I have been given a raw file that holds several jpg images. I have to go through the file, find each jpg image, and put those images each in a separate file. So far I have code that can find each where each image begins and ends. I also have written code that names several file names I can use to put the pictures in. It is an array: char filename[] , that holds the names: image00.jpg - image29.jpg .
What I cannot figure out is how to open a file every time I find an image, an then close that file and open a new one for the next image. Do I need to use fwrite()? Also, each image is in blocks of 512 bytes, so I only have to check for a new image every 512 bytes once I find the first one. Do I need to add that into fwrite?
So, to summarize my questions, I don't understand how to use fwrite(), if that is what I should be using to write to these files.
Also, I do not know how to open the files using the names I have already created.
Thanks in advance for the help. Let me know if I need to post any other code.
Use fopen(rawfilename, "rb"); to open the raw file for reading. and fread to read from it.
Use fopen(outfilename, "wb"); to open output file for writing and fwrite to write to it.
As mentioned in my comment, you are assigning char *[] to char*, use char filename[] = "image00.jpg"; instead.
Don't forget to close each file after you finish its processing (r/w) (look at fclose() at the same site of other links)
Decide how much bytes to read each time by parsing the jpeg header. Use malloc to allocate the amount of bytes needed to be read, and remember, for each allocation of buffer you need to free the allocated buffer later.
Pretty much any book on C programming should cover the functions you need. As MByD pointed out, you'll want to use the functions fopen(), fwrite(), and fclose().
I imagine your code may include fragments that look something like
/* Warning: untested and probably out-of-order code */
...
char **filename = {
"image00.jpg", "image01.jpg", "image02.jpg",
...
"image29.jpg" };
...
int index = 0;
const int blocksize = 512; /* bytes */
...
index++;
...
FILE * output_file = fopen( filename[index], "wb");
fwrite( output_data, 1, blocksize, output_file );
fclose(output_file);
...
Related
I have been trying to write a file from memory in C, more specifically an executable file. Every time I try to use fputs it detects a '00' in memory after a bit and stops writing. But there is still the rest of the file that it has to write. In the file that I am trying to write there are '00's all over the place for padding. I have some code below for reference:
char *buffer;
buffer = malloc(size);
// ...
FILE *file;
file = fopen("somename","w");
fputs(buffer,file);
fclose(file);
Is there any way I would be able to have '00's in memory without fputs taking it as an EOF?
Thanks!
you should use 'fwrite' take the place of fpus
I'm trying to write a program in C that will read from /proc/stat, wait a second, then read from /proc/stat again a second later, then do some math to find the "current" CPU usage, as well as the usage for each core. I think that the easiest way to do this is to save the results of /proc/stat to a file, as well as a second file for the second "state" of /proc/stat. I would then read from it and calculate from there. For some reason, though, I have no clue why neither of my files are being created.
int main() {
FILE *state0;
FILE *state1;
state0 = fopen("/proc/stat", "w+");
delay();
state1 = fopen("/proc/stat", "w+");
char *con0 = malloc(500);
fprintf(state0, con0);
fclose(state0);
fclose(state1);
return 0;
}
And help is greatly appreciated
The issue is, when you try to save a file by calling:
fprintf(state0, con0);
You're trying to write the memory allocated at con0 to state0, which is a pointer to /proc/stat.
This of course fails because you don't have permissions to write to /proc/stat, which you shouldn't.
To open these files for reading, as tadman mentioned, you want to open the files in "r" mode:
FILE *state0;
FILE *state1;
state0 = fopen("/proc/stat", "r");
delay();
state1 = fopen("/proc/stat", "r");
What you'd want to do is create a separate file, for example:
FILE *newfile;
newfile = fopen("/tmp/procstattemp", "w");
and write to this file using:
fprintf(newfile, con0);
Also, you're allocating memory for con0, but you haven't written anything to this buffer.
If you want to save /proc/stat to a file, you'll need to read the file and copy it into con0 before writing your temporary file.
