Lecture 4 - More on File I/O

Question 1

access()

Answer 1

• int access(char *file_name, int mode);
• used to check that a file can be opened with the specified mode
• modes: R_OK (check for read access), W_OK (check for write access), F_OK (check for existence)
• returns zero on success
• MUST #include <unistd.h> for access</unistd.h>

Question 2

feof()

Answer 2

• when we’re reading from a file, we need to determine when we’ve hit the end
• returns nonzero if we have hit EOF

Question 3

how to check if you ran out of disk space when writing a file

Answer 3

• ferror() looks for various errors (including EOF)
• call it after every read or write to figure out if something bad happened
• If you can correct it, use clearerr() to clear the file pointer’s internal error flag

Question 4

%[]

Answer 4

• used in scanf() to match and store a set of character from input into a string
• put list of characters that it should accept in-between (ex. 0-9, A-Z, 0-9A-z, %29s)

Question 5

fscanf() return values

Answer 5

• will return the number of variables it successfully scanned or -1 if we’ve hit the end of file
• may return 0 if there’s a blank line at the end of the file
• to catch any error, check if the return value is less than the number of variables scanned

Question 6

ftell()

Answer 6

• long ftell (FILE *stream)
• returns the current file position indicator of a file stream (how far you’ve read/written
• returns current position in the file in bytes or -1L if an error occurs

Question 7

fseek()

Answer 7

• int fseek(FILE *stram, long offset, int whence);
• a pointer to a FILE object that identifies the file stream
• the offset (number of bytes) to move to the file (can be negative)
• returns 0 if successful, -1 if there’s an error

Question 8

fgets()

Answer 8

• char *fgets(char *str, int n, FILE *stream);
• used to read a line of text from a file or standard input
• *str is pointer where the read string will be stored
• n is the max num characters to read including \0
• stream is the stream to read from
• returns str on success, NULL on failure

Question 9

fputs()

Answer 9

• int fputs(const char *str, FILE *stream);
• functions that write a string to a file or stdout
• str is string to be written, the stream is what it is written to
• returns non negative number on success, -1 on failure

Question 10

assert()

Answer 10

• used in debugging to catch logical error by checking conditions at runtime
• if condition is false, assert() prints an error message and terminates the program