize/2 );
        }
        else
        {
        printf("Failed");
        }
        retVal2 = stat( "c:\test.txt", &buf );
        if ( !retVal2 )
                {
                retSize2 = buf.st_size;
                if( retSize2 == (retSize/2 ) )
                        {
                        printf("\nSize after: %d\n", retSize2);
                        printf("Truncate passed");
                        return 0;
                        }
                else
                        {
                        printf("Failed");
                        return -1;
                        }
                }
        else
                {
                printf("Failed");
                return -1;
                }
}

Output

Size before: 50
Size after: 25
Ttruncate Passed

         
//example for ftruncate
#include<unistd.h>
#include<stdio.h>
int test_ftruncate()
{
//assuming that the file exists and has some //data in it
   int fp = open("c:\test.txt", O_RDWR);
   int retVal, retVal2, retSize, retSize2;
   struct stat buf;
   if(fp != -1)
   {
     retVal2 = fstat( fp, &buf );
     if ( !retVal2 )
     {
        retSize = buf.st_size;
        printf("Size before: %d", retSize);
        retVal = ftruncate( fp, retSize/2 );
        close(fp);
     }
     else
     {
        printf("Failed");
     }
     fp = open("c:\test.txt", O_RDONLY);
     if((fp != -1) && (!retVal))
     {
        retVal2 = fstat( fp, &buf );
        if ( !retVal2 )
        {
          retSize2 = buf.st_size;
          if( retSize2 == (retSize/2 ) )
          {
            printf("\nSize after: %d\n", retSize2);
            printf("Ftruncate Passed");
          }
          else
          {
            printf("Failed");
          }
     }
     else
     {
       printf("Failed");
     }
  }
}

Output

Size before: 100
Size after: 50
Ftruncate Passed

Errors

The truncate and ftruncate succeed unless:

[`EBADF`]
	The fd argument is not a valid descriptor for a regular file.
[`EINVAL`]
	The fd argument references to an object other than a file.
[`EINVAL`]
	The fd descriptor is not open for writing.

Bugs

These calls should be generalized to allow ranges of bytes in a file to be discarded.

Use of truncate to extend a file is not portable.

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