CSI 402 Lecture 2 Working with Files (Text and Binary)
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1 CSI 402 Lecture 2 Working with Files (Text and Binary) 1 / 30
2 AQuickReviewofStandardI/O Recall that #include <stdioh> allows use of printf and scanf functions Example: int i; scanf("%d", &i); printf("value of i = %d\n", i); Bu ered I/O: printf("reached Point A\n"); The message may not appear right away 6 2 / 19
3 Review of standard I/O (continued) Bu ering : Used for e ciency Program Memory Buffer Device Bu er has finite capacity Output written to device when bu er is full Reduces synchronization overhead between CPU and device Flushing the bu er: printf("reached Point A\n"); fflush(stdout); Note: The output device stdout is bu ered 6 3 / 19
4 Introduction to File I/O Ref: Chapter 11 of Deitel & Deitel Notes: A file is a sequence of bytes File operations are normally bu ered for e ciency reasons Declaring file variables: FILE *f, *g; Remarks: Variables f and g are file pointers The type FILE is defined in stdioh 6 6 / 19
5 Introduction to File I/O (continued) Opening a file: Function fopen with the following prototype: FILE * fopen (const char *filename, const char *mode); Some common modes: "r" :Read "w" :Write "a" :Append Function returns pointer to file if successful; otherwise, returns NULL Important Note: Always check the pointer value returned by fopen for NULL 6 7 / 19
6 Descriptions of modes (a) Read: The file is opened for reading; can t be written The file must exist; otherwise, the call to fopen fails (b) Write: The file is opened for writing; it can t be read If the file exists, it is truncated; otherwise, a new (empty) file is created (c) Append: Allows for writing at the end of the file If the file exists, it is opened for appending; otherwise, a new (empty) file is created 6 8 / 19
7 Code segment using fopen #include <stdioh> #include <stdlibh> FILE *f; if ((f = fopen("indat", "r")) == NULL){ /* File open failed */ printf("can t open indat\n"); exit(1); } else { /* File opened successfully */ } Note: We need <stdlibh> to use the exit function 6 9 / 19
8 Closing a File Function fclose Prototype: int fclose (FILE *f); Flushes the file bu er before closing it Function returns 0 if successful; otherwise, returns EOF Good Programming Practice In your programs, close each file explicitly 6 10 / 19
9 Review of fscanf (a) Function fscanf: Similar to scanf (used for reading from stdin) Parameters: First parameter: Pointer to an input file (The file must be open for reading) Second parameter: String specifying formats (as in scanf) Subsequent parameters: Pointers to variables into which values will be read from the file Returns a negative integer if an error occurs while writing to the file; otherwise, returns the number of characters written to the output file 9 / 30
10 Review of fprintf (b) Function fprintf: Similar to printf (used for writing to stdout) Parameters: First parameter: Pointer to an output file (The file must be open for writing) Second parameter: String specifying formats (as in printf) Subsequent parameters: Expressions whose values must be written to the file (as in printf) Returns a negative integer if an error occurs while writing to the file; otherwise, returns the number of characters written to the output file 10 / 30
11 Additional Remarks (a) Flushing the bu er for files: Write operations to files are bu ered for e We can use fflush for files also ciency reasons Example: int x; FILE *outfile; fprintf(outfile, "Output = fflush(outfile); %d\n", x); 6 15 / 19
12 Additional Remarks (continued) Writing to stderr: Use fprintf to write to stderr Example: #define MAX_LEN 20 char fname[20]; fprintf(stderr, "Fatal: Can t open file = %s\n", fname); exit(1); 6 16 / 19
13 Positioning in Files Ref: Chapter 11 of Deitel & Deitel EOF File Offset Input files: File offset gives the number of the byte to be read next It is set to zero when file is opened (using "r" mode) Value of file offset increases as bytes are read from file 2 3 / 20
14 Positioning in Files (continued) Output files: File offset gives the number of the byte to be written next It is set to zero when file is opened (with mode "w") File offset increases as bytes are written to file Note: For both input and output files, the current value of file offset can be obtained using ftell function 2 4 / 20
15 Library Function ftell Part of stdioh Prototype: long ftell(file *fp) Returns the offset for the file specified by fp; returns -1L in case of error Example: FILE *fp; long pos; /* Open file, etc */ pos = ftell(fp); printf("offset = %ld\n", pos); 2 5 / 20
16 Library Function fseek Also part of stdioh To move around in a file Prototype: int fseek (FILE *fp, long offset, int origin) fp specifies the (input or output) file offset (which may be negative) specifies the amount of movement How offset is used depends on the parameter origin 2 6 / 20
17 Library Function fseek (continued) Parameter origin can have any of the following three values (constants) SEEK_SET: offset specified relative to the beginning of the file SEEK_CUR: offset specified relative to the current position SEEK_END: offset specified relative to the end of the file Function fseek returns 0 if successful and a non-zero value otherwise 2 7 / 20
