replacer.c

#import "fishhook.h" // from https://github.com/facebook/fishhook

// Replace write and writev, the two "chokepoint" functions that writes to stderr and stdout will probably pass through
static int (*originalWritev)(int fd, const struct iovec *v, int n);
static int (*originalWrite)(int fd, const void *buf, size_t size);

void checkForBadConstraintsMessage(int fd, const char *string, size_t size) {
    if (strnstr(string, "UIViewAlertForUnsatisfiableConstraints", size)) {
        // Write it to the console anyways before crashing
        originalWrite(fd, string, size);
        abort();
    }
}

void writeWillOccur(int fd, const char *string, size_t size) {
    if (fd != STDERR_FILENO && fd != STDOUT_FILENO) {
        return;
    }
    // #########################################
    // #########################################
    // ######### Insert code at this point. for example, you could call checkForBadConstraintsMessage(string, size)
    // #########################################
    // #########################################
}

int myWritev(int fd, const struct iovec *v, int n) {
    for (int i = 0; i < n; i++) {
        writeWillOccur(fd, v[i].iov_base, v[i].iov_len);
    }
    // Keep this as the last line, or else make sure to restore errno right before this function returns
    return originalWritev(fd, v, n);
}

int myWrite(int fd, const void *buf, size_t size) {
    writeWillOccur(fd, buf, size);
    // Keep this as the last line, or else make sure to restore errno right before this function returns
    return originalWrite(fd, buf, size);
}

...

// This does the function replacement. Any calls before this will call the original functions and not ours
rebind_symbols((struct rebinding[2]){{"write", myWrite, (void *)&originalWrite}, {"writev", myWritev, (void *)&originalWritev}}, 2);