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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