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Those functions can read or write any type of data to files. In C, a file can refer to a disk file, a terminal, a printer, etc. That is, a file represents a concrete device with which you want to exchange information. Before you can operate on the file, you have to open that file.
After you finish the information exchange with it, you have to close that opened file. The data flow you transfer from a program to a file, or the other way around, is called a stream , which is a series of bytes or characters. In other words, all streams have the same behavior regardless of whatever device they are associated with.
There are two formats of streams. The first one is the text stream , which consists of text lines. The second format is the binary stream , which is a series of bytes that represent internal data such as numbers, structures or arrays. Binary streams are primarily used for non-textual data, where the appearance of the content inside the file is nor important it does not care if they are not seen as plain text.
A buffer is a memory area that is temporarily used to store data before it is sent to its destination. Since the smallest object that can be represented in C is a character, access to a file is permitted at any character or byte boundary. Any number of characters can be read or written from a movable point, known as the file position indicator. The characters are read, or written, in sequence from this point, and the position indicator moved accordingly.
The position indicator is initially set to the beginning of a file when it is opened, but can also be moved explicitly not just because a read or write has been performed. This section focuses on how to open and close a disk data file and how to interpret the error messages that can be returned by these two functions.
The FILE structure is the structure that controls files and it is defined in the header file stdio. The following defines a file pointer:. In the FILE structure there is a member which represents the file position indicator. The function fopen opens a file and associates a stream with that opened file. You need to specify the method of opening the file and the filename as arguments. Here filename is a char pointer that references a string containing a filename.
The fopen function returns a pointer of type FILE. If an error occurs during the procedure to open the file, the fopen function returns NULL. When you use the a character and the file already exists, the contents of the file will be preserved and new data that you write will be added to the end. If the file does not already exist, it will be created. This is in contrast to w ; this mode discards any data that may already be in the file if the file does not exist, it creates a new one.
When you use r , the file must already exist; if it does not, the call will fail and returns NULL. After a disk file is read, written, or appended with some new data, you have to disassociate the file from the specified stream. This is done by calling the fclose function:. If fclose closes a file successfully, it returns 0. Otherwise, the function returns EOF. Normally, this function fails only when the disk is removed before the function is called or there is no more space left on the disk.
Otherwise, the data saved in the file may be lost. In addition, failing to close a file when you are done with it may prevent other programs from accessing the file later. The next program shows how to open and close a text file, checking the values returned from the functions:. In order to open a file using its file descriptor fd , you must use the fdopen function. This function behaves as fopen , but instead of opening a file using its name or path, it uses the file descriptor:.
A way to get a file descriptor is with mkstemp:. This function generates a unique temporary file name from template string. The last six characters of template must be "XXXXXX" and these are replaced with a string that makes the filename unique. The file is then created for reading or writing. Since the string will be modified, template must not be a string constant, but should be declared as a character array. The mkstemp function returns the file descriptor of the temporary file or -1 on error.
Read or write one character or byte at a time, with functions like fgetc and fputc. Read or write one line of text that is, one character line at a time, with functions like fgets and fputs.
Read or write one block of characters or bytes at a time, with fread and fwrite. In this course we are going to focus in the last way, block reading and writing, which is quite useful both for text and binary files.
Here ptr is an array in which the data is stored. The fread function returns the number of elements that are actually read during the attempt, if an error occurs or an EOF is encountered. If we get an error, as the fread function does not distinguish between end-of-file and another type of error, you must use feof and ferror to determine what happened we will see it in a moment.
The syntax for the fwrite function is:. The function returns the number of elements actually written; therefore, if no error has occurred, the value returned by fwrite should equal the third argument in the function. The return value may be less than the specified value if an error occurs.
As we have seen before, we have the feof function to determine when the end of a file is encountered. This function returns 0 if the end of the file has not been reached; otherwise, it returns a nonzero integer. It returns 0 if no error has occurred; otherwise, it returns a nonzero integer. The following program read one block of characters at a time and write it in another file.
The reason is that you append a null character in line 35 after the last character read so that you ensure the block of characters saved in buffer is treated as a string and can be printed out on the screen properly by the printf function. In the last section you have learned how to read or write data sequentially to an opened disk file. However, in some other cases, you need to access particular data somewhere in the middle of a disk file.
The fseek function offers you the possibility to move the file position indicator to the spot you want to access in a file. If it is successful, the function returns 0 ; otherwise, the function returns a nonzero value.
You can obtain the current value of the file position indicator by calling the ftell function. The value returned by ftell represents the number of bytes from the beginning of the file to the current position pointed to by the file position indicator. If the function fails, it returns —1L that is, a long value of minus 1. Sometimes you might want to reset the file position indicator and put it at the beginning of a file. To do this you have the rewind function. There exists some further functions to manipulate files that can be useful.
The remove function removes a file given its filename. This function returns 0 if success; otherwise, it returns a nonzero value. The rename function changes the name of the file identified by oldname to newname. We have the possibility to move the file to another directory if we set a path into the newname , not just a filename:. If it fails for any reason, the original file is unaffected. Check with these questions that you understand how to use files in C.
Open an existing file to read and append data at the end. Creates a binary file to read and append data at the end. Whatever it happens with the file when is open, it has to be closed; therefore, the function call fclose fptr in line 11 must be located outside the if statement, after line The program tries to write in a file that has been opened just to be read. Code is correct, there is nothing wrong with it.
You open the file fptr , whose size is bytes. Which of these pairs of sentences is equivalent? You open the file fptr. You start reading byte by byte, till the end of file. At that point, which of these sentences is correct?:. The file position indicator has remained unchanged at the beginning of the file; it can just be moved with functions rewind or fseek.
The file position indicator is located at the end of the file; it has been moving byte by byte, while reading. The file position indicator has remained unchanged at the beginning of the file; it just moves when writing, not when reading.
Home Skip to content Skip to navigation. September - January Reading and writing with files. Table of Contents 1. What is a file? What is a stream? Pointers of FILE 2. Opening a file 2. Closing a file 2. Opening a file with a file descriptor 3.