C fseek ftell

The fseek function enables you to treat a file like an array and move directly to any particular byte in a file opened by fopen. To see how it works, let's create a program see Listing Borrowing from the earlier examples, it uses a command-line argument to get the name of the file it will read.

Note that fseek has three arguments and returns an int value. The ftell function returns the current position in a file as a long value. If you run the program from a command-line environment, this program expects the filename to be in the same directory or folder as the executable program. If you run the program from an IDE, the program looks depend on the implementation. We need to discuss three topics: how fseek and ftell work, how to use a binary stream, and how to make the program portable.

The first of the three arguments to fseek is a FILE pointer to the file being searched. The file should have been opened by using fopen. The second argument to fseek is called the offset.

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This argument tells how far to move from the starting point see the following list of mode starting points. The argument must be a long value. It can be positive move forwardnegative move backwardor zero stay put. The third argument is the mode, and it identifies the starting point. Under ANSI, the stdio.

Older implementations may lack these definitions and, instead, use the numeric values 0L1Land 2Lrespectively, for these modes. Recall that the L suffix identifies type long values, or the implementation might have the constants defined in a different header file.

When in doubt, consult your usage manual or the online manual. The value returned by fseek is if everything is okay, and -1 if there is an error, such as attempting to move past the bounds of the file.

The ftell function is type longand it returns the current file location. Under ANSI, it is declared in stdio.Programmer can seek to any offset of an array by changing offset.

fseek ftell 使い方

However seeking to a file offset is not the same as the size of a file often too large to store in primary memory. File functions works on current file position. This position can be changed by fseek. Programmer need to provide a desired offset and origin. About our authors : Team EQA.

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c fseek ftell

How to seek to a position of a file? How to know current position? How to rewind position back to beginning?

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Overview file is like byte array located in secondary memory device. FSEEK Fseek allows programmer to change current file position to a particulate offset with reference to origin.

It is specified by one of the following constants.

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Otherwise, it returns non-zero value. If a read or write error occurs, the error indicator ferror is set. Return Value On success, the current value of the position indicator is returned.

On failure, -1L is returned, and errno is set to a system-specific positive value. Draw on printer Memory leaks.There is no need to read each record sequentially, if we want to access a particular record. C supports these functions for random access file processing.

Syntax: fseek file pointer, displacement, pointer position ; Where file pointer It is the pointer which points to the file.

fseek, _fseeki64

This is the number of bytes which are skipped backward if negative or forward if positive from the current position. This is attached with L because this is a long integer. Pointer position: This sets the pointer position in the file.

c fseek ftell

From this statement pointer position is skipped 5 bytes forward from the current position. This function returns the value of the current pointer position in the file.

The value is count from the beginning of the file. Syntax: ftell fptr ; Where fptr is a file pointer. This function is used to move the file pointer to the beginning of the given file. Syntax: rewind fptr ; Where fptr is a file pointer. Random Access To File There is no need to read each record sequentially, if we want to access a particular record. Value pointer position 0 Beginning of file. Previous Next.Join Stack Overflow to learn, share knowledge, and build your career.

Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. If ftell f tells us the current file position, the size here should be the offset from the end of the file to the beginning. Should not ftell f only give us the position of the end of the file?

File positions are like the cursor in a text entry widget: they are in between the bytes of the file. This is maybe easiest to understand if I draw a picture:. This is a hypothetical file. It contains four characters: abcand d. Each character gets a little box to itself, which we call a "byte".

The fifth box has been crossed out because it's not part of the file yet, but but if you appended a fifth character to the file it would spring into existence. The valid file positions in this file are 0, 1, 2, 3, and 4.

c fseek ftell

There are five of them, not four; they correspond to the vertical lines before, after, and in between the boxes. When you open the file assuming you don't use "a"you start out on position 0, the line before the first byte in the file. When you seek to the end, you arrive at position 4, the line after the last byte in the file.

Because we start counting from zero, this is also the number of bytes in the file. This is one of the several reasons why we start counting from zero, rather than one.

In no particular order:. On Windows, if you open a file in text mode, the numbers you get from ftell on that file are not byte offsets from the beginning of the file; they are more like fgetpos cookies, that can only be used in a subsequent call to fseek.

If you need to seek around in a text file on Windows you may be better off opening the file in binary mode and dealing with both DOS and Unix line endings yourself — this is actually my recommendation for production code in general, because it's perfectly possible to have a file with DOS line endings on a Unix system, or vice versa.

Adventures in Random Access: fseek() and ftell()

I don't know what the Windows equivalent is. If your file contains characters that are beyond ASCII, then the number of bytes in the file is very likely to be different from the number of characters in the file.

If you are using C's "wide streams" and "wide characters", then, just like text streams on Windows, the numbers you get from ftell on that file are not byte offsets and may not be useful for anything other than subsequent calls to fseek. But wide streams and characters are a bad design anyway; you're actually more likely to be able to handle all the world's languages correctly if you stick to processing UTF-8 by hand in narrow streams and characters.

