I want to read from a file by using its file descriptor. I can't use its name because of assignment rules.
I obtain it by calling open and it works fine. At this moment I know that I have to use the read() function in order to read from it. My problem is that read() function requires as an argument the number of bytes to read, and I want to read a whole line from the file each time, so I don't know how many bytes to read.
If i use for example fscanf(), it works fine with a simple string and I take back the whole line as I want. So my question is:
Is there any function like fscanf() which can be called with file descriptor and not with a file pointer?
When you say "have to use read()" I can't tell if that's your understanding of the situation given a file descriptor from open() or a restriction on some kind of assignment.
If you have a file descriptor but you're more comfortable with fscanf() and friends, use fdopen() to get a FILE * from your fd and proceed to use stdio.
Internally it uses functions like read() into a buffer and then processes those buffers as you read them with fscanf() and friends.
What you could do is read one character at a time, until you've read the entire line, and detect a '/n'. As this is homework, I won't write it for you.
A few things to be warned of, however.
You need to check for EOF, otherwise, you might end up in an infinite loop.
You should declare some buffer which you read a character, then copy it into the buffer. Not knowing what your input is, I can't suggest a size, other than to say that for a homework assignment, [256] would probably be sufficient.
You need to make sure you don't overfill your buffer in the even that you do run over it's length.
Keep reading until you find a '/n' character. Then process the line that you have created, and start the next one.
Related
Are the buffer of a C input and output different ? I am trying to implement buffering emulation in assembly and trying to do it as the C one does. I have so far implemented buffering system in my fgets function, however, I am not sure how I should implement it in case of fputs function. If the "buffer" are same, then it does make sense to implement a global variable which will contain the file descriptor last used so that in the case of a "fputs, fgets, fputs" operation, I can use the last file descriptor to flush out the buffer before reading in case of fgets. But this method also seems very costy, as I have to flush out the buffer every time regardless of the fact that I called a fputs function before. Or should I use two buffer for inputting and outputting ?
Or should I use two buffer for inputting and outputting ?
Suggest using one buffer per file handle; that should cover the common use cases — rather than buffering based on i/o direction.
How do C functions like fscanf() and fgets() remember where in the file to start reading from? For instance, when reading a file using fscanf(), it seems to remember where it last left terminated, rather than starting from the beginning of the file again. How exactly does this work?
The FILE * parameter points to a buffer and a file handle (see the fileno() function).
The actual where is remembered in the kernel in the file structure.
There is a legend that the FILE * pointer points into the file. This is not literally true, but it might as well be true for the interpretation of the beginning programmer.
In fact what happens is as follows: Every process has an array in kernel of type struct file (this type is not defined in userspace so don't go looking for it) that contains all of its open files. A handle is returned by the open() syscall that is merely an index into the array. The function fileno() retrieves the handle from the FILE * pointer returned by fopen() and can be manipulated directly. This is usually a bad idea except for accessing ioctl() or fctl() as you will end up fighting with the internal buffer in the FILE object.
One of the members of struct file is loff_t f_pos which is the exact location in bytes the kernel read() or write() stopped at. This is buffered in FILE which knows how many bytes it read ahead or pended for a later write for you.
I have an existing program where a message (for example, an email, or some other kind of message) will be coming into a program on stdin.
I know stdin is a FILE* but I'm somewhat confused as to what other special characteristics it has. I'm currently trying to add a check to the program, and handle the message differently if it contains a particular line (say, the word "hello"). The problem is, I need to search through the file for that word, but I still need stdin to point to its original location later in the program. An outline of the structure is below:
Currently:
//actual message body is coming in on stdin
read_message(char type)
{
//checks and setup
if(type == 'm')
{
//when it reaches this point, nothing has touched stdin
open_and_read(); //it will read from stdin
}
//else, never open the message
}
I want to add another check, but where I have to search the message body.
Like so:
//actual message body is coming in on stdin
read_message(char type)
{
//checks and setup
//new check
if(message_contains_hello()) //some function that reads through the message looking for the word hello
{
other_functionality();
}
if(type == 'm')
{
//when it reaches this point, my new check may have modified stdin
open_and_read(); //it will read from stdin
}
//else, never open the message
}
The problem with this is that to search the message body, I have to touch the file pointer stdin. But, if I still need to open and read the message in the second if statement (if type = 'm'), stdin needs to point to the same place it was pointing at the start of the program. I tried creating a copy of the pointer but was only successful in creating a copy that would also modify stdin if modified itself.
I don't have a choice about how to pass the message - it has to stay on stdin. How can I access the actual body of a message coming in on stdin without modifying stdin itself?
Basically, how can I read from it, and then have another function be able to read from the beginning of the message as well?
The short answer is that you can't. Once you read data from standard input, it's gone.
As such, your only real choice is to save what you read, and do the later processing on that rather than reading directly from standard input. If your later processing demands reading from a file, one possibility would be to structure this as two separate programs, with one acting as a filter for the other.
In general, you can only read bytes from stdin once. There is no fseek() functionality. To solve this problem, you can read the bytes into a buffer in your program, look at the bytes, and then pass the buffer off to another function that actually does something with the rest of the data.
Depending on your program, you may need to only read some of the data on stdin, or you may need to read all of it into that buffer. Either way, you will probably have to modify the existing code in the program in some way.
I know stdin is a FILE* but I'm somewhat confused as to what other special characteristics it has.
