Exercise 1

#include <unistd.h>
#include <stdio.h>
int main(int argc, char *argv[]){
    char buffer[256];
    while(1){
        int length = read(STDIN_FILENO, buffer, 256);
        if(length == -1){
            perror(NULL);
            return 1;
        }
        if(length == 0)
            break;
        write(STDOUT_FILENO, buffer, length);
    }
    return 0;
}

Exercise 2

#include <unistd.h>
#include <stdio.h>
int readFile(){
    char buffer[256];
    while(1){
        int length = read(STDIN_FILENO, buffer, 256);
        if(length == -1){
            perror(NULL);
            return 1;
        }
        if(length == 0)
            return 0;
        write(STDOUT_FILENO, buffer, length);
    }
}
int main(int argc, char *argv[]){
    if(readFile())
        return -1;
    lseek(STDIN_FILENO, 0, SEEK_SET);
    if(readFile())
        return -1;
    return 0;
}

Exercise 3

#include <unistd.h>
#include <stdio.h>
int main(int argc, char *argv[]){
    char buffer[256];
    while(1){
        int length = read(4, buffer, 256);
        if(length == -1){
            perror(NULL);
            return 1;
        }
        if(length == 0)
            break;
        write(STDOUT_FILENO, buffer, length);
    }
    return 0;
}

Exercise 4

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
int main(int argc, char *argv[]){
    int fd = open("/tmp/z", O_RDONLY);
    if(fd == -1){
        perror(NULL);
        return 1;
    }
    char buffer[256];
    while(1){
        int length = read(fd, buffer, 256);
        if(length == -1){
            perror(NULL);
            return 1;
        }
        if(length == 0)
            break;
        write(STDOUT_FILENO, buffer, length);
    }
    close(fd);
    return 0;
}

Exercise 5

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
int main(int argc, char *argv[]){
    int fd = open(argv[1], O_RDONLY);
    if(fd == -1){
        perror(NULL);
        return 1;
    }
    char buffer[256];
    while(1){
        int length = read(fd, buffer, 256);
        if(length == -1){
            perror(NULL);
            return 1;
        }
        if(length == 0)
            break;
        write(STDOUT_FILENO, buffer, length);
    }
    close(fd);
    return 0;
}

Exercise 6

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[]){
    int fd = open(argv[1], O_RDONLY);
    if(fd == -1){
        perror(NULL);
        return 1;
    }
    int num = 1;
    int printNumber = 1;
    while(1){
        char c;
        int count = read(fd, &c, 1);
        if(count == -1){
            perror(NULL);
            return 1;
        }
        if(count == 0)
            break;
        if(printNumber){
            char numBuf[16];
            sprintf(numBuf, "%5d ", num++);
            write(STDOUT_FILENO, numBuf, strlen(numBuf));
            printNumber=0;
        }
        write(STDOUT_FILENO, &c, 1);
        if(c == '\n')
            printNumber=1;
    }
    close(fd);
    return 0;
}

The program above is intentionally made simple rather than efficient; the program below is more efficient, but has a few more lines.

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
 
void checkPrintNl(int *shallPrint, unsigned *number){
    if(!*shallPrint)
        return;
    char buf[12];
    sprintf(buf, "%5u ", (*number)++);
    write(STDOUT_FILENO, buf, strlen(buf));
    *shallPrint=0;
}
 
int main(int argc, char *argv[]){
    int fd = open(argv[1], O_RDONLY);
    if(fd == -1){
        perror(NULL);
        return 1;
    }
    int printLineNum = 1;
    unsigned lineNum = 1;
    while(1){
        char buf[256];
        int length = read(fd, buf, 256);
        if(length == -1){
            perror(NULL);
            return 1;
        }
        if(length == 0)
            break;
        char *it = buf, *nlPos;
        while( (nlPos = memchr(it, '\n', length)) ){
            checkPrintNl(&printLineNum, &lineNum);
            int partLen = nlPos+1-it;
            write(STDOUT_FILENO, it, partLen);
            length -= partLen;
            it = nlPos+1;
            printLineNum = 1;
        }
        if(length){
            checkPrintNl(&printLineNum, &lineNum);
            write(STDOUT_FILENO, it, length);
        }
    }
    return 0;
}

Exercise 7

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
 
#define CHECK(result, textOnFail) \
if(result == -1){                 \
    perror(textOnFail);           \
    return 1;                     \
}
int main(int argc, char *argv[]){
    if(argc < 3){
        write(STDERR_FILENO, "Not enough arguments\n", 21);
        return 1;
    }
    int file1 = open(argv[1], O_RDONLY);
    CHECK(file1, argv[1]);
    int file2 = open(argv[2], O_RDONLY);
    CHECK(file2, argv[2]);
    while(1){
        char buf1[256], buf2[256];
        int len1 = read(file1, buf1, 256);
        CHECK(len1, argv[1]);
        int len2 = read(file2, buf2, 256);
        CHECK(len2, argv[2]);
        if(len1 != len2)
            break;
        if(len1 == 0){
            write(STDOUT_FILENO, "Files have the same contents\n", 29);
            return 0;
        }
        if(memcmp(buf1, buf2, len1))
            break;
    }
    write(STDOUT_FILENO, "Files differ\n", 13);
    return 0;
}

Exercise 8

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
 
int main(int argc, char *argv[]){
    ++argv; --argc; // get rid of program name; simplifies array indexes
    int *file = malloc(sizeof(int) * argc);
    int running = 0;
    for(int i = 0; i < argc; ++i){
        file[i] = open(argv[i], O_RDONLY);
        if(file[i] == -1)
            perror(argv[i]);
        else
            running = 1;
    }
    while(running){
        running = 0;
        for(int i = 0; i < argc; ++i){
            while(file[i] != -1){
                char c;
                int l = read(file[i], &c, 1);
                if(l <= 0){
                    if(l == -1)
                        perror(argv[i]);
                    close(file[i]);
                    file[i] = -1;
                    break;
                }
                if(c == '\n'){
                    running = 1;
                    break;
                }
                write(STDOUT_FILENO, &c, 1);
            }
            write(STDOUT_FILENO, (i == argc-1) ? "\n" : "\t", 1);
        }
    }
    free(file);
    return 0;
}

The program above is intentionally made simple rather than efficient; the program below is more efficient, but has more lines.

