I needed a command line utility for Bash (for both Windows and Linux) that only outputs bytes between 2 points in a file to STDOUT. head and tail weren’t really cutting it so I figured I’d throw something together. Below is the source code for the result, which I call chunk (Windows Executable).
I compiled the file as c++, but it should be c99 compatible. The file has been tested as compilable for: GCC4 on Slackware, GCC3 on Red Hat, and GCC3 on MingW (Windows [WIN32 should be defined by the compiler]).
Chunk outputs bytes between 2 points in a file to STDOUT. The parameters are:
1) The file
2) The byte offset to start at (hex is supported like 0xA)
3) The number of bytes to output. If not given, the end of the file is assumed.
The source is as follows:
//Copyright 2009 by Dakusan (http://www.castledragmire.com/Copyright). Licensed under Dakusan License v2.0 (http://www.castledragmire.com/Misc/Software_Licenses/Dakusan_License_v2.0.php).
//See http://www.castledragmire.com/Posts/chunk for more information
#define __LARGE64_FILES
#include <stdio.h>
#include <stdlib.h> //strtoull
#ifdef WIN32 //STDOUT only needs to be set to binary mode in windows
#include <io.h> //_setmode
#include <fcntl.h> //_O_BINARY
#endif
typedef unsigned long long UINT64;
const UINT64 MaxSizeToRead=1024*1024*10; //The maximum number of bytes to read at a time to our buffer (Must be < 2^31)
UINT64 GetNumberFromString(const char* S) //Extract both hexidecimal and decimal numbers from a string
{
bool IsHex=S[0]=='0' && (S[1]|32=='x'); //If string starts as 0x, then is a hex number
return strtoull(S+(IsHex ? 2 : 0), NULL, IsHex ? 16 : 10); //Hex number starts after 2 characters and uses base 16
}
int main(int argc, char *argv[], char *envp[])
{
//Determine if proper number of parameters are passed, and if not, output help info
if(argc!=3 && argc!=4)
return fprintf(stderr, "Chunk outputs bytes between 2 points in a file to STDOUT. The parameters are:\n1) The file\n2) The byte offset to start at (hex is supported like 0xA)\n3) The number of bytes to output. If not given, the end of the file is assumed.\n") & 0;
//Open the file and get its length
FILE *TheFile=fopen64(argv[1], "rb");
if(TheFile==NULL)
return fprintf(stderr, "File not found or cannot open file\n") & 0;
fseeko64(TheFile, 0, SEEK_END); //Get the length by seeking to the end
UINT64 FileSize=ftello64(TheFile);
//Determine the requested start offset
UINT64 Offset=GetNumberFromString(argv[2]), SizeToOutput;
if(Offset>=FileSize)
{
fprintf(stderr, "Offset is larger than file's size\n");
fclose(TheFile);
return 0;
}
//Determine the size to read
if(argc==3) //If no final parameter, read to the end of the file
SizeToOutput=FileSize-Offset;
else //Determine from the 3rd parameter
{
SizeToOutput=GetNumberFromString(argv[3]);
if(Offset+SizeToOutput>FileSize)
{
fprintf(stderr, "Requested size is larger than the file, truncating to end of file\n");
SizeToOutput=FileSize-Offset;
}
else if(!SizeToOutput) //If nothing to output, exit prematurely
{
fclose(TheFile);
return 1;
}
}
//Output requested data 10MB at a time from the file to STDOUT
char *Buffer=new char[SizeToOutput>MaxSizeToRead ? MaxSizeToRead : SizeToOutput]; //Only allocate as many bytes to our read buffer as is necessary
fseeko64(TheFile, Offset, SEEK_SET); //Seek to the beginning read offset of our file
#ifdef WIN32 //STDOUT only needs to be set to binary mode in windows
_setmode(_fileno(stdout), _O_BINARY);
#endif
while(SizeToOutput) //Keep reading and outputting until requested data is complete
{
UINT64 SizeToRead=SizeToOutput>MaxSizeToRead ? MaxSizeToRead : SizeToOutput; //Number of bytes to read and write
fread(Buffer, SizeToRead, 1, TheFile); //Read the data
fwrite(Buffer, SizeToRead, 1, stdout); //Write the data to STDOUT
SizeToOutput-=SizeToRead; //Decrease number of bytes we still need to read
}
//Cleanup
delete[] Buffer;
fclose(TheFile);
return 1;
}