See also:
http://www.cplusplus.com/reference/cstdio/fprintf/
http://www.cplusplus.com/reference/cstdio/fopen/
I'm using C to write some data to a file. I want to erase the previous text written in the file in case it was longer than what I'm writing now.
I want to decrease the size of file or truncate until the end. How can I do this?
If you want to preserve the previous contents of the file up to some length (a length bigger than zero, which other answers provide), then POSIX provides the truncate() and ftruncate() functions for the job.
#include <unistd.h>
int ftruncate(int fildes, off_t length);
int truncate(const char *path, off_t length);
The name indicates the primary purpose - shortening a file. But if the specified length is longer than the previous length, the file grows (zero padding) to the new size. Note that ftruncate() works on a file descriptor, not a FILE *; you could use:
if (ftruncate(fileno(fp), new_length) != 0) ...error handling...
However, you should be aware that mixing file stream (FILE *) and file descriptor (int) access to a single file is apt to lead to confusion — see the comments for some of the issues. This should be a last resort.
It is likely, though, that for your purposes, truncate on open is all you need, and for that, the options given by others will be sufficient.
For Windows, there is a function SetEndOfFile() and a related function SetFileValidData() function that can do a similar job, but using a different interface. Basically, you seek to where you want to set the end of file and then call the function.
There's also a function _chsize() as documented in the answer by sofr.
In Windows systems there's no header <unistd.h> but yet you can truncate a file by using
_chsize( fileno(f), size);
That's a function of your operating system. The standard POSIX way to do it is:
open("file", O_TRUNC | O_WRONLY);
If this is to run under some flavor of UNIX, these APIs should be available:
#include <unistd.h>
#include <sys/types.h>
int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
According to the "man truncate" on my Linux box, these are POSIX-conforming. Note that these calls will actually increase the size of the file (!) if you pass a length greater than the current length.
<edit>
Ah, you edited your post, you're using C. When you open the file, open it with the mode "w+" like so, and it will truncate it ready for writing:
FILE* f = fopen("C:\\gabehabe.txt", "w+");
fclose(file);
</edit>
To truncate a file in C++, you can simply create an ofstream object to the file, using ios_base::trunc as the file mode to truncate it, like so:
ofstream x("C:\\gabehabe.txt", ios_base::trunc);
If you want to truncate the entire file, opening the file up for writing does that for you. Otherwise, you have to open the file for reading, and read the parts of the file you want to keep into a temporary variable, and then output it to wherever you need to.
Truncate entire file:
FILE *file = fopen("filename.txt", "w"); //automatically clears the entire file for you.
Truncate part of the file:
FILE *inFile("filename.txt", "r");
//read in the data you want to keep
fclose(inFile);
FILE *outFile("filename.txt", "w");
//output back the data you want to keep into the file, or what you want to output.
I'm creating a program using lex and yacc to parse text, but i need create a parser of various content. I don't wish use the stdin, if i using FILE *yyin to specify the input, i can change the source. I need can call the function from library parse (created with lex file and yacc file) to parse this content and receive a result.
/**
* This i don't know is possible, receive a char * and return a FILE*
*/
FILE *function_parse_to_file(char* text){
FILE *fp = NULL;
/**
* is really necessary create a temporary file with content text?
*/
return fp
}
/**
* I need call from other library or application
*/
char *function_parse_from_lex(char* text){
yyin = function_parse_to_file(text);
init();
yyparse();
fclose(yyin);
}
On a POSIX-2008-compliant system (and on Linux), you can use fmemopen to get a FILE* handle on an in-memory buffer.
You can define YY_INPUT macro with three arguments: buffer, result, max_size, where:
buffer - input with buffer where to read data,
result - output to store number of bytes read
max_size - input with buffer size
Just include the macro definition in your Lex file using header or inline and it will be used instead of fread(...)
You really haven't stated your question clearly, but I am going to assume you want to create a FILE * which will return the contents of the string pointed to by the char * when data is read from it. You could simply create a pipe and then invoke fdopen on the read side. It is a bit dangerous to just write the data into the write side, since the write might block and lead to a deadlock, but you can certainly fork a child and have the child write the data into the pipe.
