System IO
CAS CS210
Ying Ye
Boston University
Files
 In Linux, all I/O devices are modeled as files
 All input and output is performed by reading and writing the
appropriate files
 When a file is opened, system returns a file descriptor, which is
used to access the file
Standard I/O
 getchar(), putchar(), scanf(), printf()......
 When each process is created, three files are opened:
 standard input
 standard output
 standard error
Standard I/O
 int getchar(void): read a character from standard input stream,
return it as int
 int putchar(int c): write the character c to standard output
stream
 Usage:
char a = getchar();
putchar(a);
 Practice: http://cs-people.bu.edu/yingy/input.c
http://cs-people.bu.edu/yingy/output.c
Standard I/O
input.c:
output.c:
char string[6];
int len = 0;
int i;
while(string[len] != '\0'){
for(i = 0; i < 5; i++)
string[i] = getchar();
len++;
}
string[i] = '\0'
int i;
printf(string);
for(i = 0; i < len; i++)
putchar(string[i]);
File I/O
 File object in Standard C library: FILE
it contains the current position indicator in the file
 Open file:
FILE *fopen(char *name, char *mode);
mode: "r": read only
"r+": read and write
e.g.
FILE *fp = fopen("file_name", "r+");
File I/O
 Read:
int getc(FILE *fp);
returns a character from the current position in file, current
position moves one byte forward
e.g. char c = getc(fp);
 Write:
int putc(int c, FILE *fp);
writes a character c to file fp, current position moves one byte
forward
e.g. char c = 'a'; putc(c, fp);
File I/O
 Close file:
int fclose(FILE *fp);
 Download: http://cs-people.bu.edu/yingy/file.c
 First, create an empty file: vim testfile, then save and quit it
 Open testfile after you run your program
File I/O
fp = fopen("testfile", "r+");
fprintf(fp, string);
//doesn't put terminator to testfile
buffer = (char *)malloc(29 * sizeof(char));
fseek(fp, 0, SEEK_SET);
int i = 0;
while(i < 28) buffer[i++] = getc(fp);
buffer[i] = '\0';
fclose(fp);