Synfire's Guide To C Programming

9. File I/O

Reading from and writting to a file is a very important aspect of programming in C. Even more so on *nix systems, because everything in *nix is a file. As you will learn in '12. sockets' even the internet is a file! but that is later on. Here you will learn how to read, write and append to a textfile.

Textfiles

Reading.c

#include <stdio.h>
#define MAX 1000

int main()
{
    FILE *f;
    /* creates a stream */

    char *string(MAX);
    f = fopen("filename", "r");
    /* links the stream to'filename' in 'r' (read) mode. */

    if(!f) /* if the stream isn't connected to the file */
        return(1); /* exit program */

    while( fgets(string, MAX, f) != NULL)
        printf("%s", string);
    /* creates a while loop and uses fgets to read from 'f' into
       'string' until it reaches a '\0' or the MAX length. */

    fclose(f);
    /* closes the stream */

    return(0);
}

Writing.c

#include <stdio.h>

int main()
{
    FILE *f;
    /* creates the stream */

    char *String[] = "This text will be written.";

    f=fopen("filename","w");
    /* opens the file in distructive write mode.
       Distructive write mode means that if the 
       file exists then it is overwritten, if it
       don't exist then it is created. */

    if(!f)  /* checks to see if all went well */
        return(1);

    fprintf(f, "%s\n", String);
    /* writes the contents of 'String' followed by
       a newline into 'filename'. */

    fclose(f);
    return(0);
}

Appending.c

#include <stdio.h>

int main()
{
    FILE *f;
    /* creates the stream */

    char *string[] = "This is appended to the end of the file.";

    f=fopen("filename","a");
    /* opens the file in APPEND mode. Append writes to the
       end of the document. */

    if(!f) /* checks to see if went well */
        return(1);

    fprintf(f, "%s\n", string);
    /* writes 'string' to the end of the file */

    fclose(f);
    return(0);
}

Now those examples should suffice for reading and writing to TEXT FILES. Thats all for this first part. Now we go on to structs the reason I have left out binary file manipulations is that honestly it goes beyond the scope of this document. But to make up for it, I am adding a socket section to the end. What can I say, I hate trying to explain binary files.

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