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
 
/** A few words of explanation: each 'read' and 'write' call involves
 * the operating system. Calling the OS is expensive. Thus, I/O is
 * usually buffered. This program buffers both input and output, but
 * to buffer output and still use OS API, one has to buffer I/O oneself.
 */
#define BUFSIZE 4096
 
// ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ OUTPUT BUFFERING ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 
struct { //< custom standard output buffer
    char b[2*BUFSIZE];
    size_t c;
} outBuf;
 
void flushOutput(){ //< writes out the standard output buffer
    if(outBuf.c != write(STDOUT_FILENO, outBuf.b, outBuf.c)){
        perror("writing to output failed");
        exit(1);
    }
    outBuf.c = 0;
}
 
/** adds data to output buffer and outputs if buffer is sufficiently filled
  * @warning: this relies on an assumption that count never exceeds BUFSIZE */
void bufferOutput(char *data, size_t count){ 
    assert(count <= BUFSIZE);
    memcpy(outBuf.b + outBuf.c, data, count);
    if((outBuf.c += count) >= BUFSIZE)
        flushOutput();
}
 
// ~ ~ ~ ~ ~ READING LINES FROM INPUT & BUFFERING INPUT ~ ~ ~ ~ ~
 
struct myFile {  //< stores state & buffer of a file
    char *name;  //< used for diagnostics when read returns -1
    int fd;      //< once the file ends (or read fails), -1 is put here
    char buf[BUFSIZE];
    int pos;     //< position where data in buf starts
    int count;   //< number of bytes in but from pos to end of data
};
 
/// reads from file to buffer, 
/// @returns 0 if any data was read, else returns 1
int readChunk(struct myFile *f){
    f->pos = 0;
    f->count = read(f->fd, f->buf, BUFSIZE);
    if(f->count<=0){
        if(f->count == -1)
            perror(f->name);
        close(f->fd);
        f->fd = -1;
        return 1;
    }
    return 0;
}
 
/** reads and outputs a line from file,
 * @returns 0 if there is any outstanding data in file, else returns 1 */
int readAndOutputLine(struct myFile *f){
    while(1){
        if(f->fd == -1)
            return 1;
        if(f->count == 0)
            if(readChunk(f))
                return 1;
        char *nlPos, *startPos = f->buf+f->pos;
        if( (nlPos = memchr(startPos, '\n', f->count)) ){
            int partLen = nlPos-startPos;
            bufferOutput(startPos, partLen);
            f->pos   += partLen+1;
            f->count -= partLen+1;
            if(f->count)
                return 0;
            return readChunk(f);
        }
        bufferOutput(startPos, f->count);
        f->count = 0;
    }
}
 
// ~ ~ ~ ~ ~ OPENING FILES, LOOPING THROUGH LINES ~ ~ ~ ~ ~
 
int main(int argc, char *argv[]){
    ++argv; --argc; // get rid of program name; simplifies array indexes
    struct myFile *myFiles = malloc(sizeof(myFiles) * argc);
    int allFilesEof = 1;
    for(int i = 0 ; i < argc; ++i){
        myFiles[i].fd = open(argv[i], O_RDONLY);
        if(myFiles[i].fd == -1)
            perror(argv[i]);
        else {
            myFiles[i].name = argv[i];
            allFilesEof &= readChunk(myFiles+i);
        }
    }
    while(!allFilesEof){
        allFilesEof = readAndOutputLine(myFiles);
        for(int i = 1 ; i < argc; ++i){
            bufferOutput("\t", 1);
            allFilesEof &= readAndOutputLine(myFiles+i);
        }
        bufferOutput("\n", 1);
    }
    free(myFiles);
    flushOutput();
    return 0;
}

Exercise 9

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
 
void handleSignal(int num) {
  write(2, "Shutting down...\n", 17);
  exit(1);
}
 
int main() {
  signal(SIGINT, handleSignal);
  while (1)
    getchar();
}

Exercise 10

#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
 
void writeDateTo(int fd) {
  struct timespec now;
  char buf[21];
  clock_gettime(CLOCK_REALTIME, &now);
  buf[20] = '\n';
  for (int i = 0; i < 10; ++i, now.tv_nsec /= 10)
    buf[19 - i] = '0' + now.tv_nsec % 10;
  buf[10] = '.';
  for (int i = 0; i < 10; ++i, now.tv_sec /= 10)
    buf[9 - i] = '0' + now.tv_sec % 10;
  write(fd, buf, 21);
}
 
void handleSignal(int num) {
  int fd = open("signal_log", O_WRONLY | O_APPEND | O_CREAT, 0666);
  writeDateTo(fd);
  close(fd);
}
 
int main() {
  signal(SIGUSR1, handleSignal);
  while (1)
    getchar();
}