Thread: system calls read and write

  1. #1
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    system calls read and write

    Hey,
    Is it even possible to write to a binary file a number as an int and not as a string? If yes how do i write it and how to i read it?

  2. #2
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    To write an integer to a file

    Code:
       int i;
      fwrite(&i,sizeof(i),1,f);
    To read an integer from a file

    Code:
       int i;
      fread(&i,sizeof(i),1,f);
    (Where 'f' is a file pointer)

    You should also check for errors and act on them...

  3. #3
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    Quote Originally Posted by hamster_nz View Post
    To write an integer to a file

    Code:
       int i;
      fwrite(&i,sizeof(i),1,f);
    To read an integer from a file

    Code:
       int i;
      fread(&i,sizeof(i),1,f);
    (Where 'f' is a file pointer)

    You should also check for errors and act on them...
    I am supposed to use read and write system calls

  4. #4
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    The *nix read and write operate on bytes, so the bytes could be the bytes of the int value in memory, or they could be the bytes of a string representation of the number; it's up to you.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
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    Quote Originally Posted by giorgosmarga View Post
    I am supposed to use read and write system calls
    Here is using the "read" and "write" system calls directly.

    Note that this program needs no header files, as it is accessing the Linux system calls directly rather than using any library functions

    Code:
    // SYSTEM CALL NUMBERS
    #define SYSCALL_READ  0
    #define SYSCALL_WRITE 1
    
    
    // STANDARD FILE DESCRIPTORS
    #define STDIN  0
    #define STDOUT 1
    
    
    long syscall(long number, ...);
    
    
    const char msg[] = "Hello world! Press Enter\n";
    int main(void) {
       char buffer[1];
       syscall(SYSCALL_WRITE, STDOUT, &msg, sizeof(msg)-1);
       syscall(SYSCALL_READ, STDIN, &buffer, 1);
       return 1;
    }
    Here's the output:
    Code:
    $ ./main
    Hello world! Press Enter
    
    
    $

  6. #6
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    Or, using the wrapper functions and worrying about endianess:
    Code:
    #include <unistd.h>
    #include <arpa/inet.h>
    
    int n = 1;
    
    // Write all bytes of n in BIG ENDIAN format.
    n = htonl( n );
    write( STDOUT_FILENO, &n, sizeof n );

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