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File system calls

System call and library function, open and its flags, read, write, lseek, close, descriptors 0, 1 and 2, the return value, errno and perror.

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  • System call: what is it?

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    A program's request to the kernel for an operation it may not do alone, such as reading a file; the processor switches from user mode to kernel mode while it is handled.

    This switch has a cost, hence library functions such as fread or printf that gather data in a buffer and only call read or write when worthwhile.

  • The file t.txt contains ABCDEF. What does it contain after this program?

    ```c
    int fd = open("t.txt", O_RDWR);
    lseek(fd, 2, SEEK_SET);
    write(fd, "xy", 2);
    close(fd);
    `

    • ABxyEF
    • ABxyCDEF
    • xyCDEF
    • ABCDEFxy
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    ABxyEF

    lseek sets the position at byte 2; write overwrites the next two bytes (C and D) without shifting anything. The file keeps its size of 6 bytes. Writing at the end would need SEEK_END or O_APPEND.

  • At the end of the file, read returns …; on error, it returns … and sets errno.

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    At the end of the file, read returns 0; on error, it returns -1 and sets errno.

    Mixing them up is a classic mistake: 0 is not an error, it is the normal stop signal for a read loop.

  • File descriptor: what is it?

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    A small integer, specific to each process, that designates an open file, a pipe, a socket or a terminal for the read, write and close calls.

    It is an index into a per-process table. The first three entries are open at startup: 0 standard input, 1 standard output, 2 standard error.

  • A program opens a file with open then calls write(fd, "ok", 2) without testing open's return. When the file is missing, the output shows write: Bad file descriptor. What is the cause and the fix?

    • write has too few bytes to write: increase its size
    • The file is not closed: add a close before the write
    • errno was reset to zero: read it again before the write
    • open returned -1, an invalid descriptor for write: test fd == -1 right after open
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    open returned -1, an invalid descriptor for write: test fd == -1 right after open

    The real fault is upstream: open failed (“No such file or directory”) and returned -1, which the program passed to write, which complained. Fixing the write would only hide the error; test open's return, print perror and exit.

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