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.
Related
I have a question about the SO question What is the difference between read() and fread()?
How do read() and fread() work? And what do unbuffered read and buffered read mean?
If fread() is implemented by calling read(), why do some people say fread() is faster than read() in the SO question C — fopen() vs open()?
fread cannot get faster than read provided (!) you use the same buffer size for read as fread does internally.
However every disk access comes with quite some overhead, so you improve performance if you minimise their number.
If you read small chunks of data then every read accesses the disk directly, thus you get slow, while in contrast fread profits from its buffer as long as there's yet data in – and on being consumed up the next large chunk is read into the buffer at once to again provide small chunks from on being called again.
What's the difference between read and fread?
fread is a standardized C programming language function that works with a FILE pointer.
read is a C function available on a POSIX compatible system that works with a file descriptor.
how do read() and fread() work?
fread calls some unknown system defined API to read from a file. To know how specific fread works you have to look at specific implmeentation of fread for your specific system - windows fread is very different from Linux fread. Here's implementation of fread as part of glibc library https://github.com/lattera/glibc/blob/master/libio/iofread.c#L30 .
read() calls read system call. https://man7.org/linux/man-pages/man2/syscalls.2.html
what do unbuffered read and buffered read mean?
fread reads data to some intermediate buffer, then copies from that buffer to the memory you passed as argument.
read makes kernel to copy straight to the buffer you passed as argument. There is no intermediate buffer.
why does some say fread() is faster than read() in this topic?
The assumption is that calling system calls is costly - what kernel does inside the system call and system call itself is costly.
Reading the data in a big like 4 kilobyte chunk into an intermediate buffer once and then reading from that buffer in small chunks within your program is faster than context switching to kernel every time to read a small chunks of data so that kernel will do repeatedly a small input/output operation to fetch the data.
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.
I'm fairly new to c so bear with me.
How do I go about finding out the number of chars read/write reads?
Can I be more specific and designate the # of chars read/write reads in an argument? If so, how?
From man(2) read:
If successful, the number of bytes actually read is returned
From man(2) write:
Upon successful completion the number of bytes which were written is returned
Now concerning:
Can I be more specific and designate the # of chars read/write reads in an argument? If so, how?
AFAIK no, but there might be some device/kernel specific ways using for example ioctl(2)
C and C++ has different IO libraries. I guess you are coding in C.
fprintf(3) returns (when successful) the number of printed characters.
scanf(3) returns the number of successfully read items, but also accept the %n specifier:
n Nothing is expected; instead, the number of characters
consumed thus far from the input is stored through the next
pointer, which must be a pointer to int.
You could also do IO line by line... (getline, snprintf, sscanf, fputs ....)
for Linux and Posix
If you call directly the read(2) or write(2) functions (i.e. syscalls) they return the number of input or output bytes on success.
And you could use the lseek(2) syscall, or the ftell(3) <stdio.h> function, to query the current file offset (which has no meaning so would fail on non-seekable files like pipes, sockets, FIFOs, ...).
See also FIONREAD
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.
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().