CODEWITHSUNDEEP
clinux
OddCommand24
OddCommand24

Reputation: 441

How to use the "write" system call using C langauge in Linux?

My code is working fine. The only error I'm getting is that after the program writes the text into the file i.e. text1.txt, the text file prints some weird symbols like /00 when I actually open it. 

int fd;
fd = open("text1.txt", O_RDWR);
char text[] = "This is my file.";
write(fd,text,sizeof(text));

Upvotes: 2

Views: 2946

Answers (1)

Dai
Dai

Reputation: 155270

  • You need to ensure that open succeeded instead of blindly writing to the file-descriptor.
    • Always check the return value of a syscall (and most C standard library functions) and check errno if the return value indicated an error.
  • Your string literal will include a hidden \0 (NULL) character after the dot.
    • Writing text directly to the file will therefore include the trailing \0 which is what you're seeing.

These issues can be rectified by:

  • Always checking the return value of a syscall - and in this case: print a helpful error message to stdout and perform any necessary cleanup (the goto closeFile; statement).

    • Because C doesn't have a native try/catch or RAII it means its difficult to write terse error-handling and cleanup code, but using goto for common clean-up code is generally acceptable in C, hence the goto closeFile statement.

  • Using strlen to get the actual length of the string.

    • Though in a pinch it's okay to use sizeof(text) - 1 provided you're in a scope where the C compiler knows the length of text as using sizeof() won't work if you cross a function boundary due to array pointer decay.

Like so:

void writeToFile() {

    int fd = open( "text1.txt", O_CREAT | O_WRONLY ); // Use `O_WRONLY` instead of `O_RDWR` if you're only writing to the file. Use `O_CREAT` to create a file if it doesn't already exist.
    if( fd == -1 ) {
        printf( "Error opening file: errno: %d - %s\n", errno, strerror( errno ) );
        return;
    }

    size_t textLength = strlen( text );
    size_t written = write( fd, text, textLength );
    if( written == -1 ) {
        printf( "Error writing text: errno: %d - %s\n", errno, strerror( errno ) );
        goto closeFile;
    }
    else if( written < textLength ) {
        printf( "Warning: Only %d of %d bytes were written.", written, textLength );
        goto closeFile;
    }
    else {
        // Carry on as normal.
    } 

closeFile:
    if( close( fd ) == -1 ) {
        printf( "Error closing file: errno: %d - %s\n", errno, strerror( errno ) );
    }
}

Upvotes: 4

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