"test.exe encountered a breakpoint"

Question

I am writing a UNIX paste clone. However I keep getting "encountered a breakpoint" messages, but VS won't tell me on what line it happened.

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

#define INITALLOC   16
#define STEP         8

int main(int argc, char *argv[])
{
    if (horzmerge(argc - 1, argv + 1) == 0) {
        perror("horzmerge");
        return EXIT_FAILURE;
    }
    getchar();
    return EXIT_SUCCESS;
}
int horzmerge(int nfiles, const char **filenames)
{
    FILE **files;
    char *line;
    int i;

    if ((files = malloc(nfiles * sizeof (FILE *))) == NULL)
        return 0;

    for (i = 0; i < nfiles; ++i)
        if ((files[i] = fopen(filenames[i], "r")) == NULL)
            return 0;

    do {
        for (i = 0; i < nfiles; ++i) {
            if (getline(files[i], &line) == 0)
                return 0;
            fprintf(stdout, "%s", line);
            free(line);
        }
        putchar('\n');
    } while (!feof(files[0]));  /* we can still get another line */

    for (i = 0; i < nfiles; ++i)
        fclose(files[i]);
    free(files);
    return 1;
}
int getline(FILE *fp, char **dynline)
{
    size_t nalloced = INITALLOC;
    int c, i;

    if ((*dynline = calloc(INITALLOC, sizeof(char))) == NULL)
        return 0;

    for (i = 0; (c = getc(fp)) != EOF && c != '\n'; ++i) {
        if (i == nalloced)
            if ((*dynline = realloc(*dynline, nalloced += STEP)) == NULL)
                return 0;
        (*dynline)[i] = c;
    }
    (*dynline)[i] = '\0';
    if (c == EOF)
        return EOF;
    return i;
}

I placed breakpoints, and saw that it was the free(line) statement in horzmerge. But sometimes the program runs fine. Sometimes it doesn't. Sometimes I get a "Heap corrupted" in getline. I've been working on this code for a week, still can't find the bug(s).

Try using something like [valgrind](http://valgrind.org/docs/manual/quick-start.html)? — Nicholas White, Dec 24 '15 at 00:54
No function prototypes? And what about `while (!feof(files[0]))` — Weather Vane, Dec 24 '15 at 00:59
@WeatherVane using the `feof` call, I can tell if I need to read more lines or not. — lost_in_the_source, Dec 24 '15 at 01:02
Have you read the man page for `feof()`? And anyway, that's only the first file in the array. — Weather Vane, Dec 24 '15 at 01:02
It is bad because the loop will be executed once more than intended, since it tells whether EOF was reached **after** an attempt to read? @WeatherVane — lost_in_the_source, Dec 24 '15 at 01:08
It does not tell you that end of file was reached. It tells you made a duff read. — Weather Vane, Dec 24 '15 at 01:11
[Why is “while ( !feof (file) )” always wrong?](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Weather Vane, Dec 24 '15 at 01:15

score 1 · Accepted Answer · answered Dec 24 '15 at 01:01

It looks to me like the line where you null-terminate the input string is capable of overrunning the buffer you calloced or realloced. That has the potential of corrupting your heap when you free that buffer.

Dont't forget to leave room for the null character at the end of the string when you allocate memory.

Null-terminated strings are like disco. They still suck forty years later.

"test.exe encountered a breakpoint"

1 Answers1