tar.c

tar.c

#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>
#include <stdbool.h>
#include <sys/stat.h>
#include <sys/mman.h>

typedef const char* string_t;

bool VfsReadFile( string_t buffer, struct stat file,
				  string_t fileName, string_t* contents, size_t* readLength)
{
	const uint32_t magicOffs = 257, nameOffs = 0,
				   sizeOffs = 124, szBlock = 512,
				   szLength = 12;
	
	bool successful = false;
	const char magic[] = "ustar";
	
	*contents = NULL;
	*readLength = 0;
	
	for ( size_t size, szContents = 0, p = 0, newOffset = 0
		; !successful && (p + newOffset + szBlock <= file.st_size)
		; size = 0
		, p += newOffset)
   {
		string_t entryName = buffer + nameOffs + p + newOffset,
				 offset = buffer + sizeOffs + p + newOffset;
		
		for(uint32_t i = 0; i< 5; i++) {
			// Check for supported TAR version
			if( buffer[i + magicOffs + p] != magic[i] ) {
				goto finished;
			}
		}
		
		for (uint32_t i = szLength-2, szWord = 1; i + 1 > 0; szWord*=8, i--) {
			if (( offset[i] >= '1' ) && (offset[i] <= '9')) {
				size += (offset[i]-'0') * szWord;
			}
		}
		
		// Offset size in bytes. Depends on file size and TAR block size; trim by block
		if (( size % szBlock )>= 0 ) {
			newOffset += (1 + size/szBlock) * szBlock;
		}
		
		if(strncmp(fileName, entryName, strlen(fileName)) == 0 ) {
			*readLength = size;
			*contents = (string_t) buffer + p + szBlock;
			
			successful = true;
		}
	}
	
finished:
	return successful;
}

int main(int args, char* arg[]) {	
	int fd;
	
	if (args - 1 == 2 ) {
		switch (fd = open( arg[1], O_RDWR)) {
			case -1:
				switch (errno) {
				case ENOENT:
					printf( "Error opening the file %s: No such file or directory.\n", arg[1] );
					break;
					
				default:
					break;
				}
				break;
			
			default: {
				struct stat file;
				fstat( fd, &file );
				
				size_t length;
				string_t contents;
				
				if (file.st_size > 0 ) {
					char* buffer = mmap(NULL, file.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
					bool result = 0;
					
					if (( result = VfsReadFile( buffer, file, arg[2], &contents, &length ))) {
						printf( "%s: File is %zx bytes in length:\n%s\n", arg[2], length, contents );
					}
					else {
						printf( "%s: No such file in archive. (%d)\n", arg[2], result );
					}
					
					munmap(buffer, file.st_size);
				}
				else {
					printf( "%s: File is empty.\n", arg[1] );
				}
			} break;
		}
		
		close(fd);
	}
	else {
		puts( "Please specify an archive name and filename to open on the commandline." );
	}
	
	return 0;
}