File I/O

File Open and Close

• File Pointer • Pointer to FILE structure

FILE *fp;

FILE *fopen(char *name, char *mode);

• Open a fileFILE *fp;

fp = fopen("filename","r");

• "r" for read, "w" for write, "a" for append.

• Close a filefclose(fp);

FILE

typedef struct {

char *fpos; /* Current position of file pointer (absolute address) */

void *base; /* Pointer to the base of the file */

unsigned short handle; /* File handle or File descriptor*/

short flags; /* Flags (see FileFlags) */

short unget; /* 1-byte buffer for ungetc (b15=1 if non-empty) */

unsigned long alloc; /* Number of currently allocated bytes for the file */

unsigned short buffincrement; /* Number of bytes allocated at once */

} FILE;

Character File I/O

• Read a character from a filec=getc(fp);/* macro,EOF for end of file or error */

c=fgetc(fp);/* function */

int ungetc(int c, FILE *fp); /* push c back to fp */

• Write a character to a fileputc(c,fp); /* macro */

fputc(c,fp); /* function */

• Check EOF or errorfeof(fp); /* non-zero if EOF */

ferror(fp);

/* non-zero if error occurred on the stream fp */

EOF

• End Of File

• Symbolic Constant, making nothing in a program depend on the specific numeric value

• Defined in stdio.h

• #define EOF -1or

• #define EOF 0

Example: Copy file1 file2

#include

int main(int argc, char *argv[])

{

FILE *fp1, *fp2;

int c;

fp1 = fopen(argv[1],"r");

fp2 = fopen(argv[2],"w");

while((c=getc(fp1))!= EOF)

putc(c,fp2);

fclose(fp1);

fclose(fp2);

return 0;

}

Formatted File I/O

int fscanf(FILE *fp, char *format, ...)

int fprintf(FILE *fp, char *format, ...)

Standard File I/O

• OS opens three files and provides (constant) file pointers to them

(declared in stdio.h). OS assigns them to a program.

• stdin : standard input connected to the keyboard

• stdout : standard output connected to the screen

• stderr : standard error connected to the screen

• OS is responsible for closing the three files.

• You can close stdin and stdout if not needed. Reassigned by freopen.

• Macros defined in terms of stdin and stdout

#define getchar() getc(stdin)

#define putchar(c) putc((c), stdout)

Error Handling – stderr and exit

#include /* cat: concatenate files, version 2 */main(int argc, char *argv[]){

FILE *fp;void filecopy(FILE *, FILE *);char *prog = argv[0]; /* program name for errors */if (argc == 1 ) /* no args; copy standard input */

filecopy(stdin, stdout);else

while (--argc > 0)if ((fp = fopen(*++argv, "r")) == NULL) {

fprintf(stderr, "%s: can't open %s\n”, prog, *argv);exit(1);

} else {filecopy(fp, stdout);fclose(fp);

}

if (ferror(stdout)) {fprintf(stderr, "%s: error writing stdout\n", prog);exit(2);

}exit(0);

}

/* filecopy: copy file ifp to file ofp */

void filecopy(FILE *ifp, FILE *ofp)

{

int c;

while ((c = getc(ifp)) != EOF)

putc(c, ofp);

}

Error Handling – stderr and exit

• exit calls fclose for each open output file to flush out any buffered output

• Return values• 0: all is well

• Non-zero : abnormal situation

Line Input and Output

• char *fgets(char *line, int maxline, FILE *fp)

/* fgets: get at most n chars from iop */char *fgets(char *s, int n, FILE *iop){

register int c;register char *cs;cs = s;while (--n > 0 && (c = getc(iop)) != EOF)

if ((*cs++ = c) == '\n’) break;*cs = '\0';

return (c == EOF && cs == s) ? NULL : s;}

Line Input and Output

• int *fputs(char *line, FILE *fp)

/* fputs: put string s on file iop */int fputs(char *s, FILE *iop){

int c;while (c = *s++)

putc(c, iop);

return ferror(iop) ? EOF : 0;}