tmpfile() -- STDIO Function (libc)

Create a temporary file
#include <stdio.h>
FILE *tmpfile(void);

The function  tmpfile() creates a file to hold  data temporarily.  The file
is opened  into binary update mode (wb+) and  is removed automatically when
it is  closed or  when the program  exits.  There is  no way to  access the
temporary file by  name.  If your program needs to  do so, it should open a
file explicitly.

tmpfile() returns  NULL if  it could  not create a  temporary file.   If it
could, it returns a pointer to the FILE associated with the temporary file.
The function exit() removes all files created by tmpfile().

Example

This example implements a primitive  file editor that can edit large files.
It uses  two temporary files to  keep all changes.  The  editor accepts the
following commands:

    dn  delete; d52 deletes line 52
    in  insert; i7 inserts line before line 7
    pn  print; p17 prints line 17
    p   print the entire file
    w   write the edited file and quit
    q   quit without writing the file

The entire temporary file is copied with each command.

#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdarg.h>

FILE *fp, *tmp[2];
int linecount;

fatal(message)
char *message;
{
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}

/*
 * Copy up to line number or EOF.
 * Return number of lines copied.
 */
static int
copy(line, *ifp, ofp)
int line; FILE *ifp, *ofp;
{
    int i, c, count;

    count = 0;
    for(c=i=1; (i<line || line==-1) && c!=EOF; i++) {
        while((c = fgetc(ifp)) != EOF && c != '\n')
            fputc(c, ofp);

        if(c == '\n') {
            count++;
            fputc('\n', ofp);
        }
    }
    return(count);
}

/*
 * Read a file until line number is read.
 * Return 1 if line is found before EOF.
 */
static int
find(line, ifp)
int line; FILE *ifp;
{
    int i, c;

    for(c=i=1; i<line && c!=EOF; i++)
        while((c = fgetc(ifp)) != EOF && c != '\n')
            ;
    return(c != EOF);
}

main(int argc, char *argv[])
{
    int i, line, args;
    char c, cmdbuf[80];

    if(argc != 2)
        fatal("usage: tmpfile filename\n");

    if((tmp[0]=tmpfile())==NULL||(tmp[1]=tmpfile())==NULL)
        fatal("Error opening tmpfile\n");

    if((fp = fopen(argv[1], "r")) == NULL)
        fatal("Error opening input file\n");

    linecount = copy(-1, fp, tmp[i = 0]);
    fclose(fp);

    /* one file pass per command */
    for(;;) {
        if(gets(cmdbuf) == NULL)
            fatal("EOF on stdin\n");

        if(!(args = sscanf(cmdbuf, "%c%d", &c, &line)))
            continue;
        fseek(tmp[i], 0L, SEEK_SET);

        switch(c) {
        /* Write edited file */
        case 'w':
            if((fp = fopen(argv[1], "w")) ==  NULL)
                fatal("Error opening file\n");
            copy(linecount + 1, tmp[i], fp);
            fclose(fp);

        /* Quit */
        case 'q':
            exit(EXIT_SUCCESS);

        /* Print entire file */
        case 'p':
            if(args == 1) {
                copy(linecount + 1, tmp[i], stdout);
                continue;
            }
            if(find(line, tmp[i]))
                copy(2, tmp[i], stdout);
            continue;

        /* Delete a line */
        case 'd':
            if(args == 1)
                printf("dn where n is a number\n");
            else if(line > linecount)
                printf("only %d lines\n", linecount);

            else {
                copy(line, tmp[i], tmp[i^1]);
                if(find(2, tmp[i]))
                    copy(-1, tmp[i], tmp[i^1]);

                linecount--;
                fseek(tmp[i], 0L, SEEK_SET);
                i ^= 1;
            }
            continue;

        /* Insert a line */
        case 'i':
            if(1 == args)
                printf("in where n is a number\n");
            else if(line > linecount)
                printf("only %d lines\n", linecount);

            else {
                copy(line, tmp[i], tmp[i^1]);
                printf("Enter inserted line\n");
                copy(2, stdin, tmp[i^1]);
                copy(-1, tmp[i], tmp[i^1]);
                linecount++;

                fseek(tmp[i], 0L, SEEK_SET);
                i ^= 1;
            }
            continue;

        default:
            printf("Invalid request\n");
            continue;
        }
    }
}

See Also

mktemp(),
libc,
tempnam(),
tmpnam()
ANSI Standard, §7.9.4.3
POSIX Standard, §8.1

Notes

If a program exits abnormally or aborts, the files created by tmpfile() may
not be removed.

© 2012 by Stephen A. Ness