C Delete last n characters from file - c

I need to delete the last n characters from a file using C code. At fist I was trying to use '\b', but it returns a Segmentation Fault. I have seen interesting answers to similar questions here and here, but I would prefer to use mmap function to do this, if it's possible. I know it could be simpler to truncate the file by creating a temp file, and writing chars to temp until some offset of the original file. The problem is I don't seem to understand how to use mmap function to do this, can't see what parameters I need to pass to that function, specially address, length and offset. From what I've read, I should use MAP_SHARED in flags and PROT_READ|PROT_WRITE in protect.
The function definition says:
void * mmap (void *address, size_t length, int protect, int flags, int filedes, off_t offset)
Here is my main:
int main(int argc, char * argv[])
{
FILE * InputFile;
off_t position;
int charsToDelete;
if ((InputFile = fopen(argv[1],"r+")) == NULL)
{
printf("tdes: file not found: %s\n",argv[1]);
}
else
{
charsToDelete = 5;
fseeko(InputFile,-charsToDelete,SEEK_END);
position = ftello(InputFile);
printf("Pos: %d\n",(int)position);
int i;
//for(i = 0;i < charsToDelete;i++)
//{
// putc(InputFile,'\b');
//}
}
fclose(InputFile);
return 0;
}

Why not use:
#include <unistd.h>
#include <sys/types.h>
int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
like for instance:
charsToDelete = 5;
fseeko(InputFile,-charsToDelete,SEEK_END);
position = ftello(InputFile);
ftruncate(fileno(InputFile), position);

Read all but n bytes from the file and write to a temporary file, close the original file, rename temporary file as original file.
Or use e.g. truncate or similar function if you have it.
Also, failure to open the file doesn't have to be that it can't be found, You should check errno on failure to see what the error is. Use e.g. strerror to get a printable string from the error code.

Unfortunately, mmap does not allow you to change size of underlying file object.
Instead, I would recommend to simply truncate your file, use something like this:
truncate(filename, new_length);

Related

#define macro causing segmentation fault or something else causing segfault?

I am using libnfs for C. I am calling nfs_read and it takes the size of bytes to read as a uint64_t variable. I have the size defined as a macro (#define 100). I mostly get a segfault (or sometimes some other error based on what value I choose, but always the same error for the specific value)for any size value greater than 24. I also tried changing #define to global uint64_t. I first had it in a header file and also moved it from header file to c file. But the result is always the same (if size is greater than 24), segfault.
But when I pass the value directly (as hard coded value) to the function nfs_read I do not get segfault, for any value of size (<24 or >24).
I have done fair number of projects in C before and have never faced such an error. Any idea what could be happening here. Thanks.
typedef struct {
int is_nfs;
int fd;
struct nfs_context *nfs;
struct nfsfh *fh;
}nfs_fd_t;
#define COUNT 100
// The open function is same as in the linked example except for I added nfs_dd_t struct to store nfs and fh
int dd_open(const char *path, int flags, mode_t mode, nfs_fd_t *nfs_fd);
ssize_t read_wrapper(nfs_dd_t *nfs_fd)
{
char * buf = malloc(COUNT);
int ret = dd_read(nfs_fd, buf, COUNT);
// follow up logic
}
ssize_t dd_read(nfs_fd_t *nfs_fd, void *buf, uint64_t count)
{
int ret;
if ((ret = nfs_read((*nfs_fd).nfs, (*nfs_fd).fh, count, (char *)buf)) < 0) {
errno = -ret;
return -1;
}
return ret;
}
nfs_dd_t is a struct containing nfs and fh. I am basically following this example only slightly modified for my need.

