read() returns invalid file descriptor ???

This is a discussion on read() returns invalid file descriptor ??? within the C Programming forums, part of the General Programming Boards category; Code: FILE *mysql; mysql = fopen("mysql.txt","rw"); char buffer[1000]; if (read(mysql,buffer,sizeof(buffer)) > 0) { printf ("%s\n",buffer); } how can this be ...

  1. #1
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    Unhappy read() returns invalid file descriptor ???

    Code:
    FILE *mysql;
    mysql = fopen("mysql.txt","rw");
    
    char buffer[1000];
    
    if (read(mysql,buffer,sizeof(buffer)) > 0) {
           printf ("%s\n",buffer);
    }
    how can this be explained? what am i doing wrong?
    read(134520840, 0xbfbea748, 1000) = -1 EBADF (Bad file descriptor)

  2. #2
    Registered User whiteflags's Avatar
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    You need to use fread() instead. To be short, file I/O functions come in groups. read() works best if you used open() to get a file descriptor from it. You used fopen(), which is fine.

  3. #3
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    read() is not a standard function in the C/C++ library.

    It is (IIRC) a windows and unix specific function. It does not accept an argument that is a pointer to a FILE: the argument is typically an integer which is a file descriptor returned by a function named open().

  4. #4
    Registered User Micko's Avatar
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    You should try to use something like this:
    Code:
    fread (buffer, sizeof buffer, 1, mysql);
    if you want to read file line by line then I suggest using fgets function!
    Last edited by Micko; 08-04-2006 at 06:49 AM.
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

  5. #5
    and the hat of wrongness Salem's Avatar
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    > mysql = fopen("mysql.txt","rw");
    Read the manual again, "rw" isn't a valid mode.
    Also look up the perror() function which will print a wordy description of why various things fail.

    Also as noted, you're using read() where you should be using fread()
    (Or fopen() where you should be using open())
    Either way, make them match up with one another.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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