Automatic Vars, Unbuff File I-O

Automatic Variables in Makefiles

Purpose

Avoid code duplication and simplify rules in Makefiles.

Manual Compilation Example

CC = clang
CFLAGS = -g
 
sorting.o: sorting.c
    $(CC) $(CFLAGS) -c sorting.c
 
insert.o: insert.c
    $(CC) $(CFLAGS) -c insert.c

What does this program do?

• It declares a variable (CC) that holds the compile and link (clang) method
• It declares a variable (CFLAGS) that holds the suffix for compiling with debugger
• sorting.o: sorting.c sets a target rule, telling the makefile that the object file sorting.o should be created from the source file sorting.c This is tedious, and can easily be automated.

Wildcards

%.o: %.c
	$(CC) $(CFLAGS) $< -o $@

• %: wildcard for filename matching
  • you can also use a variable instead of %

• $<: first dependency (the file after the colon)
• $@: target (left of the colon)
• $^: if you have multiple dependencies, this symbol refers to all dependencies

Unbuffered File I/O in C

Buffered I/O:

• fopen()
• fscanf()
• fclose()

Unbuffered I/O:

• open()
• read()
• close() → this is used in CSE 130, not in this class

System Call Functions

open()

int fd = open(const char *name, int flags);

• name: filename
• flags: O_RDONLY, O_WRONLY, O_RDWR
• returns: file descriptor, -1 (error case)

read()

ssize_T res = read(int fd, void *buf, size_t n);

• fd: file descriptor from open
• buf: buffer to receive bytes
• n: number of bytes to read
• returns:
  • res
    • not 0 or -1: number of bytes actually read
    • 0: EOF
    • -1: error

close()

int res = close(int fd);

• fd: file descriptor from open
• returns:
  • 0 (successfully closed)
  • -1 (error)

Handling File Larger Than Buffer

• Use a loop to repeatedly read characters

char buffer[16];
ssize_t result = read(fd, buffer, sizeof(buffer));

• If read() returns fewer bytes than requested:
  • Continue reading w/remaining buffer space
  • Recalculate n as space remains

Template Code for Unbuffered I/O

int main (int argc, char **argv) {
    // Check argc
    int fd = open(argv[1], O_RDONLY);
    // Check fd
    char buffer[16];
    int n = sizeof(buffer);
    // read() should be in a loop
    ssize_t result = read(fd, buffer, n);
    result = close(fd);
    return 0;
}