gvbuffstderr.c

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#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>

struct buff_s {
    int fd;
    fpos_t pos;
    char *template;
};

/* switch stderr output to temporary file buffering
 */ 
void *buffstderr(void)
{
#ifdef _WIN32
    char *p;
#else
    int t;
#endif
    struct buff_s *b;

    assert ((b = malloc(sizeof(struct buff_s))));
    assert ((b->template = strdup("/tmp/stderr_buffer_XXXXXX")));
#ifdef _WIN32
    p = mktemp(b->template);
    assert (p);
#else
    t = mkstemp(b->template);
    assert (t > 0);
#endif
    fflush(stderr);
    fgetpos(stderr, &(b->pos));
    b->fd = dup(fileno(stderr));
    freopen(b->template, "w+", stderr);
    return (void *)b;
}

/* switch stderr output back to normal,
 *    and return any buffered output in a malloc'ed buffer
 */
char *unbuffstderr(struct buff_s *b)
{
    long sz;
    char *t = NULL;
    FILE *f;

    fflush(stderr);
    sz = ftell(stderr);
    dup2(b->fd, fileno(stderr));
    close(b->fd);
    clearerr(stderr);
    fsetpos(stderr, &(b->pos));

    if (sz) {
        /* stderr has been restored; these asserts use normal stderr output */
        assert((t = malloc(sz+1)));
        assert((f = fopen(b->template, "r")));
        assert( fread(t, 1, sz, f) == sz);
        fclose(f);
        t[sz]='\0';
    }

    unlink(b->template);
    free(b->template);
    free(b);

    return t;
}

#if 0  /* FOR TESTING */
int main (int argc, char *argv[]) {

        void *buff;
        char *text;

        fprintf(stderr, "First message\n");

        buff = buffstderr();
        fprintf(stderr, "Second message\n");

        fprintf(stderr, "Third message\n");
        
        text = unbuffstderr(buff);
        fprintf(stderr, "Fourth message\n");

        if (text) {
            fprintf(stderr, text);
            free(text);
        }

        return 0;
}
#endif