NickName:CheeHow Ask DateTime:2013-08-06T13:09:04

Truncating the first 100MB of a file in linux

I am referring to How can you concatenate two huge files with very little spare disk space?

I'm in the midst of implementing the following:

Allocate a sparse file of the combined size.
Copy 100Mb from the end of the second file to the end of the new file.
Truncate 100Mb of the end of the second file
Loop 2&3 till you finish the second file (With 2. modified to the correct place in the destination file).
Do 2&3&4 but with the first file.

I would like to know if is there anyone there who are able to "truncate" a given file in linux? The truncation is by file size, for example if the file is 10GB, I would like to truncate the first 100MB of the file and leave the file with remaining 9.9GB. Anyone could help in this?

Thanks

Copyright Notice：Content Author:「CheeHow」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/18072180/truncating-the-first-100mb-of-a-file-in-linux

Answers

Sunding Wei 2014-08-01T10:43:31

Answer, now this is reality with Linux kernel v3.15 (ext4/xfs)\nRead here\nhttp://man7.org/linux/man-pages/man2/fallocate.2.html\nTesting code\n#include <stdio.h>\n#include <stdlib.h>\n#include <string.h>\n#include <stdlib.h>\n#include <fcntl.h>\n\n#ifndef FALLOC_FL_COLLAPSE_RANGE\n#define FALLOC_FL_COLLAPSE_RANGE 0x08\n#endif\n\nint main(int argc, const char * argv[])\n{\n int ret;\n char * page = malloc(4096);\n int fd = open("test.txt", O_CREAT | O_TRUNC | O_RDWR, 0644);\n\n if (fd == -1) {\n free(page);\n return (-1);\n }\n\n // Page A\n printf("Write page A\\n");\n memset(page, 'A', 4096);\n write(fd, page, 4096);\n\n // Page B\n printf("Write page B\\n");\n memset(page, 'B', 4096);\n write(fd, page, 4096);\n\n // Remove page A\n ret = fallocate(fd, FALLOC_FL_COLLAPSE_RANGE, 0, 4096);\n printf("Page A should be removed, ret = %d\\n", ret);\n\n close(fd);\n free(page);\n\n return (0);\n}\n",

Joni 2013-08-06T05:51:02

Chopping off the beginning of a file is not possible with most file systems and there's no general API to do it; for example the truncate function only modifies the ending of a file.\nYou may be able to do it with some file systems though. For example the ext4 file system recently got an ioctl that you may find useful: http://lwn.net/Articles/556136/\n\nUpdate: About a year after this answer was written, support for removing blocks from beginning and middle of files on ext4 and xfs file systems was added to the fallocate function, by way of the FALLOC_FL_COLLAPSE_RANGE mode. It's more convenient than using the low level iotcl's yourself.\nThere's also a command line utility with the same name as the C function. Assuming your file is on a supported file system, this will delete the first 100MB:\nfallocate -c -o 0 -l 100M yourfile\n\ndelete the first 1GB:\nfallocate -c -o 0 -l 1G yourfile\n",