Reading files to shared memory

I am reading a binary file that I want to offload directly to the Xeon Phi through Cilk and shared memory.
As we are reading fairly much data at once each time and binary data the preferred option is to use fread.
So if I make a very simple example it would go like this
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
_Cilk_shared uint8_t* _Cilk_shared buf;
int main(int argc, char **argv) {
printf("Argv is %s\n", argv[1]);
FILE* infile = fopen(argv[1], "rb");
buf = (_Cilk_shared uint8_t*) _Offload_shared_malloc(2073600);
int len = fread(buf, 1, 2073600, infile);
if(ferror(infile)) {
perror("ferror");
}
printf("Len is %d and first value of buf is %d\n", len, *buf);
return 0;
}
The example is very simplified from the real code but enough to examplify the behavior.
This code would then return
ferror: Bad address
Len is 0 and first value of buf is 0
However if we switch out the fread for a fgets (not very suitable for reading binary data, specially with the return value) things work great.
That is we switch fgets((char *) buf, 2073600, infile); and then drop the len from the print out we get
first value of buf is 46
Which fits with what we need and I can run _Offload_cilk on a function with buf as an argument and do work on it.
Is there something I am missing or is fread just not supported? I've tried to find as much info on this from both intel and other sites on the internet but I have sadly been unable to.
----EDIT----
After more research into this it seems that running fread on the shared memory with a value higher than 524287 (524287 is 19 bits exactly) fread gets the error from above. At 524287 or lower things work, and you can run as many fread as you want and read all the data.
I am utterly unable to find any reason written anywhere for this.
I don't have a PHI, so unable to see if this would make a difference -- but fread has it's own buffering, and while that may be turned of for this type of readind, then I don't see why you would go through the overhead of using fread rather than just using the lower level calls of open&read, like
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdint.h>
_Cilk_shared uint8_t* _Cilk_shared buf;
int main(int argc, char **argv) {
printf("Argv is %s\n", argv[1]);
int infile = open(argv[1], O_RDONLY); // should test if open ok, but skip to make code similar to OP's
int len, pos =0, size = 2073600;
buf = (_Cilk_shared uint8_t*) _Offload_shared_malloc(size);
do {
buf[pos]=0; // force the address to be mapped to process memory before read
len = read(infile, &buf[pos], size);
if(len < 0) {
perror("error");
break;
}
pos += len; // move position forward in cases where we have no read the entire data in first read.
size -= len;
} while (size > 0);
printf("Len is %d (%d) and first value of buf is %d\n", len, pos, *buf);
return 0;
}
read & write should work with shared memory allocated without the problem you are seeing.
Can you try to insert something like this before the fread calls?
memset(buf, 0, 2073600); // after including string.h
This trick worked for me, but I don't know why (lazy allocation?).
FYI, you can also post a MIC question on this forum.

How to read an integer and a char with read() function in C?

I'm working on linux, I have a file that contains a line like this:
328abc
I would like, in C, to read the integer part (328) and the characters 'a','b','c', using only the function:
ssize_t read (int filedes, void *buffer, size_t size))
This is the only thing the file contains.
I know there are better ways to do that with other functions, but I haven't coded in C for a long time, and trying to help a friend, only this function is alowed.
How do I play with the buffer to do that?
Thanks
edit:
I understand that I need to parse the buffer manually. and my question is how?
If that's the only thing in the file. This will do:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
int main() {
char buffer[6];
char intBuffer[4];
ssize_t bytesRead;
int number;
int fd;
if ((fd = open("file.txt", O_RDONLY)) == -1) {
perror("Error opening file");
exit(EXIT_FAILURE);
}
if ((bytesRead = read(fd, buffer, 6)) == -1) {
perror("Error reading file");
exit(EXIT_FAILURE);
}
memcpy(intBuffer, buffer, 3);
intBuffer[3] = '\0';
number = atoi(intBuffer);
printf("The number is %d\n", number);
exit(EXIT_SUCCESS);
}
The following code will print "The number is 328".
Is this some kind of homework?
I am asking because there are better ways to do that than using the read function.
Anyway to answer your question, read reads size bytes from the file whose file descriptor is filedes and places them to the buffer.
It does not know anything about line breaks etc. So you need to manually find where a line ends, etc. If you want to only use read, then you need to manually parse the buffer after each call to read (supposing your files contains many lines, that you want to parse).
Beware that a line may be split between two read calls, so you need to handle that case with caution.