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It only works because an implementation defines it to work. POSIX does, for one.

c fseek ftell

Windows does, also, for binary streams - but not for text streams. It's wrong to not add a caveat or warning to, "This is how you get the number of bytes in a file. I've seen it. For a binary stream, 7.Join Stack Overflow to learn, share knowledge, and build your career. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

How can I find out the size of a file I opened with an application written in C? I would like to know the size, because I want to put the content of the loaded file into a string, which I allocate using malloc. You can use stat if you know the filenameor fstat if you have the file descriptor.

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If you have the file descriptor fstat returns a stat structure which contain the file size. Have you considered not computing the file size and just growing the array if necessary? Here's an example with error checking ommitted :. This has the advantage of working even for streams in which it is impossible to get the file size like stdin. It handles all the code for loading a file, allocating memory, and handling errors. How are we doing? Please help us improve Stack Overflow.

Take our short survey. Learn more. How can I get a file's size in C? Asked 12 years, 3 months ago. Active 1 year ago. Viewed k times. Improve this question. Matteo Ugolotti 3 3 silver badges 11 11 bronze badges. Nino Nino 4, 4 4 gold badges 20 20 silver badges 15 15 bronze badges. Note that sizeof char is 1, by definition. Ya, but some esoteric platform's compiler might define char as 2 bytes - then the program allocates more than is necessary.

One can never be too sure. George an "esoteric platform's compiler" where sizeof char!As we all know that C is one of the commonly used programming languages. It also deals with working in the files and performing all the operations related to the files. C provides inbuilt library functions that are ready to use by importing the library in the program and following their syntax accordingly. Once the file pointer is moved to a specified offset, we can perform any operation like writing, reading from the file according to the programmer requirements.

So fseek is used with its all 3 arguments along with the other file functions to perform their respective tasks. It is basically a C function used for handling the file operations. By using the fseek function in C, the desired read and write operations can be performed by locating the file pointer and moving it accordingly. First, we are including the stdio. First, the file is opened using the fopen function and the file pointer is returned by it.

Now the fseek function is used to move the file pointer to the end of the file. In order to isert the data in the file, fputs function is used with the string which needs to be inserted. Now the fseek function is used in order to move the file pointer 12 characters ahead in the file. After performing all the operations, fclose function is used to close the file. The above description clearly explains the fseek function in C and how it is used in C language.

If the fseek operation is successful, it returns a zero value otherwise it would return a non-zero value. As working with the files is an important aspect of C language, it also offers various inbuilt functions like fopenfclosegetwputw. This is a guide to fseek in C.

Here we discuss an introduction to fseek in C, syntax, parameters, how does it work with programming examples. You can also go through our other related articles to learn more —. This website or its third-party tools use cookies, which are necessary to its functioning and required to achieve the purposes illustrated in the cookie policy.

By closing this banner, scrolling this page, clicking a link or continuing to browse otherwise, you agree to our Privacy Policy. Forgot Password? Popular Course in this category. Course Price View Course. Free Software Development Course. Login details for this Free course will be emailed to you. Email ID. Contact No.Understanding the difference between text mode and binary mode is important when using functions that operate on file streams.

See FIOC. Understand the difference between text mode and binary mode with file streams for more information. Subclause 7. Seeking to the end of a binary stream in binary mode with fseek is not meaningfully supported and is not a recommended method for computing the size of a file.

For a text stream, its file position indicator contains unspecified information, usable by the fseek function for returning the file position indicator for the stream to its position at the time of the ftell call. Consequently, the return value of ftell for streams opened in text mode should never be used for offset calculations other than in calls to fseek. The character ' b ' shall have no effect, but is allowed for ISO C standard conformance. For each object, size calls shall be made to the fputc function, taking the values in order from an array of unsigned char exactly overlaying the object.

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The file-position indicator for the stream if defined shall be advanced by the number of bytes successfully written. This means that the file position indicator, and consequently the file size, is directly based on the number of bytes actually written to a file. This code example attempts to open a binary file in binary mode and use fseek and ftell to obtain the file size.

On these systems, setting the file position indicator to the end of the file using fseek is not guaranteed to work for a binary stream, and consequently, the amount of memory allocated may be incorrect, leading to a potential vulnerability. This solution works only with regular files. This compliant solution uses the Windows GetFileSizeEx function to determine the size of the file on a or bit operating system:. This noncompliant code example attempts to open a text file in text mode and use fseek and ftell to obtain the file size:.

However, the file position indicator returned by ftell with a file opened in text mode is useful only in calls to fseek. Because binary files are treated the same as text files in POSIX, either compliant solution can be used for determining the size of a binary file under POSIX to determine the size of a text file as well. Understanding the difference between text mode and binary mode with file streams is critical when working with functions that operate on them.

Setting the file position indicator to end-of-file with fseek has undefined behavior for a binary stream. In addition, the return value of ftell for streams opened in text mode is useful only in calls to fseeknot for determining file sizes or for any other use.

As such, fstat or other platform-equivalent functions should be used to determine the size of a file. The POSIX standard mentions nothing about fstat ever failing to provide a valid file size although fstat can return -1 if it has a general failure, such as if it is given an invalid file descriptor. Not that this guarantees anything Do you have any more evidence of fstat returning an invalid size while still indicating success? And for an infinite file fstat will not work.


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