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The following program erases the file specified on the command line However, it first gives you a chance to change your mind A utility like this might be useful for new computer users
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/* Double check before erasing */ #include <stdioh> #include <stdlibh> #include <ctypeh> int main(int argc, char *argv[]) { char str[80]; if(argc!=2) { printf(''usage: xerase <filename>\n"); exit(1); } printf("Erase %s (Y/N): ", argv[1]); gets(str); if(toupper(*str)=='Y') if(remove(argv[1])) { printf("Cannot erase file\n"); exit(1); } return 0; }
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Flushing a Stream If you wish to flush the contents of an output stream, use the fflush( ) function, whose prototype is shown here: int fflush(FILE *fp); This function writes the contents of any buffered data to the file associated with fp If you call fflush ( ) with fp being null, all files opened for output are flushed The fflush( ) function returns zero if successful; otherwise, it returns EOF
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Page 245
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fread( ) and fwrite( ) To read and write data types that are longer than 1 byte, the C file system provides two functions: fread( ) and fwrite( ) These functions allow the reading and writing of blocks of any type of data Their prototypes are size_t fread(void *buffer, size_t num_bytes, size_t count, FILE *fp); size_t fwrite(const void *buffer, size_t num_bytes, size_t count, FILE *fp); For fread( ), buffer is a pointer to a region of memory that will receive the data from the file For fwrite( ), buffer is a pointer to the information that will be written to the file The value of count determines how many items are read or written, with each item being num_bytes bytes in length (Remember, the type size_t is defined as some kind of unsigned integer) Finally, fp is a file pointer to a previously opened stream The fread( ) function returns the number of items read This value may be less than count if the end of the file is reached or an error occurs The fwrite( ) function returns the number of items written This value will equal count unless an error occurs Using fread( ) and fwrite( ) As long as the file has been opened for binary data, fread( ) and fwrite( ) can read and write any type of information For example, the following program writes and then reads back a double, an int, and a long to and from a disk file Notice how it uses sizeof to determine the length of each data type
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/* Write some non-character data to a disk file and read it back */ #include <stdioh> #include <stdlibh> int main(void) { FILE *fp; double d = 1223; int i = 101; long 1 = 123023L; if((fp=fopen("test", "wb+"))==NULL) printf(''Cannot open file\n"); exit(1); } {
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Page 246 fwrite(&d, sizeof(double), 1, fp); fwrite(&i, sizeof(int), 1, fp); fwrite(&l, sizeof(long), 1, fp); rewind(fp); fread(&d, sizeof (double), 1, fp); fread(&i, sizeof(int), 1, fp); fread(&l, sizeof(long), 1, fp); printf("%f %d %ld", d, i, 1); fclose(fp); return 0; }
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As this program illustrates, the buffer can be (and often is) simply the memory used to hold a variable In this simple program, the return values of fread( ) and fwrite( ) are ignored In the real world, however, you should check their return values for errors One of the most useful applications of fread( ) and fwrite( ) involves reading and writing userdefined data types, especially structures For example, given this structure,
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struct struct_type { float balance; char name[80]; } cust;
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the following statement writes the contents of cust to the file pointed to by fp:
fwrite(&cust, sizeof(struct struct_type), 1, fp);
A Mailing List Example To illustrate just how easy it is to write large amounts of data using fread( ) and fwrite( ), we will rework the mailing list program first shown in 7 The enhanced version will be capable of storing the addresses in a file As before, addresses will be stored in an array of structures of this type:
Page 247 struct addr { char name[30]; char street[40]; char city[20]; char state[3]; unsigned long int zip; } addr_list[MAX];
The value of MAX determines how many addresses the list can hold When the program executes, the name field of each structure is initialized with a null By convention, the program assumes that a structure is unused if the name is of zero length The save( ) and load( ) functions, shown next, are used to save and load the mailing list database Note how little code is contained in each function because of the power of fread( ) and fwrite( ) Notice also how these functions check the return values of fread( ) and fwrite( ) for errors
/* Save the list */ void save(void) { FILE *fp; register int i; if((fp=fopen("maillist", "wb"))==NULL) printf(''Cannot open file\n"); return; } for(i=0; i<MAX; i++) if(*addr_list[i]name) if(fwrite(&addr_list[i], sizeof(struct addr), 1, fp)!=l) printf("File write error\n"); fclose(fp); } /* Load the file */ void load(void) { FILE *fp; register int i; {
Page 248 if((fp=fopen("maillist", "rb"))==NULL) { printf(''Cannot open file\n"); return; } init_list(); for(i=0; i<MAX; i++) if(fread(&addr_list[i], sizeof(struct addr), 1, fp)!=1) if(feof(fp)) break; printf("File read error\n"); } fclose(fp); }
Both functions confirm a successful file operation by checking the return value of fread( ) or fwrite ( ) Also, load( ) must explicitly check for the end of the file via feof( ) because fread( ) returns the same value whether the end of the file has been reached or an error has occurred The entire mailing list program is shown next You may wish to use this as a core for further enhancements, such as the ability to search for addresses
#define MAX 100 struct addr { char name[30]; char street[40]; char city[20]; char state[3]; unsigned long int zip; } addr_list[MAX];
void init_list(void), enter(void); void delete(void), list(void); void load(void), save(void); int menu_select(void), find_free(void);
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