print out contents of archived files in C

I have the same question posted here:
How to print the name of the files inside an archive file?
but those answers don't necessarily address the problem. I have an archived file week.a and I'd like to print out the names of the files inside that archive, called mon.txt, and fri.txt.
It should work just like the ar -t command, except I'm not allowed to use that.
What I've tried:
My first attempt was to create a for loop and print out the arguments, but then I realized the file is already archived by that point, and so that wouldn't work.
My second attempt was to look at the print_contents function of ar, which I've listed below:
static void
print_contents (bfd *abfd)
{
size_t ncopied = 0;
char *cbuf = (char *) xmalloc (BUFSIZE);
struct stat buf;
size_t size;
if (bfd_stat_arch_elt (abfd, &buf) != 0)
/* xgettext:c-format */
fatal (_("internal stat error on %s"), bfd_get_filename (abfd));
if (verbose)
printf ("\n<%s>\n\n", bfd_get_filename (abfd));
bfd_seek (abfd, (file_ptr) 0, SEEK_SET);
size = buf.st_size;
while (ncopied < size)
{
size_t nread;
size_t tocopy = size - ncopied;
if (tocopy > BUFSIZE)
tocopy = BUFSIZE;
nread = bfd_bread (cbuf, (bfd_size_type) tocopy, abfd);
if (nread != tocopy)
/* xgettext:c-format */
fatal (_("%s is not a valid archive"),
bfd_get_filename (bfd_my_archive (abfd)));
/* fwrite in mingw32 may return int instead of size_t. Cast the
return value to size_t to avoid comparison between signed and
unsigned values. */
if ((size_t) fwrite (cbuf, 1, nread, stdout) != nread)
fatal ("stdout: %s", strerror (errno));
ncopied += tocopy;
}
free (cbuf);
}
But with this route, I don't really know what a lot of that code means or does (I'm very new to C). Could someone help make sense of this code, or point me in the right direction for writing my program? Thank you.
Based on format at wikipedia.org/wiki/Ar_(Unix), the basic shape of your program will be:
fopen(filename)
fscanf 8 characters/* global header */
check header is "!<arch>" followed by LF
while not at end of file /* check return value of fcanf below */
fscanf each item in file header
print filename /* first 16 characters of file header */
check magic number is 0x60 0x0A
skip file size characters /* file contents - can use fseek with origin = SEEK_CUR */
fclose(file)
Refer to the C stdio library documentation for details of functions needed. Or see Wikipedia C file input/output
int counting(FILE *f)
{
int count=0;
rewind(f);
struct ar_hdr myheader;
fseek(f,8,SEEK_CUR);
while(fread(&myheader,sizeof(struct ar_hdr),1,f)>0)
{
long test;
test = atol(myheader.ar_size);
fseek(f,test,SEEK_CUR);
count++;
}
printf("count is : %d\n",count);
return count;
}
this code i had written to count the number of files in archive.. u can use the same to print the file names inside it as well

How do i read a file backwards using read() in c? [duplicate]

This question already has answers here:
Reading a text file backwards in C
(5 answers)
Closed 9 years ago.
I am supposed to create a program that takes a given file and creates a file with reversed txt. I wanted to know is there a way i can start the read() from the end of the file and copy it to the first byte in the created file if I dont know the exact size of the file?
Also i have googled this and came across many examples with fread, fopen, etc. However i cant use those for this project i can only use read, open, lseek, write, and close.
here is my code so far its not much but just for reference:
#include<stdio.h>
#include<unistd.h>
int main (int argc, char *argv[])
{
if(argc != 2)/*argc should be 2 for correct execution*/
{
printf("usage: %s filename",argv[0[]);}
}
else
{
int file1 = open(argv[1], O_RDWR);
if(file1 == -1){
printf("\nfailed to open file.");
return 1;
}
int reversefile = open(argv[2], O_RDWR | O_CREAT);
int size = lseek(argv[1], 0, SEEK_END);
char *file2[size+1];
int count=size;
int i = 0
while(read(file1, file2[count], 0) != 0)
{
file2[i]=*read(file1, file2[count], 0);
write(reversefile, file2[i], size+1);
count--;
i++;
lseek(argv[2], i, SEEK_SET);
}
I doubt that most filesystems are designed to support this operation effectively. Chances are, you'd have to read the whole file to get to the end. For the same reasons, most languages probably don't include any special feature for reading a file backwards.
Just come up with something. Try to read the whole file in memory. If it is too big, dump the beginning, reversed, into a temporary file and keep reading... In the end combine all temporary files into one. Also, you could probably do something smart with manual low-level manipulation of disk sectors, or at least with low-level programming directly against the file system. Looks like this is not what you are after, though.
Why don't you try fseek to navigate inside the file? This function is contained in stdio.h, just like fopen and fclose.
Another idea would be to implement a simple stack...
This has no error checking == really bad
get file size using stat
create a buffer with malloc
fread the file into the buffer
set a pointer to the end of the file
print each character going backwards thru the buffer.
If you get creative with google you can get several examples just like this.
IMO the assistance you are getting so far is not really even good hints.
This appears to be schoolwork, so beware of copying. Do some reading about the calls used here. stat (fstat) fread (read)
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
int main(int argc, char **argv)
{
struct stat st;
char *buf;
char *p;
FILE *in=fopen(argv[1],"r");
fstat(fileno(in), &st); // get file size in bytes
buf=malloc(st.st_size +2); // buffer for file
memset(buf, 0x0, st.st_size +2 );
fread(buf, st.st_size, 1, in); // fill the buffer
p=buf;
for(p+=st.st_size;p>=buf; p--) // print traversing backwards
printf("%c", *p);
fclose(in);
return 0;
}

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