C System Calls and FILE I/O library function

System calls are special types of functions available in some programming languages. They are used by programs to communicate directly with the operating system. The OS talks back to the program through the return value of the function. When a system call is made, the control is relinquished to the operating system to perform the system call and the program is blocked until the call has finished. We should always check the return value as this is the only method the operating system communicates with the program. All these functions are included under header file unistd.h, sys/stat.h and sys/types.h. The return value is an integer. Operation takes place on file descriptor.

There are more than 100 system calls implemented as a part of C library. A few of C system calls are:

1. close : This closes the file descriptor.
    Function Definition : int close(int fildes);
2. dup : It provides and alias for the provided file descriptor.
    Function Definition : int dup(int fildes);
3. dup2:  It provides and alias for the provided file descriptor and then deletes the old file descriptor.
    Function Definition : int dup2(int fildes);
4. fstat : It is used to determine information about a file based on its file descriptor.
    Function Definition : int dup(int fildes, struct stat *buf); The second parameter stores the data             about the file.
5. lseek : change the location of the read/write pointer of a file descriptor. The location can be set           either in absolute or relative terms.
    Function Definition: off_t lseek(int fildes, off_t offset, int whence);
    whence :The method in which the offset is to be interpreted (relative, absolute, etc.).
 

Value	Meaning
SEEK_SET	Offset is to be measured in absolute terms.
SEEK_CUR	Offset is to be measured relative to the current location of the pointer.
SEEK_END	Offset is to be measured relative to the end of the file.

6. lstat :  determine information about a file based on its filename.
    Function Definition: int lstat(const char *path, struct stat *buf);
7. open : open a new file and obtain its file descriptor.
    Function Definition:
                        int open(const char *path, int oflags);
                        int open(const char *path, int oflags, mode_t mode);
8. read : read data into a buffer.
    Function Definition : size_t read(int fildes, void *buf, size_t nbytes);
9. stat :  used to determine information about a file based on its file path.
    Function Definition:  int stat(const char *path, struct stat *buf);
10. write : used to write data out of a buffer.
    Function Definition : size_t write(int fildes, const void *buf, size_t nbytes);


In contrast to these library function for system calls, there are other similar library function which are used for file I/O in C.  The operation takes place on pointers of type FILE(i.e. streams) and an integer is returned They are:

1. fopen: opens the filename pointed to, by filename using the given mode.
    Function Definition :  FILE *fopen(const char *filename, const char *mode);
2. fprintf: sends formatted output to a stream.
    Function Definition : int fprintf(FILE *stream, const char *format, ...)
3. fscanf:  reads formatted input from a stream.
    Function Definition: int fscanf(FILE *stream, const char *format, ...)
4. fputc: writes a character (an unsigned char) specified by the argument char to the specified stream     and advances the position indicator for the stream.
    Function Definition : int fputc(int char, FILE *stream)
5. fgetc: gets the next character (an unsigned char) from the specified stream and advances the               position indicator for the stream.
    Function Definition: int fgetc(FILE *stream)

Two other functions are used for I/O operations on binary files i.e. streams :

1. fread : reads data from the given stream into the array pointed to, by ptr.
    Function Definition : size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)
2. fwrite :  writes data from the array pointed to, by ptr to the given stream.
    size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)