18 A Related Function: rewind Part of stdioh Prototype: int rewind (FILE *fp) Sets file offset to 0 (ie, gets us back to the beginning of a file) rewind(fp) is equivalent to fseek(fp, 0L, SEEK_SET); Program Example: Handout / 20
19 Moving Outside File Boundary Function fseek allows any offset value; it doesn t check whether specified move is within the file For illegal moves, effect is implementation dependent On most Unix systems: Function fseek does not move the offset value below the beginning of the file File offset can be changed to a value beyond the end of file However, trying to read from a non-existent position produces EOF For an output file, fseek allows forward jumps ; positions where nothing was written contain \0 2 9 / 20
20 Random Access Files Random Access: Example: For files: Access time is independent of position Array : Provides random access List : Does not provide random access (Provides sequential access) Random access: Fast access Applications: Airline reservation systems, Banking systems, etc 2 10 / 20
21 Random Access Files in C No explicit support (Functions fread and fwrite from stdioh are used) Common method: Make all records to be of the same size Example: Size of each record = 50 bytes Starting position of Record i = (i 1) Size of record 2 11 / 20
22 Formatted and Unformatted Files Formatted Files: Also called text files; they can be viewed/edited using a standard text editor Can be produced by a C program using formatted write (ie, using fprintf) Example for formatted write: FILE *ofp; int num = -25; fprintf(ofp, "%d", num); No of bytes written to the file = 3 Note: The number of bytes written to the output file depends on the value of the integer 2 12 / 20
23 Formatted and Unformatted Files (continued) Unformatted files: Also called binary files; they cannot be viewed/edited using standard text editors To produce unformatted files, C program must use unformatted write using the fwrite function Function fwrite: Prototype: size_t fwrite (const void *p, size_t size, size_t nent, FILE *fp) Writes bytes from memory to a file p: Gives the starting address in memory 2 13 / 20
24 Description of fwrite (continued) size: nent: Gives the size (ie, number of bytes) of each entry Gives the number of entries to be written fp: Pointer to the output file Writes the specified number of entries (starting from the specified memory address) to the output file Returns the number of entries written (If this value is less than nent, it is an indication of error) 2 14 / 20
25 Description of fwrite (continued) Some Technicalities: FILE *ofp; int num = -25; &num: Starting address of num (Type: int *) (const void *) &num: Type casts address to const void * sizeof(num): Size of the entry (ie, no of bytes) to be written No of entries to be written: 1 Now, the call to fwrite is as follows: fwrite((const void *) &num, sizeof(num), 1, ofp); Note: We must check the return value of fwrite to ensure that no errors occurred 2 15 / 20
26 Difference Between fprintf and fwrite Example: (Assume int uses 4 bytes) int num = -2017; FILE *out_f1, *out_f2; -- Open file out_f1 (out1fmt) Open file out_f2 (out2ufmt) -- /* Formatted write */ fprintf(out_f1, "%d", num); /* Unformatted write */ fwrite((const void *) &num, sizeof(num), 1, out_f2); -- Close files -- File out1fmt: Size = 5 bytes A text file: can be examined/edited using a text editor Can be read using fscanf 2 16 / 20
27 Difference Between fprintf and fwrite (continued) File out2ufmt: Size = 4 bytes A binary file: cannot be examined using a text editor Can be read from using fread (a function for reading unformatted files) Formatted Read: Uses fscanf FILE *ifp; int num; fscanf(ifp, "%d", &num); Unformatted read: Uses fread 2 17 / 20
28 Description of Function fread Prototype: size_t fread (void *p, size_t size, size_t nent, FILE *fp) Reads bytes from file into memory p: Gives the starting address for reading into memory size: read nent: fp: Gives the size (ie, number of bytes) of each entry to be Gives the number of entries to be read Pointer to the input file Reads the specified number of entries from the input file into memory starting from the specified memory address Returns the number of entries read (If this value is less than nent, it is an indication of error) 2 18 / 20
29 Program Examples 1 Creating a random access file: Handout 22 2 Writing to a random access file: Handout 23 3 Reading from a random access file: Handout 24 Examples of Binary Files: Compiled versions of C programs (ie, files with extension o ) Executable versions of C programs (eg file aout ) Compressed files File archives (eg files created using tar command in Unix) 2 19 / 20
30 Suggested Exercises 1 Study Handout 21 carefully to understand the use of functions fseek and ftell 2 Study Handouts 22, 23 and 24 carefully to understand the use of functions fread and fwrite 3 Study the other program examples in Chapter 11 of Deitel & Deitel 2 20 / 20
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