6. File Input/Output
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| 6.1 File Open |
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 C allows us to read data from text/binary files using fopen(). |
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| FILE ∗fopen(char name[], char mode[]) |
– mode can be "r" (read only),"w" (write only),"a" (append) among other options. "b" can be appended for binary files.
– fopen returns a pointer to the file stream if it exists or NULL otherwise.
– We don’t need to know the details of the FILE data type.
– Important: The standard input and output are also FILE *datatypes (stdin, stdout).
– Important: stderr corresponds to standard error output(different from stdout). |
| int fclose(FILE ∗fp) |
– closes the stream (releases OS resources).
– fclose() is automatically called on all open files when program terminates. |
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| 6.2 File Input |
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| int getc(FILE ∗fp) |
– reads a single character from the stream.
– returns the character read or EOF on error/end of file.
– Note: getchar simply uses the standard input to read a character.
We can implement it as follows: #define getchar() getc(stdin) |
| char[] fgets(char line[], int maxlen, FILE ∗fp) |
– reads a single line (upto maxlen characters) from the input stream (including linebreak).
– returns a pointer to the character array that stores the line (read-only)
– return NULL if end of stream. |
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| Example: fgets |
/* fgets: get at most n chars from a file */
char *fgets(char *s, int n, FILE *fp)
{
register int c;
register char *cs;
cs = s;
while (--n > 0 && (c == getc(fp)) != EOF)
if ((*cs++ = c) == ‘\n’)
break;
*cs = ‘\0’;
return (c == EOF && cs == s) ? NULL : s;
}
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| 6.3 File Output |
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| int putc(int c, FILE ∗fp) |
– writes a single character c to the output stream. returns the character written or EOF on error.
– Note: putchar simply uses the standard output to write a character.
We can implement it as follows: #define putchar(c) putc(c, stdout) |
| int fputs(char line[], FILE ∗fp) |
– writes a single line to the output stream.
– returns zero on success, EOF otherwise. |
| int fscanf(FILE ∗fp, char format[], arg1, arg2) |
– similar to scanf, sscanf
– reads items from input stream fp. |
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| Example: fputs |
/* fputs: put string s on file fp */
int fputs(char *s, FILE *fp)
{
int c;
while (c = *s++)
putc(c, fp);
return ferror(fp) ? EOF : 0;
} |
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| Example: concatenate files |
#include <stdio.h>
/* cat: concatenate files */
int main(int argc, char *argv[])
{
FILE *fp;
void filecopy(FILE *, FILE *);
if (argc == 1) filecopy(stdin, stdout);
else {
while(--argc > 0) {
if ((fp = fopen(*++argv, "r")) == NULL) {
printf("cat: can’t open %s\n", *argv);
return 1;
}
else {
filecopy(fp, stdout);
fclose(fp);
}
}
}
return 0;
}
/* filecopy: copy file ifp to file ofp */
void filecopy(FILE *ifp, FILE *ofp)
{
int c;
while ((c = getc(ifp)) != EOF)
putc(c, ofp);
} |
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