On the other hand, there's no real reason not to create a temporary file. Assuming you are going to unlink the file after you read it, there's very little chance of the data ever going to disk (the OS will keep it in memory) If you're really concerned to can use a path on a ram disk.
I have opened one file with following way:
fp = fopen("some.txt","r");
Now in this file the 1st some bytes lets say 40 bytes are unnecessary junk of data so I want to remove them. But I cannot delete that data from that file, modify or
create duplicates of that file without that unnecessary data.
So I want to create another dummy FILE pointer which points to the file and when I pass this dummy pointer to any another function that does the following operation:
fseek ( dummy file pointer , 0 , SEEK_SET );
then it should set the file pointer at 40th position in my some.txt.
But the function accepts a file descriptor so i need to pass a file descriptor which will treat the file as those first 40 bytes were never in the file.
In short that dummy descriptor should treat the file as those 40 bytes were not in that file and all positioning operations should be with respect to that 40th byte counting as the is 1st byte.
Easy.
#define CHAR_8_BIT (0)
#define CHAR_16_BIT (1)
#define BIT_WIDTH (CHAR_8_BIT)
#define OFFSET (40)
FILE* fp = fopen("some.txt","r");
FILE* dummy = NULL;
#if (BIT_WIDTH == CHAR_8_BIT)
dummy = fseek (fp, OFFSET*sizeof(char), SEEK_SET);
#else
dummy = fseek (fp, OFFSET*sizeof(wchar_t), SEEK_SET);
#endif
The SEEK_SET macro indicates beginning of file, and depending on whether you are using 8-bit characters (ASCI) or 16-bit characters (eg: UNICODE) you will step 40 CHARACTERS forward from the beginning of your file pointer, and assign that pointer/address to dummy.
Good luck!
These links will likely be helpful as well:
char vs wchar_t
http://www.cplusplus.com/reference/clibrary/cstdio/fseek/
If you want, you can just convert a file descriptor to a file pointer via the fdopen() call.
http://linux.die.net/man/3/fdopen
fseek ( dummy file pointer , 0 , SEEK_SET );
In short that dummy pointer should treat the file as there is no that 40 byte in that file and all position should be with respect to that 40th byte as counting as it is 1st byte.
You have conflicting requirements, you cannot do this with the C API.
SEEK_SET always refers to the absolute position in the file, which means if you want that command to work, you have to modify the file and remove the junk.
On linux you could write a FUSE driver that would present the file like it was starting from the 40th byte, but that's a lot of work. I'm only mentioned this because it's possible to solve the problem you've created, but it would be quite silly to actually do this.
The simplest thing of course would be just to abandon this emulating layer idea you're looking for, and write code that can handle that extra header junk.
If you want to remove the first 40 bytes of a file on the disk without creating another file, then you can copy the content from the 41th byte and onwards into a buffer, then write it back at offset -40. Then use ftruncate (a POSIX library in unistd.h) to truncate at (filesize - 40) offset.
I wrote a small code with what i understood from your question.
#include<stdio.h>
void readIt(FILE *afp)
{
char mystr[100];
while ( fgets (mystr , 100 , afp) != NULL )
puts (mystr);
}
int main()
{
FILE * dfp = NULL;
FILE * fp = fopen("h4.sql","r");
if(fp != NULL)
{
fseek(fp,10,SEEK_SET);
dfp = fp;
readIt(dfp);
fclose(fp);
}
}
The readIt() is reading the file from the 11 byte.
Is this what you are expecting or something else?
I haven't actually tried this, but I think you should be able to use mmap (with the MAP_SHARED option) to get your file mapped into your address space, and then fmemopen to get a FILE* that refers to all but the first 40 bytes of that buffer.
This gives you a FILE* (as you describe in the body of your question), but I believe not a file descriptor (as in the title and elsewhere in the question). The two are not the same, and AFAIK the FILE* created with fmemopen does not have an associated file descriptor.