Well, it's opened for reading. But it's not guaranteed to be seekable, so you'll want to read in its contents entirely, then handle the resulting string (or list of strings, or whatever).
You should use and take advantage of buffering (<stdio.h> provides buffered I/O, but see setbuf).
My suggestion is to read your stdin line by line, e.g. using getline. Once you've read an entire line, you can do some minimal look-ahead inside.
Perhaps you might read more about parsing techniques.
I am struggling to know the difference between these functions. Which one of them can be used if i want to read one character at a time.
fread()
read()
getc()
Depending on how you want to do it you can use any of those functions.
The easier to use would probably be fgetc().
fread() : read a block of data from a stream (documentation)
read() : posix implementation of fread() (documentation)
getc() : get a character from a stream (documentation). Please consider using fgetc() (doc)instead since it's kind of saffer.
fread() is a standard C function for reading blocks of binary data from a file.
read() is a POSIX function for doing the same.
getc() is a standard C function (a macro, actually) for reading a single character from a file - i.e., it's what you are looking for.
In addition to the other answers, also note that read is unbuffered method to read from a file. fread provides an internal buffer and reading is buffered. The buffer size is determined by you. Also each time you call read a system call occurs which reads the amount of bytes you told it to. Where as with fread it will read a chunk in the internal buffer and return you only the bytes you need. For each call on fread it will first check if it can provide you with more data from the buffer, if not it makes a system call (read) and gets a chunk more data and returns you only the portion you wanted.
Also read directly handles the file descriptor number, where fread needs the file to be opened as a FILE pointer.
The answer depends on what you mean by "one character at a time".
If you want to ensure that only one character is consumed from the underlying file descriptor (which may refer to a non-seekable object like a pipe, socket, or terminal device) then the only solution is to use read with a length of 1. If you use strace (or similar) to monitor a shell script using the shell command read, you'll see that it repeatedly calls read with a length of 1. Otherwise it would risk reading too many bytes (past the newline it's looking for) and having subsequent processes fail to see the data on the "next line".
On the other hand, if the only program that should be performing further reads is your program itself, fread or getc will work just fine. Note that getc should be a lot faster than fread if you're just reading a single byte.
What I need to do is use the read function from unistd.h to read a file
line by line. I have this at the moment:
n = read(fd, str, size);
However, this reads to the end of the file, or up to size number of bytes.
Is there a way that I can make it read one line at a time, stopping at a newline?
The lines are all of variable length.
I am allowed only these two header files:
#include <unistd.h>
#include <fcntl.h>
The point of the exercise is to read in a file line by line, and
output each line as it's read in. Basically, to mimic the fgets()
and fputs() functions.
You can read character by character into a buffer and check for the linebreak symbols (\r\n for Windows and \n for Unix systems).
You'll want to create a buffer twice the length of your longest line you'll support, and you'll need to keep track of your buffer state.
Basically, each time you're called for a new line you'll scan from your current buffer position looking for an end-of-line marker. If you find one, good, that's your line. Update your buffer pointers and return.
If you hit your maxlength then you return a truncated line and change your state to discard. Next time you're called you need to discard up to the next end of line, and then enter your normal read state.
If you hit the end of what you've read in, then you need to read in another maxline chars, wrapping to the start of the buffer if you hit the bottom (ie, you may need to make two read calls) and then continue scanning.
All of the above assumes you can set a max line length. If you can't then you have to work with dynamic memory and worry about what happens if a buffer malloc fails. Also, you'll need to always check the results of the read in case you've hit the end of the file while reading into your buffer.
Unfortunately the read function isn't really suitable for this sort of input. Assuming this is some sort of artificial requirement from interview/homework/exercise, you can attempt to simulate line-based input by reading the file in chunks and splitting it on the newline character yourself, maintaining state in some way between calls. You can get away with a static position indicator if you carefully document the function's use.
This is a good question, but allowing only the read function doesn't help! :P
Loop read calls to get a fixed number of bytes, and search the '\n' character, then return a part of the string (untill '\n'), and stores the rest (except '\n') to prepend to the next character file chunk.
Use dynamic memory.
Greater the size of the buffer, less read calls used (which is a system call, so no cheap but nowadays there are preemptive kernels).
...
Or simply fix a maximum line length, and use fgets, if you need to be quick...
If you need to read exactly 1 line (and not overstep) using read(), the only generally-applicable way to do that is by reading 1 byte at a time and looping until you get a newline byte. However, if your file descriptor refers to a terminal and it's in the default (canonical) mode, read will wait for a newline and return less than the requested size as soon as a line is available. It may however return more than one line, if data arrives very quickly, or less than 1 line if your program's buffer or the internal terminal buffer is shorter than the line length.
Unless you really need to avoid overstep (which is sometimes important, if you want another process/program to inherit the file descriptor and be able to pick up reading where you left off), I would suggest using stdio functions or your own buffering system. Using read for line-based or byte-by-byte IO is very painful and hard to get right.
Well, it will read line-by-line from a terminal.
Some choices you have are:
Write a function that uses read when it runs out of data but only returns one line at a time to the caller
Use the function in the library that does exactly that: fgets().
Read only one byte at a time, so you don't go too far.
If you open the file in text mode then Windows "\r\n" will be silently translated to "\n" as the file is read.
If you are on Unix you can use the non-standard1 gcc 'getline()' function.
1 The getline() function is standard in POSIX 2008.
Convert file descriptor to FILE pointer.
FILE* fp = fdopen(fd, "r");
Then you can use getline().